THE IRON SNAIL

This gastropod, whose scientific name is Chrysomallon squamiferum Chen et al., 2015 [the genus name means "golden hair," for the pyrite coating often found on the shell; the species name means “scale-bearing.”] This snail is unique as it is the only known extant animal that incorporates iron sulfide into its shell and into its sclerites (= small to microscopic-sized hard-body parts). It was discovered in April, 2001 in the Indian Ocean, where it lives in the deep-sea hydrothermal-vent areas. 

Ocean-floor map of a portion of the Indian Ocean showing the approximate locations of the three sites (yellow dots) where C. squamiferum has been found (to date). Note: These locations all occur on oceanic ridges, mainly at or near the “Rodriques Triple Junction”-ridge complex, east of Madagascar. The map is a portion of a “National Geographic Society map” (1967).

This deep-sea hydrothermal-vent gastropod lives at depths between 2,415 to 2,460 m [7,923 to 7,923 ft.). It lives adjacent to both acids and reducing-vent fluids on the walls of “black smoker” chimneys, or directly on diffuse sites where fluids are flowing out. There is high concentration of hydrogen-sulfide gas and low concentration of oxygen. The temperature of the waters/fluids is between 2 and 10° C. The substrate rock is troctolite and mid-oceanic basalt.

This gastropod is an obligate symbiotroph that obtains all of its nutrition from the chemoautotrophy of its endosymbiotic bacteria. It is not a filter feeder. Its radula (in the mouth area) is small. It is likely that this gastropod has a planktonic dispersal stage, which is in accordance with its scattered locations (see Map above).

Oblique side view of an actual shell of C. squamiferum and a small piece of pyrite that was once attached to the gastropod animal but fell off later. Aperture of shell is 28 mm wide. This shell was retrieved from a depth of 2,440 m, at the base of a black smoker in the Kairei Vent Field, at the Rodriques Triple Junction, west of Madagasgar, in the Indian Ocean. Image courtesy of L.T. Groves, Collection Manager, Natural History Museum of Los Angeles County, California. 

                               A

                               B

Two views (sketches by the author) of C. squamiferum (A) oblique top view of shell and animal. (B) Apertural view of an empty shell. This snail is up to 45.5 mm in maximum shell dimension. Its coiled shell consists of three whorls; its spire is compressed; and its long aperture has an elliptical shape.

The shell of this gastropod has three layers: outer layer of iron sulfides, middle organic layer = periostracum, and innermost layer aragonite [= calcium carbonate]. The sides of its foot are armored (hence the common name “scaly foot,”) with hundreds of iron-mineral sclerites composed of the minerals: pyrite (iron sulfide) and greigite (an iron sulfate). This latter biomineral is only known to occur in this gastropod. Chrysomallon squamiferum also has an operculum (a “trap door” than closes off the opening of the shell), but it becomes less functional as the animal gets bigger.

Associated fauna found with this gastropod includes sea anemones, bivalve mussels (e.g., Bathymodiolus), several other gastropods [e.g., Lepetodrillus and Pseudorirmula) and slit limpets], annelids, crustaceans, echinoderm holothurians, and fish.

To date, Chrysomallon does not have a fossil record although fossil-hydrothermal vents (at ridges and next to subduction zones) are well documented in the literature.

Classification

Phylum Mollusca

Class Gastropoda

Order Neomphalida

Family Peltospiridae

Genus Chrysomallon Chen, Linse, Copley, and Rogers, 2015

Species Chrysomallon squamiferum Chen et al., 2015

 

Reference Cited:

Chong, C. K. Linse, J.T. Copley, and A.D. Rogers. 2015. The “scaly-foot gastropod”: a new genus of hydrothermal vent-endemic gastropod (Neomphalina: Peltospiridae) from the Indian Ocean. Journal of Molluscan Studies 81:322–334. (pdf is free and readily accessible).