If you tell someone at a party that you’re a marine biologist, they’ll probably ask you about whales or dolphins. But our watery cousins are just one part of a vast subject, from microscopic plants and bacteria to the largest animals on Earth.

Several UK universities offer first degrees in marine biology. Alternatively, science, maths or engineering graduates can become marine biologists by taking a Masters degree. Such backgrounds can be useful, because understanding life in the oceans often requires understanding the oceans themselves, which means a grasp of physics, chemistry and geology. Engineering skills are also highly prized when wrestling with a piece of uncooperative equipment on the high seas.

You don’t have to be able to dive, though there are opportunities if you do or are interested in doing so. But scuba diving hardly scratches the oceans’ surface. Marine biologists dip instruments from ships, use robot submarines or dive in manned submersibles. And with more than 95 per cent of the oceans yet to be fully explored, the chances of finding something that no human has ever witnessed are pretty good.

It is in the deep oceans that perhaps the most exciting discoveries are being made. Over half our planet is covered by water more than 3000 metres deep, and life exists at all depths, down to the bottom of the deepest trenches. But this vast habitat is also the one about which we know the least.

Marine biologists used to think that deep-sea life was always slow and scarce, until the discovery of volcanic vents on the ocean floor in the late 1970s. Scattered along a 60,000-kilometre chain of undersea volcanoes, these vents spew out mineral-laden water heated to over 300C. The vents are home to meadows of metre-long tubeworms with no mouths and swarms of strange shrimp—a proliferation of life to rival that of a coral reef.

The abundance of animals around deep-sea vents rewrites some of the rules of biology. Life on Earth is usually solar-powered: plants use sunlight to grow, herbivores eat the plants and then carnivores eat the herbivores and each other. But life at vents starts with chemicals in the hot fluids. Bacteria use these chemicals to grow, in just the way plants use sunlight. The bacteria then provide food for the animals.

The inaccessibility of the deep oceans makes visits necessarily scarce and short-lived – a typical submersible dive lasts about 8 hours. However, by leaving instruments behind and comparing the changes between visits, marine biologists are beginning to piece together the lifecycles of individual species and the deep oceans’ ecology.

Despite depths of 2500 metres or more, the vents are subject to the same tidal rhythms that lap our coastlines. Tides are the metronome of the oceans. They provide a regular beat that some animals can use to synchronise their behaviour, particularly their reproduction. But reproduction at vents can be a complex affair. Palm worms, inhabiting vents in the north-east Pacific, have developed patterns of reproduction which reflect a highly variable environment – the temperature can vary by tens of degrees over just tens of centimetres. Worms with the right conditions for reproduction appear to synchronise their reproductive cycle. By releasing all their offspring together, the palm worms may improve the chances of their offspring surviving through safety in numbers.

But how often do the worms get it on, so to speak? Worms collected one week apart show different stages of reproductive development, but worms collected one month apart are at the same stage, suggesting that their reproductive cycle has gone full circle in that time. These early results hint at a possible monthly reproductive cycle for the palm worms, which might tie in with a monthly cycle in the tides. Such a short reproductive cycle is largely unheard of elsewhere in the deep-sea, but we should perhaps not be too surprised. Animals at vents grow far faster than their counterparts elsewhere and have plenty of energy to fund such a racy lifestyle. Some of the shallow-water relatives of the palm worms also have monthly cycles linked to tides.

So despite their strangeness, the denizens of deep-sea volcanic vents may have something in common with animals in rockpools on the seashore. But why should we care about the daily drama of sex and death so far beneath the waves? In practical terms, life around vents has already given us high-temperature enzymes for industry and may yet yield new medical treatments. It has also opened our minds to the possibility of life on other planets, such as Jupiter’s moon Europa, which may hide an ocean beneath its icy crust. But for me, perhaps its greatest legacy lies in keeping alive our sense of wonder. As John Steinbeck once wrote, an ocean without unnamed monsters would be like sleep without dreams.

Jon Copley is a Teaching Fellow at the School of Ocean and Earth Science, University of Southampton

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