On Mycale diversitynext section
The Mexican Pacific coast extends for more than 8475 kilometers. Currently, the knowledge of some groups of Porifera in this vast coast such as hadromerids is similar to what has been studied in the Caribbean Sea, the Mediterranean Sea or the northeastern Atlantic Ocean. However, other diverse sponge groups such as poecilosclerids are scarcely known.
Before this study, seven species of Mycale had been described or recorded from the Pacific coast of Mexico; Mycale (C.) fascibibula (as Carmia fascifibula), M. (A.) contax (as Carmia contax), Mycale cecilia, M. angulosa, M. aff. magnirhaphidifera, M. parishi and Oxymycale paradoxa. However, after this research, only M. contax, M. cecilia and M. aff. magnirhaphidifera have been considered valid species.
After exhaustively sampling the Mexican Pacific during the last few years, we have a good collection of Mycale, which yielded eight valid species: three species belonging to the subgenus Aegogropila - Mycale magnitoxa sp. nov, M. dickinsoni sp. nov. and M. adhaerens; three species belonging to the subgenus Carmia - M. aff. magnirhaphidifera, M. cecilia and M. contax; one species of Paresperella - M. psila; and one species of Zygomycale - M. ramulosa sp. nov.
Mycale (Oxymycale) paradoxa (De Laubenfels, 1935) is not considered a valid species here. Hajdu (1994) revised slides from the type material and found it was not a mycalid at all, but a composite of what De Laubenfels (1935) called Topsentia glabra (Topsent, 1898) contaminated by spicules of Mycale bellabellensis (Lambe, 1905). Hajdu (1994) also found out that T. glabra, sensu De Laubenfels (1935) was actually a Myrmekioderma.
Besides the eight species described in this paper, at least two more different species could inhabit Mexican waters (Pacific Ocean). The anisochelae of Mycale bellabellensis that Hajdu (1994) found when he revised the holotype of O. paradoxa were depicted under SEM convincingly enough to tentatively include it in the list of Mexican species (Fig. 2A-B). In addition, some particular anisochelae with the head and foot of a similar length, and with a wide space between both terminations have been observed in the slides made from the type material of Mycale (A.) dickinsoni sp. nov. (Fig. 2C). These spicules aren't proper of this sponge since they belong to a clearly delimited species group within Mycale (Mycale) called ‘curved assemblage’ (see Hajdu et al., 1995). In the East Pacific Ocean there are two species of this group; M. (M.) darwini Hajdu and Desqueyroux-Faúndez, 1994 from the Galapagos (Hajdu and Desqueyroux-Faúndez, 1994), and M. (M.) toporoki Koltum from California (USA) (Lee et al., 2002). However, until fresh material is collected, a specific identification of the Mexican material will not be possible.
Fig. 2. SEM images of Mycale spicules found contamining Mexican pacific species. A-D spicules from Hajdu (1994). A, B, Anisochelae I and II respectively, found by Hadju contaminating Myrmekioderma sp. (de Laubenfels, 1935) and the type of Oxymycale paradoxa; C, D, Images of anisochelae I and II of Mycale bellabellensis (Lambe, 1905; sensu de Laubenfels 1932) for comparison purpose (from Hajdu 1994); E, Anisochelae found contamining the specimen type of Mycale dickinsoni.
The diversity of Mycale-species in the Mexican Pacific Ocean (10 species?), together with the five species reported in the Mexican Caribbean and Gulf coasts: M. (A.) arndti Van Soest, 1984, M. (Arenochalina) laxissima (Duchassaing and Michelotti, 1864), M. (C.) microsigmatosa Arndt, 1927, M. (Z.) angulosa (Duchassaing and Michelotti, 1864), and M. (M.) laevis (Carter, 1882) (Gómez and Green, 1984; Green et al., 1986; Lehnert, 1993, and other authors), comprise a total of 15 species, which is a diversity similar to that of Brazil where 15 species are currently known (see Hajdu et al. 1994, 1995). Only the Caribbean, which has been profusely studied, presents a higher diversity of Mycale (17 species, Hajdu and Rützler, 1998), although particular areas, such as the mangroves on the barrier reef of Belize have yielded only eight species. In the South American coast of the Pacific Ocean, largely unexplored for sponges, only seven species are known so far (Hajdu and Desqueyroux-Faúndez, 1994).
Thus, we can consider that the Mycale fauna of Mexico is highly diverse, with at least 15 species known, mostly from shallow water, for which is expected that the study of deep fauna will yield more species.
Regarding the presence of the Caribbean M. aff. magnirhaphidifera in the east Pacific Ocean
From the ten species found in Mexican Pacific waters, only one, M. (C.) aff. magnirhaphidifera is known in the West Atlantic.
The possible amphiamericanism of M. (C.) magnirhaphidifera is difficult to explain. Currently, the Isthmus of Panama represents a barrier of fresh water across what was once a large neotropical marine environment approximately three million years ago (Coates and Obando, 1996; Craig et al., 2004). However, the two freshwater lakes that the boats have to cross have proved insufficient as a fresh-water barrier (Jones and Dawson, 1973). Several species of marine fishes have survived during the crossing through the canal (McCosker and Dawson, 2004), and several invertebrates were found in the locks when they were drained for cleaning, which show that they have been transported through the canal by associating with fouling material on the underside of ships (Hildebrand, 1939). In fact, morphologically similar invertebrate species from the Atlantic and the Pacific are presently found on both sides of the Isthmus of Panama (Weinberg and Starczak, 1989; Knowlton and Mills, 1992; Knowlton et al., 1993).
If we consider the presence of M. (C.) magnirhaphidifera in the East Pacific Ocean as an invasion, this means that this species survived in fresh water during a period required for a ship to cross of the canal (close to 8 hours (McCosker and Dawson, 1975). The influence of the salinity, in the particular case of marine sponges, has been documented only in a few cases, which show that some marine sponges, particularly clionaids, can live in a salinity of 1.5-2.0, and they can also recover from several days of exposure to salinities as low as 1 (Hopkins, 1956; Hartman, 1958). The presence of some Caribbean Clionids species in the East Pacific Ocean such as C. amplicavata or C. flavifodina could have this explanation (Carballo et al., 2004a), but the confirmation that some sponges can survive during the crossing is supported by the presence of the sponge Haliclona permollis in the locks when they were drained (Hildebrand, 1939). Currently, the true identity of this species is uncertain because H. permollis is a junior synonym of Haliclona (Reniera) cinerea, which is not yet reported reliably from the NW Atlantic (Porifera DataBase), but whatever species it may be its presence shows that it has been transported through the canal by associating with fouling material on the underside of ships. The mechanism by which sponges can endure low salinity has not been studied enough, but it is known that the species Microciona prolifera occurs naturally in salinities ranging from 7 to 38 (De Laubenfels, 1947) and uses free amino acids for osmotic compensation (Knight et al., 1992). Even though there are species that can support several hours in emersion (Carballo et al., 2004b).
However, despite the possibility that M. aff. magnirhaphidifera has survived the crossing of the Panama Canal, it is not possible to decide whether Pacific M. magnirhaphidifera has recently been introduced into the Pacific, or on the contrary, it forms part of a sister-species pair which was separated together with the closure of the natural water way between the Caribbean and the eastern Pacific. De Laubenfels (1936b) found a large group of similar species that lived in both coasts of Panama, which suggested that they may have been introduced from one to the other side through the canal. Independently of the true identity of the De Laubenfels’ records, when putative transisthmian populations of the sponge Spirastrella cf. mollis were studied using allozyme, morphological, and cytological data together, it was found that in reality they were two divergent lineages; the Caribbean lineage, which was named S. hartmani, and the Pacific lineage, which named S. sabogae (Boury-Esnault et al., 1999).
In conclusion, given the limited number of morphological characters available to study marine sponges, it is not possible to decide whether Pacific M. aff. magnirhaphidifera is an invasive species in the Pacific, or if it is part of a sibling species. Such facts will only be demonstrated if we combine the classical methods of morphological measurement with the extensive use of genetic approaches.