|The three known populations of
Kairei, Longqi, Solitaire (left to right).
Chen et al., 2015
Chrysomallon squamiferum, common name the scaly-foot gastropod, is a species of deep-sea hydrothermal vent snail, a marine gastropod mollusc in the family Peltospiridae. This vent endemic gastropod is known only from deep-sea hydrothermal vents in the Indian Ocean, where it has been found at depths of about 2,400 m (1.5 mi). The shell is of a unique construction with three layers; the outer layer consists of iron sulphides, the middle layer is equivalent to the organic periostracum found in other gastropods, and the innermost layer is made of aragonite. The foot is also unusual, being armored with iron-mineralised sclerites. The snail's tissues house a symbiotic gammaproteobacteria from which it appears to obtain its nourishment.
According to WoRMS, "the name Chrysomallon or Crysomallon squamiferum was used in several databases and academic papers prior to 2015. However, the name was first validly published in the sense of the International Code of Zoological Nomenclature by Chen et al. (2015)".
The 'scaly-foot gastropod' is an iconic vent endemic gastropod known only from the deep-sea hydrothermal vents of Indian Ocean, around 2,400 metres (1.5 mi) to 2,800 metres (1.7 mi) deep. This species was discovered in 2001, living on the bases of black smokers at the Kairei hydrothermal vent field, on the Central Indian Ridge, just north of the Rodrigues Triple Point and about 2,420 metres (7,940 ft) below the surface. It has subsequently also been found in Solitaire field, Central Indian Ridge (off Mauritius)  and Longqi (aka. Dragon) field, Southwest Indian Ridge. Longqi field serves as the type locality and all type material have originated from this vent field.
With shell length that averages at around 35 millimetres (1.4 in) and exceeds 45 millimetres (1.8 in) in large individuals, it is a very large peltospirid compared to most others which are below 15 millimetres (3⁄5 in) in shell length. The snail's foot is very unusual in that it is armored with iron-mineralised sclerites, composed of iron sulphides greigite and pyrite. No other animal is known to use iron sulfides in this way.
The Solitaire population has white sclerites instead of black due to lack of iron in them, most likely due to differences in the vent fluid composition. The purpose of sclerites has been speculated to be protection or detoxification, but their true function is yet unknown. The sclerites of Kairei population is strongly magnetic due to the greigite (sulfur equivalent of magnetite) content and stick to magnets.
The snail's shell is also unusual. The shell structure is composed of three layers. The outer layer is about 30 μm thick, and is also made of iron sulphides, containing greigite Fe3S4. This makes this gastropod the only metazoan known so far that employs such material in its skeleton. The middle layer is equivalent to the organic periostracum found also in other gastropods, and is also the thickest of the three (about 150 μm). The innermost layer is made of aragonite, a form of calcium carbonate that is commonly found both in the shells of molluscs and in various corals. Each layer appear to contribute to the effectiveness of the snail's defence in different ways. The middle organic layer appears to absorb the mechanical strain and energy generated by a squeezing attack (as by the claws of a crab), making the shell much tougher. The organic layer also acts to dissipate heat. The United States military is currently funding research on the armor of the snail in hopes of developing insights into new military armor designs.
It is a chemosymbiotic holobiont hosting a thioautotrophic (i.e., sulfur-oxidising) gammaproteobacterial endosymbiont in a much enlarged oesophageal gland, and appear to rely on these for nutrition. The surface of sclerites also host a diverse variety of epibionts.
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