Scaly-foot gastropod, Sea pangolin, Volcano snail
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 2,400–2,900 m (1.5–1.8 mi). 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 sulfides, 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.
A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton. Phytoplankton are usually photosynthet...
Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
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starts withThe 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.
The scaly-foot gastropod is a vent-endemic gastropod known only from the deep-sea hydrothermal vents of the Indian Ocean, which are around 2,780 metres (1.73 mi) in depth. The species was discovered in 2001, living on the bases of black smokers in the Kairei hydrothermal vent field, 25°19.239′S 70°02.429′E / 25.320650°S 70.040483°E / -25.320650; 70.040483, on the Central Indian Ridge, just north of the Rodrigues Triple Point. The species has subsequently also been found in the Solitaire field, 19°33.413′S 65°50.888′E / 19.556883°S 65.848133°E / -19.556883; 65.848133, Central Indian Ridge, within the Exclusive Economic Zone of Mauritius and Longqi (means "Dragon flag" in Chinese) field, 37°47.027′S 49°38.963′E / 37.783783°S 49.649383°E / -37.783783; 49.649383, 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 700 km (430 mi). The distance between Solitaire and Longqi is about 2,500 km (1,600 mi). 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 0.02 square kilometres (0.0077 sq mi).
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:
This species inhabits the hydrothermal vent fields of the Indian Ocean. It lives adjacent to both acidic and reducing vent fluid, on the walls of black-smoker chimneys, or directly on diffuse flow sites.
The depth of the Kairei field varies from 2,415 to 2,460 m (7,923 to 8,071 ft), and its dimensions are approximately 30 by 80 m (98 by 262 ft). The slope of the field is 10° to 30°. The substrate rock is troctolite and depleted mid-ocean ridge basalt. The Kairei-field scaly-foot gastropods live in the low-temperature diffuse fluids of a single chimney. The transitional zone, where these gastropods were found, is about 1–2 m (3–7 ft) in width, with temperature of 2–10 °C. The preferred water temperature for this species is about 5 °C. These snails live in an environment which has high concentrations of hydrogen sulfide, and low concentrations of oxygen.
The abundance of scaly-foot gastropods was lower in the Kairei field than in the Longqi field. The Kairei hydrothermal-vent community consists of 35 taxa, including sea anemones Marianactis sp., crustaceans Austinograea rodriguezensis, Rimicaris kairei, Mirocaris indica, Munidopsis sp., Neolepadidae genus and sp., Eochionelasmus sp., bivalves Bathymodiolus marisindicus, gastropods Lepetodrilus sp., Pseudorimula sp., Eulepetopsis sp., Shinkailepas sp., and Alviniconcha marisindica, Desbruyeresia marisindica, Bruceiella wareni, Phymorhynchus sp., Sutilizona sp., slit limpet sp. 1, slit limpet sp. 2, Iphinopsis boucheti, solenogastres Helicoradomenia ? sp., annelids Amphisamytha sp., Archinome jasoni, Capitellidae sp. 1, Ophyotrocha sp., Hesionidae sp. 1, Hesionoidae sp. 2, Branchinotogluma sp., Branchipolynoe sp., Harmothoe ? sp., Levensteiniella ? sp., Prionospio sp., unidentified Nemertea and unidentified Platyhelminthes. Scaly-foot gastropods live in colonies with Alviniconcha marisindica snails, and there are colonies of Rimicaris kairei above them.
The Solitaire field is at a depth of 2,606 m (8,550 ft), and its dimensions are approximately 50 by 50 m (160 by 160 ft). The substrate rock is enriched mid-ocean ridge basalt. Scaly-foot gastropods live near the high-temperature diffuse fluids of chimneys in the vent field. The abundance of scaly-foot gastropods was lower in the Solitaire field than in the Longqi field. The Solitaire hydrothermal-vent community comprises 22 taxa, including: sea anemones Marianactis sp., crustaceans Austinograea rodriguezensis, Rimicaris kairei, Mirocaris indica, Munidopsis sp., Neolepadidae gen et sp., Eochionelasmus sp., bivalves Bathymodiolus marisindicus, gastropods Lepetodrilus sp., Eulepetopsis sp., Shinkailepas sp., Alviniconcha sp. type 3, Desbruyeresia sp., Phymorhynchus sp., annelids Alvinellidae genus and sp., Archinome jasoni, Branchinotogluma sp., echinoderm holothurians Apodacea gen et sp., fish Macrouridae genus and sp., unidentified Nemertea, and unidentified Platyhelminthes.
The Longqi vent field is in a depth of 2,780 m (9,120 ft), and its dimensions are approximately 100 by 150 m (330 by 490 ft). C. squamiferum was densely populated in the areas immediately surrounding the diffuse-flow venting. The Longqi hydrothermal-vent community include 23 macro- and megafauna taxa: sea anemones Actinostolidae sp., annelids Polynoidae n. gen. n. sp. “655”, Branchipolynoe n. sp. “Dragon”, Peinaleopolynoe n. sp. “Dragon”, Hesiolyra cf. bergi, Hesionidae sp. indet., Ophryotrocha n. sp. “F-038/1b”, Prionospio cf. unilamellata, Ampharetidae sp. indet., mussels Bathymodiolus marisindicus, gastropods Gigantopelta aegis, Dracogyra subfuscus, Lirapex politus, Phymorhynchus n. sp. “SWIR”, Lepetodrilus n. sp. “SWIR”, crustaceans Neolepas sp. 1, Rimicaris kairei, Mirocaris indica, Chorocaris sp., Kiwa n. sp. “SWIR”17, Munidopsis sp. and echinoderm holothurians Chiridota sp. The density of Lepetodrilus n. sp. “SWIR” and scaly-foot gastropods is over 100 snails per m² in close distance from vent fluid sources at Longqi vent field.
The scaly-foot gastropod is an obligate symbiotroph throughout post-settlement life. Throughout its post-larval life, the scaly-foot gastropod obtains all of its nutrition from the chemoautotrophy of its endosymbiotic bacteria. The scaly-foot gastropod is neither a filter-feeder nor uses other mechanisms for feeding. The radula and radula cartilage are small, respectively constituting only 0.4% and 0.8% of juveniles' body volume, compared to 1.4% and 2.6% in the mixotrophic juveniles of Gigantopelta chessoia.
For identification of trophic interactions in a habitat, where direct observation of feeding habits is complicated, carbon and nitrogen stable-isotope compositions can be measured. There are depleted values of δ13C in the oesophageal gland (relative to photosynthetically derived organic carbon). Chemoautotrophic symbionts were presumed as a source of such carbon. Chemoautotrophic origin of the stable carbon isotope 13C was confirmed experimentally.
This gastropod is a simultaneous hermaphrodite. It is the only species in the family Peltospiridae that is so far known to be a simultaneous hermaphrodite. It has a high fecundity. It lays eggs that are probably of lecithotrophic type. Eggs of the scaly-foot gastropod are negatively buoyant under atmospheric pressure. Neither the larvae nor the protoconch is known as of 2016, but it is thought that the species has a planktonic dispersal stage. The smallest C. squamiferum juvenile specimens ever collected had a shell length 2.2 mm. The results of statistical analyses revealed no genetic differentiation between the two populations in the Kairei and Solitaire fields, suggesting potential connectivity between the two vent fields. The Kairei population represents a potential source population for the two populations in the Central Indian Ridge. These snails are difficult to keep alive in an artificial environment; however, they survived in aquaria at atmospheric pressure for more than three weeks.
The scaly-foot gastropod is not protected. Its potential habitat across all Indian Ocean hydrothermal vent fields has been estimated to be at most 0.27 square kilometres (67 acres), while the three known sites at which it has been found, between which only negligible migration occurs, add up to 0.0177 square kilometres (4.4 acres), or less than one-fifth of a football field.
The population at the Longqi vent field may be of particular concern. The Southwest Indian Ridge, within which it is located, is one of the slowest-spreading mid-ocean ridges, and the low rate of natural disturbances is associated with ecological communities that are likely more sensitive to and recover more slowly from disruptions. Slow-spreading centers may also create larger mineral deposits, making those sensitive areas primary targets for deep-sea mining. Furthermore, by genetic measures the population at Longqi is poorly connected to those at the Kairei and Solitaire vent fields, over 2000km away within the Central Indian Ridge.
The Solitaire Vent Field falls within the exclusive economic zone of Mauritius, while the other two sites are within Areas Beyond National Jurisdiction (commonly known as the high seas) under the authority of the International Seabed Authority, which has granted commercial mining exploration licenses for both. The Kairei Vent Field is under a license to Germany (2015-2030), the Longqi Vent Field to China (2011-2026). As of 2017, no conservation measures are proposed or in place for any of the three sites.
It has been listed as endangered species in the IUCN Red List of Threatened Species since July 4, 2019.
