Animals found around hydrothermal vents – from snails covered in metal plates to hairy crabs – have unusual adaptations to survive with no sunlight and extreme pressure.
I’ve always been fascinated by the sea and the animals that can live in an environment so very different to mine. Watching humpback whales off the coast of Cape Cod, Massachusetts, captivated me as a child and I still get a thrill of excitement whenever I see any marine animals, whether that’s seals off the English coast, technicolour fish on the Great Barrier Reef or spinner dolphins and turtles in Hawaii.
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| The scaly-foot gastropod Dr Chong CHEN |
But the mysteries of the deep are just as intriguing. When we talk about the deep sea, we mean anything below about 200 metres, where light from the surface begins to fade. Studying the life living there is difficult: few can visit and many animals lifted to the surface don’t survive the journey.
For a long time, the deep sea was assumed to be too barren to support life. But in the 1970s, scientists discovered hydrothermal vents where water spews out of the seabed at up to 400°C (750°F). It remains liquid at that temperature only because of the huge pressure from the kilometres of water pressing down from above. This vent water is filled with elements like sulphur and iron that it picked up as it was travelling through rock. As it comes into contact with the cold seawater, chemical reactions cause these dissolved particles to solidify and create chimneys that can tower above the seabed.
Fascinating ecosystems arise in these chemical-rich environments. Bacteria use energy from the hydrogen sulphide spewing out of the vents to convert carbon dioxide or methane into sugars – the process is called chemosynthesis, and it’s a bit similar to photosynthesis. Some of the coolest creatures have evolved ways to farm these bacteria on or inside their bodies so they have easy access to this food source.
One of my favourites is the scaly-foot gastropod (Chrysomallon squamiferum). It looks so alien with its shiny black shell and small, metallic plates on its foot.
Like other molluscs that live near vents, the scaly-foot gastropod relies on chemosynthetic bacteria. Unlike all but one of the others, it doesn’t host them in its gills. Instead, it has evolved a modified sac, called an oesophageal gland, in its gut to house them. To make sure the bacteria have everything they need, the snail has evolved a huge heart relative to its body size – about 4 per cent of body volume – that allows it to pump plenty of oxygen and hydrogen sulphide to the bacteria from its gills. It is the perfect host.
What about those metallic plates? They are made from iron sulphide – it is the only animal known to incorporate metal into plates on its body in this way. An early suggestion was that they are a sort-of chain mail, protecting the snail from predators. But more recently, it has been suggested that they form as a result of the gastropod trying to get rid of toxic sulphur-enriched waste. Tiny channels in the scales link the snail’s tissue with the seawater. These allow sulphur travelling from inside its body to meet iron diffusing in from the water and form the iron sulphide coating.
Interestingly, another gastropod that lives at vents, called Gigantopelta, has also evolved an enlarged heart and oesophageal gland to host bacteria. However, it doesn’t have a scaly foot or metallic shell, and it hosts two species of chemosynthetic bacteria to the scaly-foot gastropod’s one. Genetic analysis has shown them to be only distantly related, as are the bacteria they host, making this a lovely example of convergent evolution.
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| A hydrothermal vent Dr Chong CHEN |
Another vent favourite is the yeti crab (Kiwa hirsuta). Rather than let the microbes live inside their bodies, they grow them in strings that dangle off their hairy claws – hence the name. When they’re hungry they simply swipe their claws at a string and suck up a snack.
Other vent residents include tubeworms that host bacteria in an organ called a “trophosome” that takes up most of their body, shrimp with a heat-sensing eye on their back and carnivorous eelpouts, all vying for the best position between the scalding, hydrogen sulphide-rich vent water and bitterly cold, oxygenated seawater.
Away from these towering vent cities are vast swathes of seabed that for a long time people assumed were devoid of life. But in May, New Scientist reported on a paper that found evidence for more than 5500 species living in the Clarion-Clipperton zone, a region of the seabed 4000 metres below the surface and coated in metal nodules that mining companies would like to harvest. As many as 92 per cent are thought to be new to science.
There is also the water column, where fish have been found at all depths, from the twilight zone where the last glimmer of sunlight reaches, through the midnight and hadal zones to the deepest point of the ocean, the Mariana trench. The more we look, the more we find – each with fascinating adaptations to their environment. Earlier this year, for example, New Scientist reported on an angler fish that glowed with bioluminescent and, seen for the first time, fluorescent light. It must be a terrifying end for any unlucky animals attracted to its teeth by these lures.
Unfortunately, all this life is under threat. As the saying goes: out of sight, out of mind. Through deep-sea mining, deep-sea fishing and climate change, humans are affecting ecosystems that have been left to their own devices for thousands of years. The scaly-foot gastropod, for example, is already classed as endangered, known to live only at three sites that are threatened by the prospect of mining.
Even though it’s hard to keep the deep sea in mind in our day-to-day lives, I would hope we could live more sustainably and leave these alien worlds to prosper.