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Contributions to Zoology, 86 (3) – 2017

Another brick in the wall: population dynamics of a symbiotic species of Oxydromus (Annelida, Hesionidae), described as new based on morphometry

Daniel Martin1, Miguel A. Meca2, João Gil3, Pilar Drake4, Arne Nygren5

1.  Centre d’Estudis Avançats de Blanes (CEAB-CSIC) – Carrer d’Accés a la Cala Sant Francesc 14. 17300 Blanes, Girona, Catalunya, Spain. E-mail:

2.  Centre d’Estudis Avançats de Blanes (CEAB-CSIC) – Carrer d’Accés a la Cala Sant Francesc 14. 17300 Blanes, Girona, Catalunya, Spain.

3.  Centre d’Estudis Avançats de Blanes (CEAB-CSIC) – Carrer d’Accés a la Cala Sant Francesc 14. 17300 Blanes, Girona, Catalunya, Spain

4.  Instituto de Ciencias Marinas de Andalucía (ICMAN-CSIC), Avenida República Saharaui 2, Puerto Real 11519, Cádiz, Spain

5.  Sjöfartsmuseet Akvariet, Karl Johansgatan 1-3, 41459, Göteborg, Sweden

Keywords: Bivalvia,Cádiz Bay,Hesionidae,Iberian Peninsula,NE Atlantic Oxydromus,symbiosis,Tellinidae. D97B28C0-4BE9-4C1E-93F8-BD78F994A8D1


Oxydromus humesi is an annelid polychaete living as a strict bivalve endosymbiont (likely parasitic) of Tellina nymphalis in Congolese mangrove swamps and of Scrobicularia plana and Macomopsis pellucida in Iberian saltmarshes. The Congolese and Iberian polychaete populations were previously considered as belonging to the same species, the latter showing regular distribution, intra-specific aggressive behaviour, and complex host-entering behaviour. The fresh Iberian samples enabled us to undertake consistent morphometric analyses, as well as to further analyse the characteristics of the association and the population dynamics of the Iberian population hosted by S. plana. Among the morphological differences between the Congolese and Iberian specimens, leading to the description of the latter as Oxydromus okupa sp. nov., the most important are: 1) longer cephalic appendages, 2) greater distance between the eyes, 3) larger dorsal cirrostyle in relation to the corresponding dorsal lobe and cirrophore. Moreover, dorsal and ventral lobes are similar in length, with the tip of the former reaching the tip of the latter in O. okupa sp. nov., while the dorsal lobe is much shorter than the ventral one in O. humesi. Mature adults of O. okupa sp. nov. occurred during the whole study period, with a higher percentage of ripe females in spring and, particularly, in summer. Numerous host specimens showed the symbiont’s most preferred shell length (>26 - 36 mm). However, the prevalence was very low (usually <5%) and showed a clear seasonal pattern, being lower during spring/summer. This suggests that males are able to leave their hosts during this period, most likely to improve fertilization by directly entering or approaching a host occupied by a ripe female, while females usually remain inside. Based on the new results, the current knowledge of symbiotic Hesionidae and their relationships with invertebrate hosts is updated and discussed.


The Hesionidae is a well-known family of polychaetes whose representatives are most often free-living in shallow waters, although some occur in the deep-sea (Summers et al., 2015) and some lives as symbionts of other invertebrates (Martin and Britayev, 1998; Miller and Wolf, 2008; De Assis et al., 2012; Martin et al., 2012, 2015; Britayev et al., 2013; Chim et al., 2013).

Oxydromus humesi (Pettibone, 1961), which is one of these symbiotic hesionids, has been reported as a strict bivalve-associate that is hosted by Tellina nymphalis Lamarck, 1818 in Loango (Congo mangrove swamps) and by Scrobicularia plana (Da Costa, 1778) and Macomopsis pellucida (Spengler, 1798) (reported as Macoma cumana (O.G. Costa, 1830)) in Cádiz Bay, Iberian Peninsula (Pettibone, 1961; Martin et al., 2012, 2015). The Iberian hosts show similar morphologies and modes of life. However, in Cádiz Bay, M. pellucida occurs mainly in the subtidal (E. Pascual, pers. observ.), whilst it is very rare in the intertidal (Subida et al., 2013). In turn, S. plana occurs in intertidal saltmarshes (like Río San Pedro), where it shows very high abundances (Subida et al., 2011; Drake et al., 2014).

Based on occasional sampling of mainly S. plana associates, it has been inferred that the association apparently affects negatively the metabolism of the host (and so it is considered to be closer to a parasitism), that infestations did not occur in hosts measuring less than 20 mm in shell length, that all symbionts seem to be adults (which are not sexually dimorphic), and that there are no relationships between worm’s size and shell length (Martin et al., 2012, 2015). However, nothing is known of the seasonal characteristics of the association, as well as of the life cycle of the symbiont. Also, when re-describing O. humesi, Martin et al. (2015) discussed the possibility of having different species at the two known locations due to an hypothetical biogeographic isolation of the respective populations. The fact that no fresh specimens were available from the Congolese population (i.e. the original specimens were formalin-fixed) prevented a molecular approach. Morphologically, animals of the two known populations appeared to be indistinguishable and a preliminary comparison of chaetal morphology and appendage measurements based on the available specimens was therefore inconclusive (Martin et al., 2015), which lead us to undertake a more robust study based on morphometry.

Therefore, the following questions were addressed: Were hosts < 20 mm infested during the seasonal cycle? Did the symbionts prefer any host size among those > 20 mm? Were the prevalence and intensity of infestation constant along time? Had the symbiont a seasonal trend in reproductive traits or size-class structure? These observations were compared with the few available data on symbionts infesting M. pellucida. Based on the new results, the current knowledge on the symbiotic relationships involving hesionid polychaetes worldwide is summarized and discussed.

Material and Methods

Collection details and sampling were explained by Martin et al. (2015). Specimens of the host bivalves were collected: monthly, from April 2011 to May 2012 and in January 2013, at Río San Pedro, 36°31’56.28”, 6°12’53.28” (S. plana); in January 2013 at Río San Pedro mouth, 36°31’47.28”, 6°14’52.80” (M. pellucida). The specimens of S. plana from May 2011 were damaged during sampling. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. Possible non-biotic influences in the observed patterns could not be determined because no environmental data were recorded during the study period.

In the laboratory, the collected specimens of S. plana and M. pellucida were opened to estimate the infestation intensity and prevalence. All obtained worms, and the bivalves harbouring them, were counted. The longest shell diameter of the hosts was measured (length, in mm) using callipers. Samples of S. plana (i.e. 100 to 300 specimens) were collected monthly and measured (independently of the presence of the symbiont) to define the seasonal size structure of the host population.

The width (in µm) of the tenth segment (parapodia included) was used as a proxy for symbiont size (WW). WW was measured under a Nikon SMZ645 stereomicroscope equipped with a micrometric ocular. Ripe females were identified by the presence of intracoelomic oocytes, which were usually visible through the body wall. In preserved specimens, the worm tissues become more opaque than in living ones, which usually hindered observation of the oocytes. Their presence was confirmed by placing the worms in a Petri dish and then cutting their body wall and applying a gentle pressure with the forceps so that present oocytes would spill from the coelomic cavity for observation. Digital images were taken with a CMEX camera, linked to a Zeiss Stemi 2000-c, stereomicroscope, using the ImageFocus 4.0 software by Euromex.

The sizes of the Iberian symbionts and their hosts were used to estimate their monthly size-class frequencies. The frequency of infested bivalves and that of the whole population was estimated separately and then expressed as percentages for comparison. The worm/host size and the ripe females/prevalence relationships were assessed by correlation analyses, while differences in percentage of females and prevalence during warm (i.e. March to September) and cold (i.e. October to February) periods were assessed by one-way Analysis of Variance (one-way ANOVA).

For morphometric purposes, preserved specimens of the Congolese population were kindly loaned by the United States National Museum of the Smithsonian Institution (USNM). Twenty-five specimens from the Iberian population, representative of all available sizes, were selected for the analyses, while for the Congolese population, 25 specimens were selected on the basis of the quality of the available specimens (as all of them were similar in size). The following characters were selected and measured (Fig. 1): WW, worm width without parapodia (WWP, µm), worm length (WL, µm), number of segments (NS), head width (HW, µm), head length (HL, µm), length of lateral antenna (LA, µm), length of palpophore (PP, µm), length of palpostyle (PS, µm), distance between anterior eyes (DAE, µm), distance between posterior eyes (DPE, µm), distance between anterior and posterior eyes (DAPE, µm), length of dorsal lobe (DL, µm), length of dorsal cirrophore (DCP, µm), length of dorsal cirrostyle (DCS, µm), length of posterior neurochaetal lobe (PNCL, µm), and length of ventral cirri (VC, µm) (measured at the 10th parapodia level). As cirrostyles alternate short and long along the body (Martin et al., 2015), the measurements were recorded for two parapodia (from chaetiger 10 to 30, depending on the specimen) bearing long and short cirrostyles, respectively, and indicated by adding L and S to the end of the acronym (e.g. “DCSL” and “DCSS” meaning dorsal cirrostyle from parapodia having long and short cirrostyles, respectively). Special care was addressed to avoid measurements on damaged appendages. The eyespot diameter, which is highly variable among the specimens of the two species (probably as a result to different individual responses to fixation) was not considered a valid taxonomic character and was not used in the statistical analyses. This also led us to measure all characters based on the eyes at the centre of each eyespot (Fig. 1). WWP, WL and NS were measured as WW, while the remaining characters were measured under a Motic BA210 binocular microscope equipped with a TOUPCAM™ U3CMOS digital camera, managed through the ToupView 3.7 software.


Fig. 1. Oxydromus okupa sp. nov. Schemes of anterior end (a), two mid-body segments (b) and one mid-body parapodia (c), showing the measurements used in the morphometric analyses. WW: worm width with parapodia (µm); WWP: worm width without parapodia (µm); HW: head width (µm); HL: head length (µm); LA: length of lateral antenna (µm); PP: length of palpophore (µm); PS: length of palpostyle (µm); DAE: distance between anterior eyes (µm); DPE: distance between posterior eyes (µm); DAPE: distance between anterior and posterior eyes (µm); DL: length of dorsal lobe (µm): DCP: length of dorsal cirrophore (µm); DCS: length of dorsal cirrostyle (µm); PNCL: length of posterior neurochaetal lobe (µm); VL: ventral lobe; VC: length of ventral cirri (µm); L: long cirrostyle; S: short cirrostyle.

The inter-population differences were analysed for three different datasets: 1) raw data (direct measurements), 2) size-independent data; 3) taxonomically relevant character proportions (i.e., WL/WW, NS/WW, WWP/WW, DCPL/DLL, DCSL/DLL, DCSL/DCPL, VCL/PNCLL, DCPS/DLS, DCSS/DLS, DCSS/DCPS, VCS/PNCLS, HL/HW, LA/HL, PP/HL, PS/HL, PS/PP, DAE/HW, DPE/HW, DAPE/HL, DAE/DPE). Size-dependency of both measured characters and character proportions was assessed by Pearson correlation. Size-independent measurements were used without transformation, while size-dependent ones were divided by worm size (WW). The inter-population differences for the averaged measurements were estimated by one-way ANOVAs. Since multiple F-tests were carried out within each data set, the level of statistical significance was set at p < 0.05 and adjusted according to false discovery rate procedure (Benjamini and Hochberg, 1995). To avoid biases due to size differences, the description of the new species was based on the size-independent data as well as on the character proportions.

All character sets (i.e., raw data, size-independent data, proportions) were analysed by Principal Component Analysis (PCA) based on normalised data. The significance of the inter-population differences was explored by one-way analysis of similarity (ANOSIM) based on Euclidean distance resemblance matrices, while the contribution of each measured character to the distance within and between the two populations was assessed by the Similarity Percentages analysis (SIMPER) based on Euclidean distance. Forward stepwise discriminant analyses (FSDA) were used to obtain discriminant functions for the two populations under study, using the three datasets and assuming equal intra-population covariance matrices and threshold significance levels of 0.05 both to include and to remove variables. The similarity of the intra-population covariance matrices was assessed by the Box test (based on an asymptotic approach to the Fisher’s F index) and the differences between population-averaged vectors were assessed by the Rao approach to the Wilks’ Lambda test. The classification matrices were built by considering the sensitivity percentage (i.e. well classified positive events) as belonging to the Iberian population, and the specificity percentage (i.e., well classified negative events) as belonging to the Congolese population. Furthermore, to determine the probability that one of the observed specimens would effectively belong to one of the two populations, we also performed a cross-validation in which each observation was removed from the original matrix and the model and the forecast successively estimated, allowing a better adjust to the model (Huberty, 1994).

Pearson correlation analyses, one-way ANOVAs and discriminant analyses were performed with the XLSTAT software (2015.5.01.23039, copyright by Addinsoft 1995-2016). All remaining analyses were conducted using the PRIMER software, version 6.1.11, copyright by PRIMER-E Ltd. 2008 (Clarke and Warwick, 2001; Clarke and Gorley, 2006).



Raw data. The PCA based on raw data shows a strong size-dependency of most characters, with the first axis (Eigenvalue = 12.0) explaining 54.6% of the variation and the second one (Eigenvalue = 3.52) only 16.0% (Fig. 2A). All characters of the Iberian population were size-dependent (except LA and PS), while the only size-dependent characters in the Congolese population were WL, NS, WWP, VCL, DLS and VCS (Table 1A). The size-relationship in the PCA plot was also more evident for the Iberian population, whose specimens showed a wider size-range representativeness than the Congolese ones. Nevertheless, the two populations clearly form separate groups in the PCA plot, and turned to be significantly different (ANOSIM, Global R = 0.435, significance level = 0.1%).


Fig. 2. Principal Component Analyses plots. a. Based on raw data. b. Based on size independent data. c. Based on measurement proportions.


Table 1. Relationships with worm size (as body width with parapodia) in Oxydromus okupa sp. nov., with bold characters indicating significant differences. A. Morphometric measurements. B. Taxonomically relevant proportions. Measurement abbreviations as in Fig. 1 . Coeff: Pearson correlation coefficient; p: significance level.

The intra-population average distances for the Iberian and Congolese worms were 29.9% and 7.10%, respectively, whilst the inter-population dissimilarity was 48.67%. PS, LA and PP, and NS, WL and PP most contributed to the Iberian and Congolese intra-population similarity, respectively, whilst LA, PS and PP most contributed to the inter-population dissimilarity (SIMPER, Table 2).


Table 2. List of the ten most contributing raw measurements to the intra-population similarities and inter-population dissimilarity based on the SIMPER analyses. Measurement abbreviations as in Fig. 1. Av.Value: Average value; Av.Sq.Dist: average square distance; Sq.Dist/SD: square distance divided by standard deviation; Contrib%: percentage of contribution; Cum.%: cumulative percentage of contribution.

All averaged character measurements showing significant differences were higher in the Iberian than in the Congolese population, except for NS and DLS, with the most remarkable difference being at LA (one-way ANOVA, Table 3).


Table 3. Comparative table of the raw morphometric measurements (µm) in the Iberian and Congolese populations. Differences expressed as percentages, with bold characters indicating significant differences (according to Benjamini & Hochberg, 1995) that may be higher (normal text) or lower (italics) in the Iberian than in the Congolese population, respectively. Measurement abbreviations as in Fig. 1. Min: minimum; Max: maximum; Mean: average ± standard deviation; F: Fisher’s F index; p: significance level.

Based on raw data, the discriminant function included the variables DLS, PP, PNCLL and LA (Table 4). This function correctly classified the 100% of individuals from both localities (Fig. 3) and the success probability in the cross validation was of 98%.


Table 4. Results of the discriminant analyses based on raw data, size-independent data and measurement proportions. F: Fisher’s F index; Lambda: Rao approach to the Wilks’ Lambda test; p: significance level; Coeff.: Standardized coefficients for the variables included in the inter-population discriminant functions, arranged in a decreasing order, according to their contribution to the inter-population discrimination; Congo, Iberian: coefficients of the selected variables in the classification functions for the Congolese and Iberian populations, respectively. Intercept: Intercept of the classification functions for each dataset. Measurement abbreviations as in Fig. 1.

Size-independent data. For the purpose of this analysis, and taking into account that the size range was well represented in the Iberian population, only LA and PS raw data were considered as size-independent (Table 1A). The PCA plot based on size-independent data revealed again two clearly different groups corresponding to the two studied populations (Fig. 2B). Despite the axes being less representative than those obtained for the raw data (Eigenvalues = 6.61 and 3.49, variation explained = 31.5% and 16.6%, respectively for axis 1 and 2), the two populations were more clearly distinguishable and showed more significant differences than the raw data (ANOSIM, global R = 0.539, significance level = 0.1%).

Based on these size-independent characters, the intra-population average distance within the Iberian and Congolese populations was 21.0% and 11.1%, respectively, whilst the inter-population distance was 50.3%. PS, DAPE and LA, and WL, NS, VCS, and PP most contributed to the Iberian and Congolese intra-population similarity, respectively, whilst the Congolese vs. Iberian dissimilarity was mainly explained by LA, DAPE, DLS, PP, and PS (SIMPER, Table 5).


Table 5. List of the ten most contributing size-independent measurements to the intra-population similarities and inter-population dissimilarity based on the SIMPER analyses. Measurement abbreviations as in Fig. 1. Av.Value: Average value; Av.Sq.Dist: average square distance; Sq.Dist/SD: square distance divided by standard deviation; Contrib%: percentage of contribution; Cum.%: cumulative percentage of contribution.

When comparing the averaged character measurements, all those showing significant differences were higher in the Iberian than in the Congolese population (particularly, LA, PS, PP and DAPE), except for NS, WWP, DLL, and DLS in particular (one-way ANOVA, Table 6).


Table 6. Comparative table of the size-independent morphometric measurements in the Iberian and Congolese populations. Differences expressed as percentages, with bold characters indicating significant differences (according to Benjamini and Hochberg, 1995) that may be higher (normal text) or lower (italics) in the Iberian than in the Congolese population. Measurement abbreviations as in Fig. 1. Min: minimum; Max: maximum; Mean: average ± standard deviation ; F: Fisher’s F index; p: significance level.

Based on size-independent data, the discriminant function included the variables WL, NS, DLS, PP, PNCLL, and LA (Table 4). This function correctly classified the 100% of individuals from both localities (Fig. 3) and the success probability in the cross validation was 100%.


Fig. 3. Scatterplots for individual scores and centroids of the discriminant functions obtained by the discriminant analyses carried out on the three datasets: raw data, size-independent data and measurement proportions. IB: Iberian population; CO: Congolese population.

Character proportions. Nine of the 20 character proportions analysed showed significant negative (n = 7) or positive (n = 2) correlations with size, indicating allometric relationships in the Iberian population, while only one was significantly negatively correlated with size in the case of the Congolese population (Table 1B).

In the PCA based on character proportions, Axes 1 (eigenvalue = 5.38) and 2 (eigenvalue = 3.13) explained 26.9% and 15.6% of the variation, respectively. The PCA plot also highlighted a marked clustering for the individuals of the two populations under study (Fig. 2C), the results being slightly less discriminant than the previous ones but equally highly significant (ANOSIM, Global R = 0.421, significance level = 0.1%).

The average intra-population distance for the Iberian and Congolese populations were 21.4% and 11.1%, respectively, whilst the average inter-population distance was 46.0%. DAPE/HL, DCSS/DLS, and LA/HL, and WL/WW, NS/WW, DCSS/DLS, and LA/LH were the most informative proportions for the intra-population similarity in Iberian and Congolese worms, respectively, whilst the intra-population dissimilarity was mainly explained by DCSL/DLL, LA/HL, NS/NW, and DAPE/HL (SIMPER, Table 7).


Table 7. List of the ten most contributing measurement proportions to the intra-population similarities and inter-population dissimilarity based on the SIMPER analyses. Measurement abbreviations as in Fig. 1. Av.Value: Average value; Av.Sq.Dist: average square distance; Sq.Dist/SD: square distance divided by standard deviation; Contrib%: percentage of contribution; Cum.%: cumulative percentage of contribution.

All averaged character proportions showing significant differences, except for NS/WW and WWP/WW, were higher in the Iberian than in the Congolese population, most of them with differences higher than 10% and particularly higher than 20% in the case of DCSS/DLS (one-way ANOVA, Table 8).

Based on character proportions, the discriminant function included the variables WL/WW, NS/WW, DCSS/DLS, PS/HL, and DAE/DPE (Table 4). This function correctly classified the 100% of individuals from both localities (Fig. 3) and the success probability in the cross validation was 96%.

Population size-structure

A total of 275 worms were collected during this study, 246 infesting S. plana (241 during the seasonal monitoring, five in January 2013), 26 in M. pellucida (plus three outside the host, but in the same container) (Table 9). Sizes ranged from 1.8 to 3.6 mm. Small-sized worms seem to be better represented during late autumn, but also in winter, while large worms were more or less constantly present throughout the study period, except during late winter and early spring (Fig. 4).


Table 9. Oxydromus okupa sp. nov. Synthesis of the monthly captures, total and infested number of hosts, percentage of ripe females, and prevalence during the study period.


Fig. 4. Oxydromus okupa sp. nov. Size-frequencies of the specimens hosted by Scrobicularia plana from April 2011 to May 2012, and by Macomopsis pellucida in January 2013. 2011: April, n = 16; June, n = 22; July, n = 15; August, n = 10; September, n = 17; October, n = 11; November, n = 28; December, n = 49. 2012: January, n = 23; February, n = 20; March, n = 8; April, n = 7; May, n = 15; 2013: January, n = 26.

Ripe females occurred during the whole period except in April 2012, and were always among the largest size-classes (2.2 to 3.6 mm) (Fig. 4). The highest percentages occurred in mid spring and summer, being August 2011 the single month during which the proportion ripe females vs. non-sexed adults sexes was 1:1 (Fig. 5; Table 9). The lowest percentages occurred in autumn and winter (Fig. 5; Table 9).


Fig. 5. Oxydromus okupa sp. nov. Monthly prevalence of ripe females vs. total prevalence in the infested Scrobicularia plana.

A total of 6,917 specimens of S. plana and 39 of M. pellucida were collected (Table 9). Length in S. plana ranged from 20 to 40 mm, with the exception of one infested host measuring 40.7 mm. However, bivalves with intermediate lengths (i.e., 26-36 mm) occurred during the whole year, being always the most abundant and also the most infested ones (Fig. 6). The most balanced size class frequency distribution occurred in September 2011. In M. pellucida, the size-class range was restricted to 20-30 mm length, and the most infested ones were slightly smaller (20-28 mm) than in S. plana (Fig. 6).


Fig. 6. Size-frequencies of the total vs. infested Scrobicularia plana from April 2011 to May 2012 and Macomopsis pellucida in January 2013 (n = 39). Monthly total number of individuals of S. plana: 2011: April, n = 300; June, n = 200; July, n = 94; August, n = 100; September, n = 108; October, n = 99; November, n = 99; December, n = 124. 2012: January, n = 100; February, n = 98; March, n = 150; April, n = 102; May, n = 100.

Overall, there was a non-significant size correlation between O. okupa sp. nov. and the host S. plana (Pearson coefficient = 0.123, p = 0.067). The monthly trends were also non-significant (Pearson coefficient = -0.375 to 0.351, p = 0.103 to 0.818), except for a positive correlation in April 2012 (Pearson coefficient = 0.870, p = 0.011). Conversely, O. okupa sp. nov. / M. pellucida symbiont-host pairs (26) collected in January 2013 showed a significant, positive size correlation (Pearson coefficient = 0.400, p = 0.021).

Infestation characteristics

The studied population of O. okupa sp. nov. showed a seasonal variability in prevalence (Fig. 5; Table 9). The highest percentages occurred during late autumn-mid winter. The highest peak was in December 2011 (i.e. 19.2%), in coincidence with the highest number of symbionts (Table 9). Intermediate prevalences occurred in November 2011 (i.e. 7.2%), January 2012 (i.e. 8.2%) and February 2012 (i.e. 6.1%). The remaining prevalences were always < 5% and the lowest one occurred in April 2011 (i.e. 1.6%). The intensity of the infestation was always a single worm per host. In the case of the specimens associated with M. pellucida, the prevalence was 66.7% (Table 9), reaching 89.3% in hosts > 20 mm long (Fig. 6).

There was a non-significant correlation between the percentage of ripe females and prevalence in S. plana along the studied period (Pearson coefficient = -0.407, p = 0.168). However, the percentage of ripe females was, on average, significantly higher (one-way ANOVA, F = 9.253, p = 0.011) in the warm than in the cold period, reaching 31.0±4.8% and 7.5±6.1%, respectively, while the prevalence showed exactly the contrary pattern, 3.2±1.3% vs. 8.9±1.7% (one-way ANOVA, F = 7.066, p = 0.022) (Fig. 5).


Morphometry and taxonomy

Molecular analyses currently tend to displace morphometry in studies trying to discriminate within closely related morphotypes for taxonomic purposes (Blaxter, 2004; Hebert and Gregory, 2005; Godfray, 2007). However, most taxonomists agree that combining the different approaches, applying the so-called Integrative Taxonomy, is the most effective strategy to build a stable and robust taxonomy (Will et al., 2005; Padial et al., 2010). Hence, the importance of morphology must not be forgotten in the genomics era (Giribert, 2015). Moreover, it is not always possible to obtain adequate material for genetic studies because the used preservation methods are destructive, while there is no fresh material available. In such cases morphometry may be the only approach allowing to identify distinguishing characters within species complexes.

Among polychaetes, many taxonomically robust morphological characters may be defined by measurements and/or proportions, some of them being size-dependent (Ben-Eliahu, 1987; Fauchald, 1991; Sigvalda­dóttir and Mackie, 1993). This approach has been used independently of molecular analyses to successfully resolve the taxonomy of several sibling species complexes, often leading to the description of new species (Orrhage and Sundberg, 1990; Fauchald, 1991; Blake, 2000; Koh and Bhaud, 2003; Koh et al., 2003; Martin et al., 2003, 2006, 2009; Ford and Hutchings, 2005; Garraffoni and de Garcia Camargo, 2006; Glasby and Glasby, 2006; Lattig et al., 2007; Hernández-Alcántara and Solís-Weiss, 2014; Coutinho et al., 2015). However, Ford and Hutchings (2005) were the first to consider the use of the statistical dissimilarities derived from the SIMPER routine of the PRIMER software (Clarke and Warwick, 2001; Clarke and Gorley, 2006), based on a matrix of morphometric measurements, as a robust support to distinguish between morphologically close species. Accordingly, they described three new species whose average dissimilarities ranged between 11-19%, later finding morphological evidences supporting the erection of the new species. Our morphometric approach, in turn, clearly discriminates the two populations of “O. humesi” which, despite their close morphologies, showed average dissimilarities ranging from 30 to 36% in the SIMPER, thus almost three times higher than the Australian species of Owenia. A detailed comparison of character variability, particularly those that were responsible for the intra-population similarity and the inter-population dissimilarity, led to a reliable way to solve this question. There were significant differences in size range between the Iberian and the Congolese populations (more restricted in the later, leading us to find less size-correlated parameters in Congolese worms). Our approach to compare the two populations led to evident and consistent differences in the appendage measurements and relative proportions (Tables 6, 8) that are considered as robust enough to formally describe the Iberian specimens as the new species O. okupa sp. nov. In addition, we further validate these differences by means of discriminant analyses, whose respective discriminant functions were able to identify the members of the two species with a reliability of 96 to 100% in all tests.

Infestation characteristics

Host/symbiont size relationships. Positive host/symbiont size relationships may either indicate 1) active size segregation behaviour, as reported both for the symbiotic coral dwelling crab Trapezia bidentata (Forskål, 1775) (Adams et al., 1985) and the fish Gobiodon histrio (Valenciennes, 1837) (Hobbs and Munday, 2004), which seem to be able to migrate from colony to colony to choose one of an appropriate size, or 2) parallel growth of hosts and symbionts, as reported both for the nemertean Malacobdella grossa (Müller, 1776), hosted by Arctica islandica (Linnaeus, 1767) (Sundet and Jobling, 1985) and the pontoniin shrimp Anchistus custos (Forskål, 1775), hosted by the bivalve Pinna bicolor Gmelin, 1791 (Britayev and Fahrutdinov, 1994). In turn, the absence of host/symbiont size correlation could be caused by symbionts that either colonize the hosts during a single phase of their life-history (e.g. juveniles, adults), that are highly mobile (thus, colonizing the available host independently of their size), or growth faster than the host, even if the infestation starts with juveniles colonizing small-sized hosts (Britayev et al., 2007).

Among symbiotic polychaetes, positive host/symbiont size-relationships are not common (Martin and Britayev, 1998) and, when occurring, they may be modified by differential behaviour linked to, for instance, reproductive activities. This seems to be the case of Branchipolynoe seepensis Pettibone, 1986, an obligate symbiont of deep-sea hydrothermal vent mytilids of the genus Bathymodiolus, whose females have a longer life span than males and remain inside the same host along their whole life, while males leave their original hosts for mating and then colonize the available hosts independently of their size. Consequently, positive host/symbiont size correlations occur only for females and for non-mature males (Britayev et al., 2007). In most known cases, however, the relationship between size structure of symbiotic polychaetes and their hosts is unclear (Martin et al., 1991, 1992; Emson et al., 1993; Rozbaczylo and Cañete, 1993; Britayev and Zamyshliak, 1996) so that the life histories of the former have been considered to be independent of that of the latter, with the hosts tending to live longer and commonly hosting successive symbionts (Martin and Britayev, 1998).

The absence of host/symbiont size-relationships previously reported for the intertidal population of O. okupa sp. nov. (Martin et al., 2012) was here confirmed by the absence of significant correlations found along the whole study period. The only exception was April 2012, which we consider as a spurious relationship due to the low number of collected worms (n<10) (Fig. 4; Table 9). However, other months with a similar number of individuals (e.g. n = 8 - 11 in March, October, and August) (Fig. 4; Table 9) showed non-significant relationships. These results support Martin et al. (2012) in that the worms do not grow together with their hosts in their intertidal environment. In turn, O. okupa sp. nov. was reported to infest specimens of S. plana > 20 mm in shell length and to be more frequent in intermediate-sized shells, 26-36 mm long (Martin et al., 2012), which has also been confirmed along the whole period of study. Limiting host sizes have been reported for the starfish Asterias rathbuni (Verrill, 1909) in Vostok Bay (Sea of Japan), whose specimens with disc radii lower than 35 mm were not inhabited by Arctonoe vittata (Grube, 1855), and those with radii up to 90 mm harboured one single symbiont, whereas the largest starfishes could host up to four polychaetes (Britayev et al., 1989). Another example occurs in Chile, where the scale-worm Harmothoe commensalis Rozbaczylo and Cañete, 1993 did not occur in specimens of the clam Gari solida (Gray, 1828) < 60 mm in shell length (Rozbaczylo and Cañete, 1993). The size-limit for S. plana as a host of O. okupa sp. nov., also implied that there were no small juveniles infesting small-sized hosts, which allows us to discard the fast-growing hypothesis for this new species.

On the other hand, the infested specimens of M. pellucida collected in a subtidal environment during January 2013 included a considerable amount of small-sized hosts (Fig. 6). Moreover, there was a significant positive size correlation between hosts and symbionts. This relationship may be caused either by the symbiont selecting the host to be infested according to the appropriateness of its size, or by host/symbiont parallel growth, which cannot be assessed in light of our present data. Despite the low number of collected bivalves, the existing size relationship and the very high infestation rates (Table 9), suggest together that M. pellucida and the subtidal environment could be the preferred host and habitat for O. okupa sp. nov., with the symbiont selecting the most suitable hosts when conditions are optimal.


Prevalence of the infestation. The prevalence of commensal polychaetes is highly variable and has been considered as a species-specific characteristic. However, a commensal species may also show different prevalences, which may vary according to bathymetric, spatial and temporal (i.e. intra- and inter-annual) patterns (Martin and Britayev, 1998). Studies including temporal trends are, however, scarce. With the exception of the impressive fidelity of the year-to-year counts of a regular seasonal trend reported for the infestation prevalence of Ophiocoma echinata (Lamarck, 1816) by Branchiosyllis exilis (Gravier, 1900) in Panamá (Hendler and Meyer, 1982), the only known data on year-to-year variability for a symbiotic polychaete are probably those on Arctonoe vittata (Grube, 1855). In Vostok Bay, this worm infests the starfish Asterias amurensis Lütken, 1871 with a progressive increase in prevalence from 0% in 1975-76 to 8.4% in 1978 and to 79.1% in 1980 (Britayev, 1991). Based on our study period, O. okupa sp. nov. seems to show a seasonal trend in prevalence, with the highest percentages occurring during the coldest seasons, i.e. from late autumn to late winter (Fig. 5; Table 9). The present dataset only covers a single year and therefore inter-annual regularities cannot be inferred.

Theoretically, we may expect an influence of the host’s population structure on that of the symbiont (Martin & Britayev, 1998). A positive relationship between host density and infestation characteristics was reported for A. vittata and its host starfish A. rathbuni in Vostok Bay (Britayev et al., 1989). The highest prevalence, mean intensity and abundance depended upon host’s density, which could be caused by the accumulation of a chemically-mediated host-cue more effectively attracting the settling symbionts. High host densities may also reduce the influence of external factors. For instance, commensals associated with less abundant host populations may experience a more relevant decrease in fitness than those harboured by dense host populations (Martin and Britayev, 1998).

Among the few relationships with known data, high infestation indexes tend to be positively correlated with the availability of large and numerous hosts to be occupied by the symbionts (Martin and Britayev, 1998). In the present case, the overall low prevalence would lead to the expectation of a low number of S. plana > 20 mm long in the study area. Conversely, S. plana was very abundant independently of size (Subida et al.,2011; Drake et al., 2014) and so numerous adequate hosts were available through the whole study period (Fig. 6; Table 9). Therefore, the prevalence seems not to be connected with the host availability. In turn, this low prevalence could be related to the extreme daily changes in temperature and/or the alternating long desiccation/immersion periods that characterize the intertidal environment, as well as with biota disturbance due to bait digging of this zone at Río San Pedro (Carvalho et al., 2013). The high prevalence in the subtidal population of M. pellucida (i.e. higher than 85% in specimens > 20 mm long), seems to confirm this hypothesis, with the permanent immersion in the subtidal environment favouring the presence of the symbiont.

The fact that no specimens of S. plana < 18 mm in shell length were found during the annual cycle, is in agreement with all previous data obtained in punctual samplings (Martin et al. 2012, 2015). The present approach did not clarify this peculiarity of the studied bivalve population. However, the high density of specimens restricted to the narrow intertidal area at Río San Pedro led to postulate the existence of a negative interaction between the established population and the settling larvae, which could be actively ingested by their conspecific adults during their normal filter/suspension feeding activities (i.e. passive cannibalism) (Cargnin-Ferreira, 2005; Santos et al. 2011).

The association of a symbiont with various hosts in the same locality could affect the prevalence, which strongly depends on its level of affinity for the different hosts. Although the symbiotic population of O. okupa sp. nov. at Río San Pedro only infested S. plana, this factor could not be eventually discarded due to the presence of the nearby subtidal population of M. pellucida at the opening of Río San Pedro.


Infestation intensity. The infestation intensity of symbiont polychaetes has been more widely reported than the prevalence. It may range from one to hundreds of symbionts per host, but is clearly dominated by the association of a single symbiont per host either due to the usually low symbiont densities or to the influence of an intraspecific aggressive behaviour (Martin and Britayev, 1998). Like the prevalence, intensity may also oscillate within the same population due to seasonal changes in relative abundances, linked or not to reproduction and recruitment events. A common situation for symbionts with 1:1 regular distributions like O. okupa sp. nov., is that adults may occasionally share the host with one to several juveniles, as previously reported for polychaete species of the genera Acholoe, Adyte or Branchiosyllis, among others (Martin and Britayev, 1998).

In the case of O. okupa sp. nov., Martin et al. (2012, 2015) reported single findings of one male and one female and one male, one female and one small worm, likely a juvenile sharing the same host, while the constant regular distribution of one single symbiont per host was attributed to intraspecific aggressive behaviour. There are several cases of symbiotic polychaetes with male and female couples living together associated with the same host, such as the Mediterranean fish parasite Ichthyotomus sanguinarius Eisig, 1906 (Eisig, 1906; Culurgioni et al., 2006) or the deep-sea hexactinellid sponge symbiont Robertianella synophthalma McIntosh, 1885, reported as Harmothoe hyalonemae in Martin et al. (1992). However, in both cases, most hosts harboured couples, while in O. okupa sp. nov. couples seemed to be the exception.


Little is known about the life cycle of most commensal polychaetes and, when known usually does not differ much from that of their free-living relatives. Thus, it is expected to find planktonic larvae (responsible of dispersal and colonization) and benthic adults (with a somewhat reduced mobility) with the single main difference that the symbiotic mode of life replaces the free-living one during the benthic phase (Martin and Britayev, 1998). Larval settlement may occur on the bottom, being then followed by a juvenile migration towards the respective hosts (Davenport and Boolootian, 1966). However, it seems more likely that chemically mediated cues (either generated by the host or by the own symbiotic adults) driving larval settlement could be the most widespread behaviour among symbiotic polychaetes (parasites included) (Martin and Britayev, 1998).

As it occurs for the free-living species, this basic life-cycle scheme may vary in many ways as a result of the adaptation to a symbiotic mode of life. For instance, it can be simplified by reducing (or even eliminating) the free-living pelagic stage or become more complex by having one or more intermediate hosts, which are occupied when the preferred ones are not available or because they have more room to host several juveniles during growing period (Martin and Britayev, 1998). Herewith, intraspecific competition and aggression may play a major role in the associated relocation processes. In the case of O. okupa sp. nov., the only information known on the juveniles is that they have not been found inside S. plana, neither free-living, which has been confirmed during the studied period.

The highest number of small worms found inside S. plana occurred in late autumn, which could be considered as an indication of new symbiont’s recruitment into the host bivalves (after the population being more actively reproducing during summer). In turn, large adults tended to disappear around mid-winter. This may suggest that the life span of O. okupa sp. nov. may be of one year, with the adults dying after reproducing. However, sampling during successive years would be required to confirm this hypothesis, as well as to assess the regularity of the recruitment events.

The life cycle of O. okupa sp. nov. may be limited by the tidal regime characteristic of its intertidal habitat, while the highly abundant population living in M. pellucida seems to indicate that recruitment may occur mainly in the more favourable subtidal conditions. Thus, we propose a possible scenario in which the symbionts mainly inhabit certain areas of the Bay not submitted to periodical desiccation by tides, living in association with M. pellucida (but we cannot discard other possible unknown hosts), then reaching intertidal areas such as those at Río San Pedro either as larvae through tidal currents or by adult migration. Therefore, the intertidal area here studied could be at the limit of the ecological distribution of this bivalve endosymbiont in the Cádiz Bay region.

The absence of juveniles inside the studied hosts suggests that this phase may be free-living and that the colonization of S. plana occurs during the benthic phase of the life cycle of O. okupa sp. nov., whose adults are able to move up along Río San Pedro with tides. However, neither juveniles, nor adults have been found in the sediments surrounding the studied areas (P. Drake, per. observ.), likely because they may be quite rare.

The possible life cycle of O. okupa sp. nov. may thus consist of 1) a planktonic larval phase settling on soft bottoms, 2) free-living juveniles, and 3) adults able to select (whenever possible) and enter the hosts at a given size (i.e., >1.6 mm wide according the present results). If so, phase 3 may involve thigmotaxis and the highly specific host entering behaviour described by Martin et al. (2015). In fact, a comparable life cycle was described for the polychaete Neanthes fucata (Savigny in Lamarck, 1818) in hermit crabs, by Gilpin-Brown (1969) off Plymouth. The planktotrophic larvae of this nereidid settle directly on soft bottoms. The juveniles live in tubes for several months feeding on detritus and small benthic animals exactly as many of their free-living relatives. Then, 4-month old worms start to develop the ability to recognize the presence of potential hosts by the substratum vibrations produced by the hermit crab legs bouncing on the sediment surface, which triggers a characteristic host-entering behaviour that allows the worms to crawl on the hermit crab shell following the shell spirals by thigmotaxis (Gilpin-Brown, 1969). This complex life cycle uses different mechanisms that characterize the free-living nereidids (such as thigmotaxis or mucus production) as specific adaptations to the commensal mode of life, while the worm itself has no relevant morphological adaptations. Therefore, in addition to the similarity of the hypothesized life cycle of O. okupa sp. nov., both species also share the lack of evident morphological adaptations (maybe except for the reduction of the central antennae in the hesionid, whose significance in terms of adaptation to the symbiotic mode of life remains unclear) and an equivalent, highly specific host entering behaviour.

The mechanism of host-recognition behaviour in O. okupa sp. nov. is currently unknown, and the presence of a host-factor has not been demonstrated (Martin et al., 2012, 2015). However, it is well known that the bivalves hosting O. okupa sp. nov. may alternate between direct water filtration and deposit feeding by tapping on the sediment surface with their inhalant siphons. Thus, we may hypothesize that the symbiont, like N. fucata, may recognize the presence of a potential host by the movements of the inhalant siphon. Despite aforementioned similarities between the known hosts of O. okupa sp. nov., there is no direct evidence of the symbiont entering into M. pellucida (or any other potential host) in a similar way as into S. plana.

Relationships between life–cycle and infestation characteristics

The infestation in O. okupa sp. nov., seemed to be connected with the reproduction. The months with lower prevalence (i.e., mid spring and summer) showed a markedly higher percentage of ripe females (>30%) (Fig. 4, 5; Table 9), reaching a 50% in August 2011, while the coldest month (i.e. October 2011 to February 2012) showed a high prevalence coupled with low percentages of ripe females (Fig. 4, 5; Table 9). The marked seasonal pattern found in the population of O. pugettensis (Johnson, 1901) infesting the starfish Patiria miniata (Brandt, 1835) at Dana Point (California) seemed also to be connected with the commensal reproductive dynamics (Lande and Reish, 1968). This species reached the highest (≥80%) and lowest (≤30%) prevalence in November-December and prior to the decrease of water temperature in summer, respectively. Its abundance was maximum in winter (2-3 worms per host) and minimum at mid-summer (<0.5 worms per host).

Despite sampling on different years is certainly required to infer regularities in the relationships between prevalence and life cycle in O. okupa sp. nov., these observations suggests that ripe females may have a reduced mobility (i.e., they tend to remain inside the host during their whole life), whereas males could be more mobile and thus leave their host, likely to increase fertilization success. Taking into account that O. okupa sp. nov. has a 1:1 regular distribution, the higher mobility attributed to males may contribute to an increase fertilization success by increasing the possibility of male/female partnership. Two possible mechanisms may explain this: 1) males may directly enter a host occupied by a ripe female, or 2) males may approach a host when females are releasing their sexual products. In both cases, we suggest that chemical cues may be involved. A similar behaviour was previously reported for B. seepensis, and, as mentioned above, also contributed to explain the lack of host-bivalve size relationships for adult males (Britayev et al., 2007). A similar situation was also reported for Haplosyllides floridana Augener, 1922, whose male stolons were never found inside the host sponge Neofibularia nolitangere (Duchassaing & Michelotti, 1864) and, conversely, were found free-swimming in the water column (Martin et al., 2009). Accordingly, the highest prevalence in the studied population of O. okupa sp. nov. occurred when both males and females occupied their respective hosts.

Despite this general pattern of reproduction vs. prevalence, O. okupa sp. nov. seems to reproduce actively during the whole year, but with an increasing effort during spring-summer and a higher intensity in summer. The absence of ripe females in April 2012 contrasted with the almost 40% found in 2011. However, ripe females occurred both before and after April 2012, allowing us to suggest that its absence in April 2012 could have been biased by the low number of symbionts found in this month (Fig. 4; Table 9). Persistent high temperatures can stimulate oocyte growth, subsequently causing the advancement of reproductive period in polychaetes, as reported for the Mediterranean populations of Eupolymnia nebulosa (Montagu, 1819) (Cha et al., 1997). However, our data does not allow assessing whether environmental constraints such as differences in temperature could affect the reproductive cycle of O. okupa sp. nov. during the studied period.

Current knowledge on hesionid symbionts

Thirty out of 170 currently known hesionid species live as commensals of other invertebrates (Martin and Britayev, 1998; Miller and Wolf, 2008; De Assis et al., 2012; Martin et al., 2012, 2015; Britayev et al., 2013; Chim et al., 2013), representing around 18% of all the known hesionid species and about 6% of the known symbiotic polychaetes (Table 10). Symbiosis seems to be restricted to the clade Ophiodrominae, which includes the genus Oxydromus. It has multiple origins within the family Hesionidae because Oxydromus and Gyptis, two of its most representative genera with symbiotic species, are not closely related phyllogenetically (Ruta, et al., 2007). Moreover, both are species-rich genera and there is no evidence indicating whether commensalism arose once or multiple times within each of them.


Table 10. List of the currently known symbiotic species within the family Hesionidae (sensu lato).

Commensal hesionids are involved in about 65 different associations (Table 10). Except for the polyxenous species of Oxydromus, such as for O. flexuosus (Delle Chiaje, 1827) and O. pugettensis (Johnson, 1901) with up to 10 and 11 hosts, respectively, most symbiotic hesionids are monoxoneus, occurring in only one (15 species) and two (6 species) hosts (Table 10). Hesionid hosts include species from very different taxonomic groups, a variety among symbiotic polychaetes only comparable to that of polynoids (Martin and Britayev, 1998). The most common hosts are, however, echinoderms (particularly starfishes and sea urchins) and polychaetes (Table 10).

Only one genus, Oxydromus, includes symbiotic species living in association with bivalves. These are O. pugettensis, O. humesi and O. okupa sp. nov. Although nothing is known on the relationships between O. pugettensis and its bivalve host, the polychaete seems to be able to detect at a certain distance the presence of at least two of its host starfishes, P. miniata and Luidia foliolata (Grube, 1866) (Davenport et al., 1960). Also there are some indications of mutualistic behaviour in their relationships with one of its echinoid hosts, the sand dollar Clypeaster humilis (Leske, 1778) (Storch and Niggemann, 1967). The existing analyses of the behaviour of O. okupa sp. nov. in experimental conditions do not prove the existence of a host-factor in its relationships with S. plana, like in the case of O. pugettensis and its host starfishes. In turn, the species shows an elaborated and complex host-entering behaviour, which leads the worm to enter inside the host bivalve mainly through the inhalant siphon (Martin et al., 2015). Moreover, the presence of O. okupa sp. nov. caused a significant reduction in the soft-body biomass of the infested hosts, compared to the non-infested ones, which may imply affectation of the host’s metabolism according to Bierbaum and Ferson (1986) thus leading to a relationship closer to parasitism (Martin et al., 2012). Such a negative influence has been previously reported for other symbiotic polychaetes living in the mantle cavity of bivalves, such as the deep-sea hydrothermal vent polynoids B. seepensis (Britayev et al., 2007). However, contrary to B. seepensis, in the case of O. okupa sp. nov. no damages in the tissues of S. plana were observed (Martin et al., 2012). In B. seepensis, tissue damages were considered as a secondary effect of the worm’s feeding inside the host caused by powerful jaws of the polynoid, and not a voluntary ingestion. Oxydromus okupa sp. nov. lacks jaws so that, even in the case of having a feeding mode similar to that B. seepensis, the hesionid seems to be able to avoid causing involuntary damage to the host tissues.

Despite the overall growing knowledge on symbiotic hesionids, O. okupa sp. nov. is probably the best known representative of the group to date. Nevertheless, further studies are required to complete the knowledge on this species, which becomes apparent if we compare it with O. humesi, with which it shares a similar morphology and a closely related host that is several thousands of kilometres away from Cadiz Bay. In other words, the present study is just another brick in the wall, which hopefully will encourage further research on the complex relationships between the symbiotic species of Oxydromus and their hosts.


The authors would like to thank José Antonio Cuesta and Alberto Arias, from the research group RNM108 of the ICMAN-CSIC, for their help during sampling and sorting the specimens, and Marta Calleja Julià for contributing to the initial morphometric analyses as a part of her high school practical work at the CEAB. Maria García, from the SEM service at the CEAB-CSIC, kindly helped with the SEM observations of the specimens. The authors would also like to thank the two anonymous reviewers and the Editors of the journal for their insightful comments, which highly contributed to improve the quality of the manuscript. The ‘Parque Natural Bahía de Cádiz’ kindly provided sampling facilities. MAM highly acknowledges Pr. Jordi Colomer, his academic tutor at the University of Girona, for the advice and recommendations during the “Treball de Fi de Grau”.

This study was funded by the Ministrerio de Educación, Cultura y Deporte (grant number CTM2013-43287-P) and the Agència de Gestió d’Ajuts Universitaris i de Recerca of the Generalitat de Catalunya through the Consolidated Research Group on Marine Benthic Ecology (grant number 2014SGR120) to Daniel Martin, and by the Consejería de Economía, Innovación, Ciencia y Empleo of the Junta de Andalucía, through the Research Group programme of the Plan Andaluz de Investigación Desarrollo e Innovación’ (grant number RNM108) to Pilar Drake.

Received: 16 November 2016

Revised and accepted: 28 March 2017

Published online: 23 October 2017

Editor: B.W. Hoeksema


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In this section, the specimens of Oxydromus living symbiotically with the tellinid bivalves Scrobicularia plana and Macoma cumana in the Atlantic coasts of the Iberian Peninsula are described as a new species. The new species was previously confused with the congeneric Oxydromus humesi, which also lives symbiotically with a tellinid bivalve, Tellina nymphalis, in Congolese Atlantic coasts. To allow comparisons, a morphometric diagnosis of O. humesi and a general morphological and morphometric descriptions of the new species are provided. The type series of the two species are deposited at the Smithsonian Museum (USNM), the Museo nacional de Ciencias Naturales of Madrid (MNCN), the Annelida Polychaeta research collection of the Centre d’Estudis Avançats de Blanes (CEAB.A.P.), the Museu Nacional de História Natural e da Ciência of Lisboa (MUHNAC), the Biological Research Collection of the Departamento de Biologia of the Universidade de Aveiro (DBUA), the Senckenberg Forschungsinstitut und Naturmuseum of Frankfurt (SMF) and the National Museum of Wales, Cardiff (NMW).

Taxonomic account

Family Hesionidae Grube, 1850

Genus Oxydromus Grube, 1855

Oxydromus humesi (Pettibone, 1961) 8453A918-89A8-4EEB-B78D-F9C452A39FA5


Parasyllidea humesi Pettibone, 1961. Pettibone (1961): figs. 4a-4h.

Oxydromus humesi (Pettibone, 1961). Martin et al. (2015) 434-441, figs. 4 (A, C, E), 5 (C, D, E, H) - 6 (A, C) - 7.


Material examined. Paratypes: USNM 30012, ca. 50 specimens fixed in formalin and preserved in 70% ethanol, Loango, Republic of Congo. (West Africa, approximately 4°37’42.6”, 11°49’30.0”); intertidal, mangrove muddy bottoms.


Morphometric diagnosis. Based on the average proportions being significant different from O. okupa sp. nov. (Table 8); “long” and “short” refer to parapodia of chaetigers 10 and 15, bearing long and short cirrostyles, respectively. General morphological description same as O. okupa sp. nov. Ratios number of segment and body width (without parapodia) vs. body width (with parapodia) ca. 0.03 and 0.5, respectively. Lateral antennae, palpophores and palpostyles ca. 0.5, 0.25 and 0.4 as long as prostomium length, respectively. Distance between anterior and posterior eyespots ca. 0.21 as long as prostomium length. Distance between anterior eyespots ca. 1.06 as long as that between posterior ones. Long and short dorsal cirrostyles 9 and 8 as long as cirrophores and 1.5 and 1.2 as long as dorsal lobes, respectively. Ventral lobe twice as long as dorsal one. Serration near blade tip less than twice short as blade width.


Table 8. Comparative table of the morphometric measurement proportions in the Iberian and Congolese populations. Differences expressed as percentages, with bold characters indicating significant differences (according to Benjamini and Hochberg, 1995) that may be higher (normal text) or lower (italics) in the Iberian than in the Congolese population. Measurement abbreviations as in Fig. 1. Min: minimum; Max: maximum; Mean: average ± standard deviation; F: Fisher’s F index; p: significance level.

Oxydromus okupa Martin, Meca and Gil sp. nov. D97B28C0-4BE9-4C1E-93F8-BD78F994A8D1

Figures 7-8-9


Fig. 7. Oxydromus okupa sp. nov. Light microscopy micrographs of the holotype in dorsal view. a. Whole worm. b. Anterior mid-body. c. Anterior end. Scale bars are µm.


Fig. 8. Light microscopy micrographs of paratypes in dorsal view. Oxydromus okupa sp. nov. a. Whole worm. b. Anterior mid-body. c. Anterior end. Oxydromus humesi. d. Whole worm. e. Anterior mid-body. f. Anterior end. Scale bars are µm.


Fig. 9. Oxydromus okupa sp. nov. a. Chaetiger 10, posterior view. b. Chaetiger 30, posterior view. c. Typical articulated chaeta. d. Tip of articulated chaeta. Oxydromus humesi. e. Chaetiger 10, posterior view. f. Chaetiger 30, posterior view. g. Tip of articulated chaeta. Scale bar is µm.


Parasyllidea humesi [non Pettibone, 1961]. Martin et al. (2012): 147-148, fig. 2b-2c;

Oxydromus humesi [non (Pettibone, 1961)]. Martin et al. (2015): 434-441, figs. 3, 4 (B, D, F), 5 (A, B, F, G), 6 (B, D), 7.


Material examined: Holotype MNCN 16.01/17760, 1 specimen. Paratypes: MNCN 16.01/17761, 24 specimens. Collected at Río San Pedro (Cádiz Bay, southern Atlantic coast of the Iberian Peninsula, 36°31’56.28”, 6°12’53.28”), intertidal muddy bottoms. All specimens fixed in a 4% formalin–seawater solution and preserved in 70% ethanol.


Additional material: Paratypes: MNCN 16.01/16090, 20 specimens, and CEAB.A.P. 854 A, 20 specimens. MUHNAC, MB29-000343 to MB29-000357, 15 specimens; DBUA0002020.01 to DBUA0002020.13, 13 specimens; SMF 24348 to SMF 24353, 6 specimens; NMW.Z.2017.010.0001 to NMW.Z.2017.010.0006, 9 specimens. Collection, fixation and preservation as for holotype.


General morphological description: Body rusty orange to tan (pale yellowish when preserved), flattened dorso-ventrally, tapering gradually posteriorly. Prostomium subrectangular, without posterior incisions, with two pairs of small reddish-brown eyes (dark brown when preserved), a median ridge, biarticulated palps and three smooth antennae inserted frontally; palpophores cylindrical, palpostyles flask-shaped, about twice the length of palpophores; median antenna very small, button like; lateral antennae conical, with tapering tips. Nuchal organs on lateroposterior prostomial margin. Facial tubercle present. Lip pads absent. Proboscis with two smooth rings and a terminal ciliated one. Jaws absent. Segment 1 dorsally reduced. Parapodial lobes and chaetae absent on segments 1-3. Long and short dorsal cirrophores distinct, ventral ones often indistinct. Dorsal and ventral cirrostyles smooth, distally tapering, slightly alternating short and long, ventral cirri shorter than dorsal, with blunt tip. Parapodia subbiramous from segment 4 onwards; notopodial lobe achaetous, half as long as neuropodial one, alternating elevated and non-elevated, with cirrophore on tip, with up to five aciculae; neuropodial lobes chaetigerous, elongated, subconical, with blunt tips, with long digitiform prechaetal lobe, rounded postchaetal lip, up to three aciculae and numerous compound neurochaetae, with heterogomph shafts, unidentate blades (middle ones much longer than dorsal and ventral) with curved, blunt tips, finely denticulate cutting edges, and a long subdistal spine reaching tip of curved end. One pair of pygidial cirri, similar to dorsal cirri. Pygidial papilla absent. Anus terminal.


Morphometric description. This description is based on the holotype (MNCN 16.01/17760); “long” and “short” refer to parapodia of chaetigers 10 and 15, bearing long and short cirrostyles, respectively). Tables 3 and 6 summarizing population ranges and averages. Body 11.6 mm long, 2 mm wide at the level of chaetiger 10, with 48 segments. Body width (without parapodia) ca. 0.35 as wide as with parapodia, ca. 5.8 as wide as body length. Ratio number of segments vs. body width (with parapodia) = 0.02. Prostomium 287 µm long and 499 µm wide; lateral antenna 226 µm long; palpostyles almost twice as long as palpophores, 150 µm and 74 µm long, respectively. Lateral antennae (226 µm long), palpophores (74 µm long) and palpostyles (159 µm long) 0.8, 0.3 and 0.5 as long a prostomium length, respectively. Distance between anterior eyespots 386 µm; distance between posterior eyespots 356 µm; distance between anterior and posterior eyespots 61 µm. Distance between anterior eyespots ca. 1.09 as long as that between posterior ones. Distance between anterior and between posterior eyespots ca. 0.7 and 0.8 as long as prostomium width, respectively. Distance between anterior and posterior eyespots ca. 0.21 as long as prostomium length. Long and short dorsal lobes similar in size, ca. 210 µm long, tips almost reaching those of ventral lobes. Long and short dorsal cirrophores 41 and 30 µm long, 0.14 and 0.2 as long a dorsal lobe. Long and short dorsal cirrostyles 530 and 512 µm long, ca. 2.4, 2.5 as long as dorsal lobes, and 17.8 and 12.8 as long as cirrophores. Long and short posterior neurochaetal lobe similar in size, ca. 136 µm long. Long and short ventral cirri 206 and 198 µm long, ca. 1.5 as long as posterior neurochaetal lobes.


Remarks: Oxydromus okupa sp. nov. and the morphologically-similar O. humesi share a virtually identical general morphological description, which resulted in them being initially identified as the same species (Martin et al. 2015). There was a single chaetal type, compound ones with heterogomph unidentate blades, which were very similar in both species. The blade serration seemed to be longer in the new species than O. humesi, although this character is highly variable and depends on the observed chaetae so that we strongly suggest not relying on it to differentiate the two species. Specimens corrected for body size (as body width including parapodia) show significant quantitative differences in body measurements (Table 6). Among these differences, the new species has significantly fewer segments and a narrower body width (without parapodia). The head is almost 4% wider, and the cephalic appendages are significantly longer in O. okupa sp. nov., with the latter including dimensions of palpophores (12% longer), palpostyles (14% longer), lateral antennae (19% longer) and distances between eyes (particularly between the anterior and posterior pairs). Some parapodial measurements are also significantly longer (6 to 9.6% long), while the dorsal lobe is the only character being shorter in O. okupa sp. nov. (8.5% and 13% shorter in parapodia bearing long and short cirrostyles, respectively). Moreover, dorsal and ventral lobes are similar in length, with the tip of the former reaching the tip of the latter in O. okupa sp. nov., while the dorsal lobe is much shorter than the ventral one in O. humesi.

Oxydromus okupa sp. nov. differs from O. humesi in body measurement proportions (Table 8). Accordingly, O. okupa sp. nov. has the body width measured without parapodia proportionally narrower with respect to the body width measured with parapodia, and it has also fewer segments. The new species also has the cephalic appendages proportionally longer with respect to prostomium length, particularly in the case of lateral antennae (more than 16% longer), and the distance between anterior and posterior eye pairs is also significantly longer, too. All ratios concerning parapodial measurements are also higher for O. okupa sp. nov., with the most marked differences being at the level of the dorsal cirrostyle vs. the corresponding dorsal lobe (15-22%) and the cirrophore (9-13.5%). Serration near blade tip as long as blade width.


Etymology: The specific epithet “okupa” refers to the Okupas (i.e., the Spanish squatters), a movement that emerged in the 80s as a response to the ever-increasing difficulty of housing opportunities. Okupation means squatting in empty buildings for the own use of the okupas, which implies self-organisation and alternative relationship mechanisms, in a parallel way as symbiotic polychaetes interact with their hosts and with the whole ecosystem.


Geographical distribution: Despite the wide distribution of S. plana in European waters (Santos et al., 2011), Oxydromus okupa sp. nov. is currently only known from the type locality, the intertidal of Río San Pedro saltmarshes, and the nearby subtidal environments of Cádiz Bay (southern Atlantic coast of the Iberian Peninsula).