The scaly-footed gastropod, Chrysomallon squamiferum, is an unusual deep-ocean inhabitant. This creature thrives in conditions hostile to most life, showcasing extraordinary adaptations. Its unique characteristics offer insights into how life flourishes in unexpected corners of our planet.
Unveiling the Scaly-Footed Gastropod
The scaly-footed gastropod, Chrysomallon squamiferum, is distinguished by its “scaly foot” and shell. Its foot is armored with hundreds of overlapping dermal sclerites, small, hard plates composed of iron sulfides like pyrite and greigite. This makes it the only known animal to incorporate these minerals into its skeleton. Newly formed sclerites are white, turning black as an iron sulfide coating develops; some populations also exhibit magnetism due to greigite.
The shell of Chrysomallon squamiferum has a unique three-layered construction. The outermost layer is a thin, 30-micrometer coating of iron sulfides, providing defense and dissipating heat. Beneath this is a thick, 150-micrometer organic middle layer, acting as a shock absorber. The innermost layer, roughly 250 micrometers thick, is aragonite, a common form of calcium carbonate.
Beyond its armor, the scaly-footed gastropod has other notable biological features. It lacks eyes, relying on facial tentacles to sense its surroundings in perpetual darkness. Its heart is unusually large, comprising about 4% of its total body volume, one of the largest proportionately in the animal kingdom. This large heart supports the high oxygen demands of symbiotic bacteria living within its body, which are central to its survival.
Life in Extreme Environments
The scaly-footed gastropod thrives exclusively in deep-sea hydrothermal vents, found at depths of 2,400 to 2,900 meters in the Indian Ocean. These environments feature extreme temperatures, with vent fluids reaching 400 degrees Celsius. However, the surrounding water where the snails live remains cooler, around 2-10 degrees Celsius, due to immense hydrostatic pressure. The vents also release toxic chemicals, particularly high concentrations of hydrogen sulfide, which would be lethal to most organisms, alongside low oxygen levels.
The gastropod’s survival is attributed to its unique energy method: chemosymbiosis. Unlike most life forms relying on photosynthesis, this snail harbors symbiotic gammaproteobacteria within its hypertrophied oesophageal gland, an enlarged part of its gut. These bacteria produce nutrients by oxidizing sulfur compounds, such as hydrogen sulfide, emitted from the vents. This system allows the snail to flourish in the complete absence of sunlight.
This symbiotic relationship makes the scaly-footed gastropod an obligate symbiotroph in its adult stage, relying entirely on these internal bacteria for nourishment. The bacteria efficiently convert toxic sulfur compounds into usable energy, providing a continuous food source for the snail. This biological strategy differs from photosynthesis, highlighting diverse ways life sustains itself. The snail’s specialized oesophageal gland and large heart facilitate this metabolic pathway and chemical circulation for its symbionts.
Discovery and Conservation
The scaly-footed gastropod was first discovered in April 2001 at the Kairei hydrothermal vent field on the Central Indian Ridge in the Indian Ocean. Though initially observed, it was formally described and named Chrysomallon squamiferum in 2015. Since then, populations have been identified at two other Indian Ocean sites: the Longqi field and the Solitaire field, all at similar deep-sea hydrothermal vents.
Despite its remote habitat, this species faces a significant threat from human activities. In 2019, the scaly-footed gastropod became the first animal listed as endangered on the IUCN Red List specifically due to deep-sea mining risks. Its habitat, hydrothermal vents, contains valuable mineral deposits like copper, gold, and manganese, increasingly sought after with advancements in mining technology.
The gastropod’s vulnerability is compounded by its restricted habitat and limited distribution. The total known area where this species resides across all three vent fields is less than 0.02 square kilometers. Mining activities, even exploratory ones, could severely damage or destroy these fragile vent ecosystems by direct physical impact or by smothering the snails with sediment plumes. Currently, two of the three known vent fields have been granted mining exploration licenses to countries like Germany and China, raising concerns for the species’ future.