page_title: Scaly-foot snail
section_title: Internal anatomy
hierarchical_section_title: Scaly-foot snail / Description / Internal anatomy
caption_attribution_description: English: 3D tomographic reconstruction of Chrysomallon squamiferum, the anatomical model of the digestive system, a top (dorsal) view. Scale bar is 250 μm. Each colour group corresponds to a specific anatomical system: grey/black - digestive tract; beige - oesophageal gland; green - nephridium; dark blue - radula; light blue - radular cartilage.
context_page_description: Chrysomallon squamiferum, commonly known as the scaly-foot gastropod, scaly-foot snail, or sea pangolin, 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. Chrysomallon 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 that also produce high-quality metal ores.
  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.
context_section_description: In Chrysomallon 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.
  The digestive system is simple, and is reduced to less than 10% of the volume typical in gastropods. The radula is "weak", of the rhipidoglossan type, with a single pair of radular cartilages. The formula of the radula is ∼50 + 4 + 1 + 4 + ∼50. The radula ribbon is 4 mm long, 0.5 mm wide; the width to length ratio is approximately 1:10. There is no jaw, and no salivary glands. A part of the anterior oesophagus rapidly expands into a huge, hypertrophied, blind-ended esophageal gland, which occupies much of the ventral face of the mantle cavity (estimated 9.3% body volume). When the snail grows, the esophageal gland is increasing isometrically with growth. The oesophageal gland has a uniform texture, and is highly vascularised with fine blood vessels. The stomach has at least three ducts at its anterior right, connecting to the digestive gland. There are consolidated pellets in both the stomach and in the hindgut. These pellets are probably granules of sulfur produced by the endosymbiont as a way to detoxify hydrogen sulfide. The intestine is reduced, and only has a single loop. The extensive and unconsolidated digestive gland extends to the posterior, filling the shell apex of the shell. The rectum does not penetrate the heart, but passes ventral to it. The anus is located on the right side of the snail, above the genital opening.
  In the excretory system, the nephridium is central, tending to the right side of the body, as a thin dark layer of glandular tissue. The nephridium is anterior and ventral of the digestive gland, and is in contact with the dorsal side of the foregut.
  The respiratory system and circulatory system consist of a single left bipectinate ctenidium (gill), which is very large (15.5% of the body volume), and is supported by many large and mobile blood sinuses filled with haemocoel. On dissection, the blood sinuses and lumps of haemocoel material are a prominent feature throughout the body cavity. Although the circulatory system in Chrysomallon is mostly closed (meaning that haemocoel mostly does not leave blood sinuses), the prominent blood sinuses appear to be transient, and occur in different areas of the body in different individuals. There are thin gill filaments on either side of the ctenidium. The bipectinate ctenidium extends far behind the heart into the upper shell whorls; it is much larger than in Peltospira. Although this species has a similar shell shape and general form to other peltospirids, the ctenidium is proportional size to that of Hirtopelta, which has the largest gill among peltospirid genera that have been investigated anatomically so far.
  The ctenidium provides oxygen for the snail, but the circulatory system is enlarged beyond the scope of other similar vent gastropods. There are no endosymbionts in or on the gill of C. squamiferum. The enlargement of the gill is probably to facilitate extracting oxygen in the low-oxygen conditions that are typical of hydrothermal-vent ecosystems.
  At the posterior of the ctenidium is a remarkably large and well-developed heart. The heart is unusually large for any animal proportionally. Based on the volume of the single auricle and ventricle, the heart complex represents approximately 4% of the body volume (for example, the heart of humans is 1.3% of the body volume). The ventricle has size 0.64 mm in juvenile animal with a shell length of 2.2 mm and the ventricle will reach size 8 mm in adults. This proportionally giant heart 
caption: A 3D reconstruction of the digestive system shows the enlarged oesophageal gland, dorsal view. Scale bar is 250 μm.
                    grey/black – digestive tract;   brown – oesophageal gland;   green – nephridium;   dark blue – radula;   light blue – radular cartilage.
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