The Coral-lovers

The free-living (but, from the look of it, not particularly mobile) coralliophiline Latiaxis mawae, photographed by Merlin Charon.

While the subjects of today's posts, the gastropods of the Coralliophilinae, might have a name that translates as 'lovers of coral', it seems likely that the corals do not love them. Members of the Coralliophilinae are a group of about 250 species of specialised predators/parasites of corals and other cnidarians. They vary in habits from free-living grazers to species that live embedded within their coral host, and as a result they vary in external appearance. However, their internal anatomy is fairly consistent, with all species lacking a radula and having a similar gut anatomy. As far as is known, they are all protandrous hermaphrodites, maturing from males to females as they grow larger. The females brood their eggs within the mantle cavity. Phylogenetic analysis has indicated a relationship with the subfamilies Rapaninae (the oyster drills) and Ergalataxinae in the Muricidae, and the recent tendency has been to treat the coralliophilines as a subfamily of the latter (Barco et al. 2010).

The endobiotic Magilus striatus, with a close-up of the coiled juvenile shell, photographed by Femorale.

Like other muricids, many of the free-living coralliophilines (such as species in the genera Latiaxis and Babelomurex) are strikingly ornamented with spiny shells. Those that live in closer association with their coral hosts, however, show a corresponding reduction in ornamentation. Members of the genus Rapa, feeding on soft corals, have inflated, relatively flat-spired shells. Most derived of all, perhaps, is the boring genus Magilus, in which the shell, after an initial coiled section, becomes uncoiled and tubular to form a channel to the outer wall of the host coral. The simplified shell morphology of the endoparasitic forms means that species can be exceedingly difficult to distinguish. A study of the genus Leptoconchus, internal parasites of mushroom corals, by Gittenberger & Gittenberger (2011) identified fourteen molecularly distinct species, each associated with a different species of coral. However, morphological variation between specimens did not reliably correlate with molecular and host-association data and could not be used to distinguish species. In comparison, a study of the host-nonspecific free-living species Coralliophila meyendorffii found that, despite the clear distinction of a larger form feeding on sea anemones vs a smaller form feeding on corals, the two forms were not resolved as molecularly distinct and there was inadequate support for the recognition of the forms as separate species (Oliverio & Mariottini 2001).

The bubble turnip Rapa rapa, photographed by Eddie Hardy.

REFERENCES

Barco, A., M. Claremont, D. G. Reid, R. Houart, P. Bouchet, S. T. Williams, C. Cruaud, A. Couloux & M. Oliverio. 2010. A molecular phylogenetic framework for the Muricidae, a diverse family of carnivorous gastropods. Molecular Phylogenetics and Evolution 56: 1025-1039.

Gittenberger, A., & E. Gittenberger. 2011. Cryptic, adaptive radiation of endoparasitic snails: sibling species of Leptoconchus (Gastropoda: Coralliophilidae) in corals. Organisms Diversity & Evolution 11: 21-41.

Oliverio, M., & P. Mariottini. 2001. Contrasting morphological and molecular variation in Coralliophila meyendorffii (Muricidae, Coralliophilinae). Journal of Molluscan Studies 67: 243-246.