The Iron Snail

The Museum has recently received specimens of the enigmatic deep-sea vent snail, Chrysomallon squamiferum, the scaly-foot snail. In this post, Dr Chong Chen explains why this species is so extraordinary.

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This is no ordinary snail. First of all, it lives in deep-sea hydrothermal vents in the Indian Ocean, more than 2,500 metres deep, just beside black smokers that are churning out superheated water exceeding 350°C. Second, it is the only known gastropod with a suit of scale armour. Thirdly, the scales as well as the shell are mineralised with iron sulfide. That’s right – these snails make a skeleton out of iron, and are the only animal so far known to do so.

Hydrothermal vents were first discovered in the Galápagos Rift as recently as 1977. This is just off the Galápagos Islands whose fauna famously inspired Charles Darwin in the development of his theory of natural selection. Vents are deep-sea ‘hot springs’ fuelled by geological activity; the hot erupting fluid is usually acidic and contains various metals, as well as hydrogen sulfide. This is what makes rotten eggs smell bad, and is toxic to most organisms. Some bacteria, however, are able to use it to produce energy in a process known as chemosynthesis.

Over geological timescales many remarkable organisms have adapted to live in these ‘toxic utopia’, and flourish by exploiting the energy produced by these bacteria. The scaly-foot snail has also harnessed the power of chemosynthesis, housing endosymbiotic bacteria – bacteria living inside another creature to mutual benefit – in an enlarged part of its gut. This produces the energy it needs. In another words – it has a food factory inside its body and doesn’t even need to feed! This is likely the reason it can grow to about 45mm in size, when most of its close relatives without endosymbionts are only 15mm or smaller.

Scaly-foot snails were first discovered in 2001, at the Kairei vent field in the Indian Ocean. Its discovery came as a great surprise as even among those animals specialised for living at vents, it was very, very strange. And cool. Although the shell of a snail is well-known to be modified into a great variety of forms, this is not the case with hard parts on the foot, and apart from an operculum (the ‘trap-door’ serving as a lid when the animal retracts to its shell) no other gastropods have other mineralised structures on the foot. Yet C. squamiferum has thousands of scales!

The shell, although not particularly exciting in form, isn’t exactly ordinary either as the outermost layer is made of iron sulfide. And so are the scales. So this entire animal is covered in iron compound, mainly pyrite (FeS₂, or ‘Fool’s gold’) and greigite (Fe₃S₄). As greigite is magnetic, the animal actually sticks to magnets. The function of the scales is postulated to be either protection or detoxification but their true use remains a mystery.

So why blog about the ‘scaly-foot’ now, if it has already been known to science for more than a decade? Well, actually, despite numerous studies and publications on its strange biology this species has never been formally described and named, until now. A recent paper by Dr Chong Chen (Department of Zoology, University of Oxford) and colleagues finally gave it the scientific name you see here – Chrysomallon squamiferum.

The Museum received a set of five specimens as part of the description process, which will serve as key references for scientists who wish to study this extraordinary species in the future.

Here’s a video of the Longqi hydrothermal field featuring Chrysomallon squamiferum in their natural habitat: