Snails Adrift: 𝘙𝘦𝘤𝘭𝘶𝘻𝘪𝘢 and 𝘑𝘢𝘯𝘵𝘩𝘪𝘯𝘢

 

Adult pair of Recluzia johnii (Holten, 1802) with eggs and bubble raft. (Photo copyright Denis Riek from here)

   Earth's oceans are incredibly vast and inhabited by nearly uncountable scores of organisms, yet the surface of the ocean itself is one of the emptiest habitats there is. The sea/sky interface is home to a tiny yet fascinating suite of organisms known collectively as the "pleuston" (alternatively "neuston"), most notably including multiple families of floating cnidarians, mats of floating macroalgae in the genus Sargassum, and even a genus of sea-skating insects (Halobates). A remarkable group of gastropods fill the niche of drifting predators: the genera Janthina and Recluzia. These gastropods belong to the Epitoniidae, a family of obligate cnidarian predators and parasites known by shell collectors as "wentletraps." Janthina are prolific predators of pleustonic hydrozoans such as the infamous man o' war Physalia physalis (Linnaeus, 1758) and unusual by-the-wind sailor Velella velella (Linnaeus, 1758). Recluzia are more specialized, feeding on the strange free floating anemones of the Minyadidae. There are eight valid species of Janthina currently recognized (three are extinct) and two species of Recluzia. The genera are easily distinguished even as empty shells by color alone. Recluzia shells are a cream/yellow color similar to many benthic epitoniids, but Janthina shells are uniquely purple/violet, often considered a camouflage adaptation to an existence shrouded in constant blue.

Indeed, life adrift has come with a range of interesting adaptations. These snails cannot swim on their own (at least not as adults); their individual survival is contingent on them remaining permanently attached to a floating raft they construct from rigid bubbles. The bubbles are made by cupping the anterior portion of the snail's "foot" above the waterline and coating the air bubble in mucus, which hardens immediately on contact with sea water. This same mucus binds the foot of the snail to the raft. Losing grip on their raft means certain death as the animal helplessly sinks into the deep. Maintaining a grip also prevents these gastropods from hiding within their shells, a basic defense strategy for most snails. A lack of body mobility has acted as selection pressure for the evolution of an enlarged and highly mobile head in Janthina. Recluzia are not so highly derived anatomically, resembling benthic epitoniids to a much higher degree than Janthina with a more elongate shell and small head.

Figures showing the modified head and general habitus of Janthina janthina (Linnaeus, 1758) from Beu (2017).

Janthina and Recluzia are so divergent from the rest of the Epitoniidae that their taxonomic affinities were mostly contentious until the early 2010s. Churchill et al. (2011) found both genera to be nested firmly within the epitoniids, dissolving the formerly recognized "Janthinidae." Despite extremely similar lifestyles, it has been suggested that these two genera may not even form a monophyletic group with each other. Beu (2017) carried out the most detailed revision of this group to date and concluded that Janthina and Recluzia took to the surface independently, surely representing one of the most remarkable examples of convergent evolution yet known. Recluzia seems to have adapted to the pleuston much more recently than Janthina as evidenced by a lack of fossil record and significantly fewer anatomical specializations. Janthina possess a fossil record dating back to the Miocene and are generally much better studied and seemingly much more common than Recluzia. Still very much unknown is what the transition from the sea floor to the sea surface looked like. Only time will tell what other secrets these fascinating mollusks are hiding.

Shells of all recognized extant species of Janthina and Recluzia from Beu (2017). 

REFERENCES

Beu, A. G. (2017). Evolution of Janthina and Recluzia (Mollusca: Gastropoda: Epitoniidae). Australian Museum.