Chrysomallon squamiferum, commonly known as the scaly-foot gastropod, scaly-foot snail, sea pangolin, or volcano snail, 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 . C. squamiferum differs greatly from other deep-sea gastropods, even the closely related neomphalines. In 2019, it was declared endangered on the IUCN Red List, the first species to be listed as such due to risks from deep-sea mining of its vent habitat.
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 at the sides with iron-mineralised sclerites.
The snail's oesophageal gland houses symbiotic gammaproteobacteria from which the snail appears to obtain its nourishment. This species is considered to be one of the most peculiar deep-sea hydrothermal-vent gastropods, and it is the only known extant animal that incorporates iron sulfide into its skeleton (into both its sclerites and into its shell as an exoskeleton). Its heart is, proportionately speaking, unusually large for any animal: the heart comprises approximately 4% of its body volume.
Taxonomy This species was first discovered in April 2001 and became known as the "scaly-foot" gastropod. The name Chrysomallon squamiferum came into use in 2003, though its formal description according to the International Code of Zoological Nomenclature was only published by Chen et al. in 2015. Type specimens are stored in the Natural History Museum, London. During the time when the name was not yet formalized, an incorrect spelling variant was "Crysomallon squamiferum".
Chrysomallon squamiferum is the type species and the sole species within the genus Chrysomallon. The generic name Chrysomallon is from the Ancient Greek language, and means "golden haired", because pyrite (a compound occurring in its shell) is golden in color. The specific name squamiferum is from the Latin language and means "scale-bearing", because of its sclerites. At first it was not known to which family this species belonged. Warén et al. classified this species in the family Peltospiridae, within the Neomphalina in 2003. Molecular analyses based on sequences of cytochrome-c oxidase I (COI) genes confirmed the placement of this species within the Peltospiridae. Morphotypes from two localities are dark; a morphotype from a third locality is white (see next section for explanation of localities). These different colored snails appear to be simply "varieties" of the same species, according to the results of genetic analysis.
Distribution
The scaly-foot gastropod is a vent-endemic gastropod known only from the deep-sea hydrothermal vents of the Indian Ocean, which are around in depth. The species was discovered in 2001, living on the bases of black smokers in the Kairei hydrothermal vent field, , on the Central Indian Ridge, just north of the Rodrigues Triple Point. The species has subsequently also been found in the Solitaire field, , Central Indian Ridge, within the Exclusive Economic Zone of Mauritius and Longqi (means "Dragon flag" in Chinese) field, , Southwest Indian Ridge. Longqi field was designated as the type locality; all type material originated from this vent field. The distance between Kairei and Solitaire is about . The distance between Solitaire and Longqi is about . These three sites belong to the Indian Ocean biogeographic province of hydrothermal vent systems sensu Rogers et al. (2012). The distance between sites is large, but the total distribution area is very small, less than .
Peltospiridae snails are mainly known to live in Eastern Pacific vent fields. Nakamura et al. hypothesized that the occurrence of the scaly-foot gastropod in the Indian Ocean suggests a relationship of the hydrothermal vent faunas between these two areas.
Research expeditions have included: 2000 – an expedition of the Japan Agency for Marine-Earth Science and Technology using the ship RV Kairei and ROV Kaikō discovered the Kairei vent field, but scaly-foot gastropods were not found at that time. This was the first vent field discovered in the Indian Ocean. 2001 – an expedition of the U.S. research vessel RV Knorr with ROV Jason discovered scaly-foot gastropods in the Kairei vent field. 2007 – an expedition of RV Da Yang Yi Hao discovered the Longqi vent field. 2009 – an expedition of RV Yokosuka with DSV Shinkai 6500 discovered the Solitaire field and sampled scaly-foot gastropods there. 2009 – an expedition of RV Da Yang Yi Hao visually observed scaly-foot gastropods at Longqi vent field. 2011 – an expedition of the British Royal Research Ship RRS James Cook with ROV Kiel 6000 sampled the Longqi vent field.
Description Sclerites
In this species, the sides of the snail's foot are extremely unusual, being armoured with hundreds of iron-mineralised sclerites; these are composed of iron sulfides including greigite and pyrite. Each sclerite has a soft epithelial tissue core, a conchiolin cover, and an uppermost layer containing pyrite and greigite. Prior to the discovery of the scaly-foot gastropod, it was thought that the only extant molluscs possessing scale-like structures were in the classes Caudofoveata, Solenogastres and Polyplacophora. Sclerites are not homologous to a gastropod operculum. The sclerites of the scaly-foot gastropod are also not homologous to the sclerites found in chitons (Polyplacophora). It has been hypothesized that the sclerites of Cambrian halwaxiids such as Halkieria may potentially be more analogous to the sclerites of this snail than are the sclerites of chitons or aplacophorans. As recently as 2015, detailed morphological analysis for testing this hypothesis had not been carried out.
The sclerites of C. squamiferum are mainly proteinaceous (conchiolin is a complex protein); in contrast, the sclerites of chitons are mainly calcareous. There are no visible growth lines of conchiolin in cross-sections of sclerites. No other extant or extinct gastropods possess dermal sclerites, and no other extant animal is known to use iron sulfides in this way, either in its skeleton, or exoskeleton.
The size of each sclerite is about 1 × 5mm in adults. Juveniles have scales in few rows, while adults have dense and asymmetric scales. The Solitaire population of snails has white sclerites instead of black; this is due to a lack of iron in the sclerites. The sclerites are imbricated (overlapped in a manner reminiscent of roof tiles). The purpose of sclerites has been speculated to be protection or detoxification. The sclerites may help protect the gastropod from the vent fluid, so that its bacteria can live close to the source of electron donors for chemosynthesis. Or alternatively, the sclerites may result from deposition of toxic sulfide waste from the endosymbionts, and therefore represent a novel solution for detoxification. But the true function of sclerites is, as yet, unknown. The sclerites of the Kairei population, which have a layer of iron sulfide, are ferrimagnetic. The non-iron-sulfide-mineralized sclerite from the Solitaire morphotype showed greater mechanical strength of the whole structure in the three-point bending stress test (12.06MPa) than did the sclerite from the Kairei morphotype (6.54MPa).
In life, the external surfaces of sclerites host a diverse array of epibionts: Campylobacterota (formerly Epsilonproteobacteria) and Thermodesulfobacteriota (formerly part of Deltaproteobacteria). These bacteria probably provide their mineralization. Goffredi et al. (2004) hypothesized that the snail secretes some organic compounds that facilitate the attachment of the bacteria.
Shell
The shell of this species has three whorls. The shape of the shell is globose and the spire is compressed. The shell sculpture consists of ribs and fine growth lines. The shape of the aperture is elliptical. The apex of the shell is fragile and it is corroded in adults.
This is a very large peltospirid compared to the majority of other species, which are usually below in shell length. The width of the shell is ; the maximum width of the shell reaches . The average width of the shell of adult snails is 32mm. The average shell width in the Solitaire population was slightly less than that in the Kairei population. The height of the shell is . The width of the aperture is . The height of the aperture is .
The shell structure consists of three layers. The outer layer is about 30μm thick, black, and is made of iron sulfides, containing greigite Fe3S4. This species is the only extant animal known to feature this material in its skeleton. The middle layer (about 150μm) is equivalent to the organic periostracum which is also found in other gastropods. The periostracum is thick and brown. The innermost layer is made of aragonite (about 250μm thick), a form of calcium carbonate that is commonly found both in the shells of molluscs and in various corals. The color of the aragonite layer is milky white.
Each shell layer appears to contribute to the effectiveness of the snail's defence in different ways. The middle organic layer appears to absorb mechanical strain and energy generated by a squeezing attack (for example by the claws of a crab), making the shell much tougher. The organic layer also acts to dissipate heat. Features of this composite material are in focus of researchers for possible use in civilian and military protective applications.
Operculum In this species, the shape of the operculum changes during growth, from a rounded shape in juveniles to a curved shape in adults. The relative size of the operculum decreases as individuals grow. About a half of all adult snails of this species possess an operculum among the sclerites at the rear of the animal. It seems likely that the sclerites gradually grow and fully cover the whole foot for protection, and the operculum loses its protective function as the animal grows.
External anatomy
The scaly-foot gastropod has a thick snout, which tapers distally to a blunt end. The mouth is a circular ring of muscles when contracted and closed. The two smooth cephalic tentacles are thick at the base and gradually taper to a fine point at their distal tips. This snail has no eyes. There is no specialised copulatory appendage. The foot is red and large, and the snail cannot withdraw the foot entirely into the shell. There is no pedal gland in the front part of the foot. There are also no epipodial tentacles.
Internal anatomy In C. squamiferum, the soft parts of the animal occupy approximately two whorls of the interior of the shell. The shell muscle is horseshoe-shaped and large, divided in two parts on the left and right, and connected by a narrower attachment. The mantle edge is thick but simple without any distinctive features. The mantle cavity is deep and reaches the posterior edge of the shell. The medial to left side of the cavity is dominated by a very large bipectinate ctenidium. Ventral to the visceral mass, the body cavity is occupied by a huge esophageal gland, which extends to fill the ventral floor of the mantle cavity.