Description & Behavior
On average the shells of adult Scaly-foot snails measure around 32 mm but can grow as large as 45.5 mm. These shells are coiled with three whorls and are globose (spherical) in shape. The spire is depressed as is the initial whorl, additionally, the whorls are tightly coiled. The shell does not have a strong lamellar (fine layered structure) sculpture but does have subtle, closely-spaced ribs and fine growth lines are present. The protoconch (larval form) has not been seen and remain undescribed. The columellar region is covered by a thin callous. The shell itself is milky white and thin with a very large, elliptic aperture. The overall shape of the shell lies between neritiform and limpet-form. The periostracum is thick, brown and lacks folds – it envelops the shell edge. The exterior of the shell is often coated by a black layer of iron sulphide.
The foot of the snail is large and covered in hundreds of overlapping hard dermal sclerites which give it its name. The size and armor of the foot prevents it from being withdrawn fully into the aperture of the shell, unsurprisingly the operculum, though present, is small, curved and bent in adults – buried under sclerites on the metapodium of the foot. These sclerites are formed of pulps of pedal tissue laminated with conchiolin. The sclerites are curved, elongate, proteinaceous and lack calcification. When newly formed, the sclerites are white but as an iron sulfide coating develops they turn black. The iron sulfide coating that often develops in Scaly-foot snails is comprised of mostly pyrite with greigite in lower proportions which makes it ferromagnetic. The sclerites are frequently 1×5 mm in adults but can reach lengths of up to 8 mm with a conchiolin and iron sulfide crust up to 0.2 mm thick.
The foot itself lacks epipodal tentacles and the anterior pedal gland. Its cephalic (head) tentacles are thick at the base, elongate and taper towards the distal tip – lacking visible eyes or pigment. The snout is tapered and thick. The sole of the foot is surrounded by a pedal (foot) flange. The shell muscle is large and horseshoe-shaped, with two anterior parts on each side connected by a narrow connective posteriorly. The flesh of the Scaly-foot snail is red.
It is speculated that the armored sclerites (plates) covering the foot of the scaly-foot snail may offer defense against crabs and predatory snails which use harpoon-like radulae to envenomate their prey. It is also thought that the formation of the iron sulphide crusts on the sclerites is facilitated by symbiotic bacteria living on the snail’s foot between the overlapping sclerites.
World Range & Habitat
The Scaly-foot snail is a deep sea gastropod living in hydrothermal vent fields between depths of 2,400 to 2,900 m. They are only known from three locations in the Indo-Pacific: the Longqi field on the South West Indian Ridge (South of Madagascar) and the Kairei and Solitaire fields on the Central Indian Ridge (East of Madagascar). Here the Scaly-foot snail can be found living a sedentary life at the bottom of black-smoker chimneys and in diffuse flows. Its total distribution area is estimated to be 0.27 km².
Feeding Behavior (Ecology)
The scaly-foot snail has an atrophied (reduced) digestive system and radula. However, the oesophageal gland is hypertrophied (about 1000x greater than in other snails) to accommodate chemosynthetic bacteria which produce its sustenance using chemicals from the surrounding environment. As such the Scaly-foot snail is an obligate symbiotroph once settled in its post larval stage.
The Scaly-foot snail shows no sexual dimorphism and is in fact a simultaneous hermaphrodite. The snail lacks copulatory organs though it does have gonads in the head-foot region and spermatophore packaging organ. The Scaly-foot snail has a simple genital slit. It is thought to have a high fecundity, laying negatively buoyant eggs which are probably lecithotrophic. It is thought that the hatched larvae have a planktonic dispersal stage which would account for the connectivity between the sites along the South-West and Central Indian Ridges.
Conservation Status & Comments
The Scaly-foot snail is endemic to three known hydrothermal vent fields along deep sea ridges in the Indian ocean, as such it has a limited distribution and a fractured range in a specialist environment upon which it is reliant for its food producing chemosynthetic bacteria. The chemical make-up of its environment is also essential to the mineral deposition strengthening its shell and sclerites. As such it is highly vulnerable to habitat depletion. As technology has developed, the mineral depositions of hydrothermal vents have become viable mining grounds for companies with the mining rights to Kairei and Lonqi being granted to Germany and China respectively. The likely disturbance in two-thirds of the Scaly-foot snail’s range has prompted the IUCN to declare the species endangered.
References & Further Research
Sigwart, J., Chen, C. & Thomas, E.A. 2019. Chrysomallon squamiferum. The IUCN Red List of Threatened Species 2019: e.T103636217A103636261. https://dx.doi.org/10.2305/IUCN.UK.2019-2.RLTS.T103636217A103636261.en Dr. Chong CHEN, Deep-sea biologist. Malacologist. Evolutionary biologist. “Mollusc collector”, photographer.
Chong Chen, Katrin Linse, Jonathan T. Copley, Alex D. Rogers, The ‘scaly-foot gastropod’: a new genus and species of hydrothermal vent-endemic gastropod (Neomphalina: Peltospiridae) from the Indian Ocean, Journal of Molluscan Studies, Volume 81, Issue 3, August 2015, Pages 322–334, https://doi.org/10.1093/mollus/eyv013
A Hot-Vent Gastropod with Iron Sulfide Dermal Sclerites, Anders Warén1,*, Stefan Bengtson1, Shana K. Goffredi2, Cindy L. Van Dover3, Science 07 Nov 2003: Vol. 302, Issue 5647, pp. 1007 DOI: 10.1126/science.1087696: https://science.sciencemag.org/content/302/5647/1007
Chen C, Copley JT, Linse K, Rogers AD, Sigwart JD. The heart of a dragon: 3D anatomical reconstruction of the ‘scaly-foot gastropod’ (Mollusca: Gastropoda: Neomphalina) reveals its extraordinary circulatory system. Front Zool. 2015;12:13. Published 2015 Jun 18. doi:10.1186/s12983-015-0105-1