Most of the host species associations for Coralliophila galea and C. caribaea found in the present study have been previously recorded (Miller, 1981; Brawley and Adey, 1982; Hayes, 1990a; Bruckner et al., 1997; Del Monaco et al., 2010; Potkamp et al., 2017). Feeding of C. galea on Orbicella annularis was found to be infrequent around 1970 (Robertson, 1970; Ott and Lewis, 1972), but later clearly impacted reef communities (Brawley and Adey, 1982; Knowlton et al., 1988, 1990; Hayes, 1990a; Bruckner et al., 1997; Baums et al., 2003b). Predation pressure by Coralliophila spp. may depend on prey preference. Multiple studies concluded that C. galea can cause much damage to Acropora spp., and to a lesser extent to Orbicella spp. (Brawley and Adey, 1982; Knowlton et al., 1988, 1990; Baums et al., 2003b). On some host species, such as O. annularis, large aggregations of C. galea snails can be found. Aggregations of C. caribaea were generally smaller. Large aggregations of snails of the genus Drupella Thiele, 1925 (Muricidae: Ergalataxinae) can have a damaging effect on Indo-Pacific reef corals (Hoeksema et al., 2013; Moerland et al., 2016; Scott et al., 2017; references herein), suggesting that large C. galea aggregations are harmful as well, confirming earlier findings (Bruckner et al., 1997; Knowlton et al., 1988, 1990; Hayes, 1990a; Baums et al. (2003a, 2003b). Predation on Alcyonacea by C. caribaea is also relevant, as gorgonians represent a large part of Caribbean and Brazilian reef communities (e.g. Preston and Preston, 1975; Sánchez et al., 1998, 2003; Dias and Gondim, 2016; Lau, 2016). However, few studies have been conducted on the ecology and prey preference of C. caribaea.
The physiology of coralliophilines, with a proboscis used in feeding, helps to optimally exploit energy from their hosts (Ward, 1965; Robertson, 1970). The Indo-Pacific C. violacea (Kiener, 1836), which usually, like its Caribbean congeners, also feeds along the coral margin, has adopted a prudent feeding strategy by exploiting energy sinks along the margin of its hosts, thereby causing minimal damage (Oren et al., 1998). Similar behaviour in Caribbean Coralliophila spp., combined with an absence of large feeding scars on most host species, suggests that C. galea and C. caribaea employ similar strategies (Martin et al., 2014). If so, the high feeding rates of C. galea on Orbicella spp. calculated by Baums et al. (2003b), which were based on the energetic requirements of C. galea rather than on feeding scars, might be an overestimation of the actual feeding rates. Coralliophila curacaoensis sp. nov. also used its proboscis for feeding on its only known host, Madracis auretenra. As this coral is common across the Caribbean, it is likely that this new snail species will also be discovered at other localities.
A large range in shell sizes was observed within Coralliophila spp. Females were on average larger than males, which is consistent with the fact that C. galea is a protandrous hermaphrodite (Baums et al., 2003a; Johnston and Miller, 2006). The same pattern was observed within C. caribaea, suggesting a similar life history, which has been suggested to be a synapomorphous trait among the Coralliophilinae (Richter and Luque, 2002, 2004). In addition, host-associated size structuring existed within both C. galea and C. caribaea populations. Differences in shell length related to host species have been observed before within C. galea (Hayes, 1990a; Bruckner et al., 1997; Baums et al., 2003a; Johnston and Miller, 2006). Ecotypes with distinctive shell lengths have also been observed in their Mediterranean congener C. meyendorffii, where small snails are associated with scleractinian hosts and large snails are associated with sea anemones (Oliverio and Mariottini, 2001b;Kružić et al., 2013). The results of the present study confirm and expand this pattern of host-associated size structuring in C. galea, and show that it also exists in C. caribaea.
Migration between host species with age could induce host-associated size structuring. In the Indo-Pacific corallivorous snail Drupella cornus (Röding, 1798), prey preference seems to change as snails age (Black and Johnson, 1994; McClanahan, 1997; Schoepf et al., 2010; Moerland et al., 2016). Age-dependent host preference would result in a clear host-associated size structuring as seen within Coralliophila spp. However, no evidence exists to support that C. galea or C. caribaea migrate between host species. As both male and female snails were found on all but one host species (for which more than one specimen was collected) of C. galea, it seems unlikely that size-related migration between specific host species also occurs in C. galea, i.e., that juvenile snails (which would all be males) are associated with different host species than adults.
Another potential mechanism behind host-associated size structuring can result from size-dependent susceptibility to predation (Johnston and Miller, 2006). Selective predation on larger snails, which on some host colonies are more exposed than smaller snails, would result in host-associated size structuring. Wells and Lalli (1977) hypothesized further that brooding females of C. galea are, compared to C. caribaea, more vulnerable to predation because of the placement of egg capsules in the mantle cavity. Predation on larger snails would therefore decrease mean shell size and skew the male to female ratio. Variation in male to female ratios, as observed in C. galea, would be expected in case of size-specific predation. However, information on predation on Coralliophila spp. is limited, resulting from laboratory experiments or anecdotal observations (Goldberg 1971; Wells and Lalli, 1977; Baums et al., 2003a; Sharp and Delgado, 2015).
Besides a large range in shell size, high intraspecific variation in shell shape was found in both C. galea and C. caribaea. Despite this high intraspecific variation, no distinct ecotypes based on shell shape could be identified within either species, though weak host-associated differences in mean shell shape and allometric patterns existed in both C. galea and C. caribaea. Differences in shell shape were subtle with strong overlap among snails associated with different host species. Tested factors explained little of the observed variation in the models of shell shape suggesting the presence of factors not considered here.
Differences in growth rate could also explain host-dependent size structuring as well as the intraspecific variation in shell shape and allometry (Kemp and Bertness, 1984; Boulding and Hay, 1993; Chiu et al., 2002; Urdy et al., 2010a, 2010b). While growth rate has not been measured in the present study, the strong correlation between average male and female size separated by host species (assuming sex-change occurs at the same relative age) is consistent with the idea that growth rates differ among snails associated with different host species, confirming previous studies (Baums et al., 2003b; Johnston and Miller, 2006). Such differences in growth rate may result from, for example, differences in nutritional quality of host tissue (Szmant et al., 1990), anti-predatory mechanisms of host species (Barnes, 1970; Brauer et al., 1970; Moore and Huxley, 1976; Glynn and Krupp, 1986; Pawlik et al., 1987; Harvell et al., 1988; Harvell and Fenical, 1989; Van Alstyne and Paul, 1992; Pawlik, 1993; O’Neal and Pawlik, 2002; Gochfeld, 2004; Lages et al., 2010), predation pressure on snails (Fraser and Gilliam, 1992; Connell, 1998; Nakaoka, 2000) or intraspecific competition (Williamson et al., 1976; Cameron and Carter, 1979), among other factors. Baums et al. (2003b) suggested that differences in environment or nutrition (and by extension, host species) played a role in the growth rate of C. galea, Johnston and Miller (2006) suggested a role for nutritional quality and secondary metabolites, as well as intraspecific competition in the population structure of C. galea. Shell morphology has also been related to vulnerability to predation (Ebling et al., 1964; Kitching et al., 1966; Vermeij, 1974, 1993; Cotton et al., 2004). Variation of these factors across the range of hosts species, with vastly different colony shapes, may therefore result in host-dependent variation in shell size, shape and allometry as observed in the present study. However, little is known about the extent to which these factors play a role in corallivores in general or in Coralliophila spp. specifically. The mechanisms behind the observed patterns in shell size and shape remain therefore largely unknown.
No host-associated genetic divergence was observed within C. galea: specimens failed to cluster by host species in the Bayesian analysis and genetic distances in snails were not significantly larger between host species than the distances within host species. Haplotypes were not correlated with host species. Johnston et al. (2012) also did not observe host-specific clustering in genetic data and only found a small genetic divergence between C. galea snails associated with Acropora palmata, Orbicella spp. and Mycetophyllia spp. Two ecotypes of the Mediterranean congener C. meyendorffii, which are associated with different host species, are also not genetically divergent (Oliverio and Mariottini, 2001b).
In contrast to C. galea, a host-associated genetic divergence was found within C. caribaea. Snails associated with alcyonaceans were genetically distinct from snails associated with scleractinians with one exception either way. The genetic divergence within C. caribaea was however relatively small. On the phylogenetic tree, the support value for the branch with nearly all snails associated with scleractinians was low. Haplotypes differed on a few loci between snails associated with scleractinians and alcyonaceans (two mutations between the haplotype cluster mostly associated with alcyonaceans and the other haplotypes for 12S, eight mutations for COI), and correlation of haplotypes with host order was not perfect. The K2P distance between specimens associated with alcyonaceans and scleractinians fell below the threshold between interspecific and intraspecific divergence as determined in the ABGD analysis. These observations all indicate that the divergence within C. caribaea is of relatively recent origin. Genetically diverged host races have mostly been described in insects (e.g. Feder et al., 1988; McPheron et al.; 1988; Powell et al., 2014), but also in sponge-associated shrimp species (Duffy, 1996). Formation of host races may be the first stage of sympatric speciation (Maynard Smith, 1966; Berlocher, 1998). Isolation among host races is required for full speciation to occur, as the host races will revert to a panmictic population in absence of reproductive isolation (Jaenike, 1981). While the genetic divergence observed in the present study suggest some reproductive isolation, more analyses are needed to assess the degree of reproductive isolation between the two putative host races found within C. caribaea (Jaenike, 1981).
Mathematical models suggested that host-associated selective forces are critical for snails to specialize (Kawecki, 1996, 1997). The absence of host-associated genetic divergence within C. galea is therefore consistent with the results from earlier prey-preference experiments, where only a weak preference for the native host species was found (Hayes, 1990b). This is especially true for species whose pelagic larval stage is long, as a long larval stage favours generalists and promotes plasticity instead of divergence and speciation (Sotka, 2005). The larval ecology of snails is therefore relevant too. Larval development varies among coralliophillid species, with planktotrophic development being considered the plesiomorphic state within the Coralliophillinae, while some evidence indicates a prolonged intracapsular or lecithotrophic development in several species (Richter and Luque, 2002; Oliverio, 2008). Veliger shells of both C. galea and C. caribaea have been illustrated by Abbott (1958) and Wells and Lalli (1977) (only C. galea). Both species are thought to have planktotrophic development, but the duration of the planktonic stage remains unknown, and is expected to be > 30 days for C. galea (Wells and Lalli, 1977; Richter and Luque, 2002; Johnston et al., 2012). Studies on the protoconch of snails may provide more insight in the larval ecology of Caribbean Coralliophila spp. (Oliverio, 2008).
For genetic divergence to be established, selection pressures must be strong enough to overcome homogenizing processes that increase gene flow between populations (Schluter, 2009; Johnston et al., 2012). The difference in host-associated genetic divergence between C. galea and C. caribaea suggests that selection pressures to specialize to either scleractinian or alcyonacean host species might be higher than the selection pressures to specialize to specific (groups of) species within these orders. Anti-predatory mechanisms of gorgonians might play a role here. Some generalist predators, such as the facultatively corallivorous polychaete Hermodice carunculata (Pallas, 1766), do feed on gorgonians (Marsden 1962; Preston and Preston 1975; Lasker 1985; Rotjan and Lewis 2008; Wolf et al., 2014), but most other species feeding on gorgonians, such as ovulid gastropods of the genus Cyphoma Röding, 1798, are specialized to this diet (Birkeland and Gregory 1975; Harvell and Suchanek 1987; Lasker and Coffroth, 1988; Lasker et al., 1988; Van Alstyne and Paul 1992; Burkepile and Hay, 2007; Chiappone et al., 2003; Reijnen et al., 2010; Schärer et al., 2010; Pinto et al., 2017; Reijnen and Van der Meij, 2017). Selection to overcome these mechanisms within C. caribaea might therefore have been strong enough to induce genetic divergence.
While specialization to (a group of) host species might have played a role in the intraspecific divergence observed within C. caribaea, the absence of a monophyletic Caribbean Coralliophila clade on the phylogenetic tree suggests that host-associated divergence was not the mechanism behind the divergence among species and that their common ancestor originated outside of the modern Caribbean. A similar pattern has been observed in the shrimp family Palaemonidae (Horká et al., 2016). Therefore, more extensive phylogenetic and phylogeographic analyses of Coralliophila spp. are needed to unravel the biogeographic patterns of this genus in the Caribbean.
The phylogenetic analyses in the present study suggest that the genus Coralliophila is polyphyletic, as C. caribaea seems to be closer related to the Indo-Pacific genus Leptoconchus than its own congeners (Fig. 15; Oliverio and Mariottini, 2001a; Oliverio et al., 2002, 2009). The genus Coralliophila is therefore in need of a taxonomic revision (Oliverio and Gofas, 2006; Oliverio, 2008; Oliverio et al., 2009).
Some overlap was observed between the distributions of intraspecific and interspecific divergence in the ABGD analysis, which is indicative of the absence of a universal threshold within the Coralliophilinae (Collins and Cruickshank, 2013). Only one other threshold value based on an ABGD analysis for muricid gastropods was found in literature: Barco et al. (2013) reported a threshold K2P distance between 0.020 and 0.025 based on a dataset of COI sequences of the genus Ocinebrina Jousseaume, 1880, which is lower than the value found in the present study. In addition, with the threshold value used in the present study, many of the Leptoconchus species included in the ABGD analysis could not be distinguished as separate species. This reinforces the lack of a universal threshold between intraspecific and interspecific divergence.
Phenotypic plasticity and genetic differentiation
While no host-associated genetic divergence has been observed within C. galea, adaptive genetic polymorphisms may still play a role, and the relative importance of both a genetic basis and phenotypic plasticity to variation in morphology remains unknown (Johnston et al., 2012). Both mechanisms are not mutually exclusive, as both genetic differentiation and phenotypic plasticity are thought to play a role in the adaptation of the snail Littorina saxatilis (Olivi, 1792) to the local habitat (Janson, 1983; Johannesson and Johannesson, 1996; Hollander et al., 2006). Reciprocal transplant experiments of C. galea among different host species showed that current host species was a more important determinant for growth than the native host species, suggesting that phenotypic plasticity plays at least some role in the morphological variation of C. galea (Baums et al., 2003b). Johnston et al. (2012) also attributed difference in growth rates at least partly to phenotypic plasticity. These results suggest that within C. galea, habitat-related phenotypic plasticity is more important than evolutionary divergence, although the intraspecific genetic variation observed within C. caribaea does not preclude a role for phenotypic plasticity within this species.