Making a skeleton out of iron: the ‘scaly-foot gastropod’

The three known forms of Chrysomallon squamiferum, from Kairei, Longqi, Solitaire vent fields (left to right)

Gastropod researcher Chong Chen writes:

Meet Chrysomallon squamiferum, or the ‘scaly-foot gastropod’ – the only metazoan (multi-cellular) animal capable of making a skeleton out of iron! Uniquely among gastropod molluscs it carries hundreds to thousands of scales on the foot like a suit of scale armour; these scales as well as the shell usually has a layer of iron sulfides. The function of these scales has been speculated to be protective or detoxification (by accumulation of sulfide waste), but their true function is yet unknown. As part of the iron armour is made of greigite (Fe₃S₄), a magnetic form of iron sulfide, this amazing animal actually sticks to strong magnets. And it also rusts. That’s right – upon contact with oxygen and water, the whole exterior slowly turns to a layer of reddish rust from metallic black.

Why doesn’t it rust in its natural environment, you may ask. Well, its environment is not exactly ordinary, either. The scaly-foot only occurs in deep-sea hydrothermal vents of Indian Ocean, more than 2500 m deep and often on ‘black smoker’ chimneys spewing out hot vent fluid exceeding 350°C. Hydrothermal vents are typically oxygen-poor environments and this might prevent the animal from rusting. The wonder doesn’t stop here – it houses endosymbiont bacteria in its enlarged oesophagus which is capable of turning chemicals in the vent fluid (poisonous to most animals) to energy, and rely on these for nutrition. So it basically keeps a food farm inside the body and does not need to feed!

Hydra, an active ‘black smoker’ vent chimney in Longqi field, Southwest Indian Ridge

With shell length that averages at around 35 mm and exceeds 45 mm in large individuals, it is a very large peltospirid snail compared to most others in the family which are below 15 mm in shell length. Originally discovered in 2001 from deep-sea hydrothermal vents of Indian Ocean, despite being a charismatic species there, it was not formally described and named until 2015. Furthermore, recent find of a white population without iron on the sclerites deepens the mystery about its sclerite formation – much more exciting secrets are waiting to be unveiled from this remarkable species!

A specimen of Chrysomallon squamiferum photographed live (Photo: David Shale)

You can watch a video of Longqi hydrothermal field featuring Chrysomallon squamiferum in their natural habitat here: