Given that hydrothermal vents—geysers on the deep ocean floor emitting mineral compounds that support a thriving ecosystem—were only discovered in 1977, it is perhaps unsurprising that novel organisms showing unique environmental adaptations are regularly reported from expeditions to these areas. In the November 7 Science, Anders Warén and colleagues at the Swedish Museum of Natural History report a gastropod mollusk from black-smoker chimneys in the Indian Ocean with its soft foot covered by a chain mail of imbricating scale-shaped iron sclerites (Science, 302:1007, November 7, 2003).

The so-far unnamed snail lives a sedentary life at the base of the black smokers—the hottest of the oceanic vents that spew mostly iron and dissolved sulfides, which combine to form iron sulfides. These compounds give the smoker its color and also provide the ingredients for the armored boot of the snail. The main component of the scales is conchiolin, a protein that is the organic basis of mollusk shells, but the outer, black layer is mineralized with pyrite (FeS₂) and greigite (Fe₃S₄), the latter conferring ferromagnetic properties to the sclerites. The interior of the sclerites is penetrated by a pulp of pedal tissue, and where the sclerite surface is overlain by adjacent sclerites, a coat of bacteria is present. Although bacteria may play a role in sulfide deposition, Warén et al. believe that this process must be directly controlled and possibly induced by the snail.

Iron sulfide is well characterized as a cell-scaffolding material in some bacteria, but was not thought to act as a cytoskeletal component in multicellular animals. A scaly scleritome, similar to that observed in the vent snail, was common among animals that appeared in the Cambrian Explosion—a period between 540 and 490 million years ago, when animals showed dramatic diversification and when most of the major groups first appear in the fossil record. Anatomical and DNA-based analysis of the phylogenetic position of the snail positions it close to other known vent gastropods of the order Neomphalina, a more recent clade endemic of modern hydrothermal vents. Warén et al. suggest that the scaly scleritome evolved recently in the vent environment from the conchiolin operculum common in other vent snails, probably to assure protection from predatory gastropods occurring in the same habitat.

“The scaly-footed gastropod demonstrates that a complex scleritome of the kind which evolved repeatedly in metazoans of the Cambrian Explosion may appear in an evolutionary instant, and it reinforces the idea that fortifying biominerals may reflect the availability of minerals in the environment where the structures evolved,” conclude the authors.