THE SEA CUCUMBER FILE
It's a sticky business being a sea cucumber, says Paul Naylor, whether it's catching food or protecting itself
Sea cucumbers are animal not vegetable, and belong to the same group as starfish and sea urchins. Imagine a starfish with no arms that has grown very tall and then fallen over! The group's characteristic bony plates act to support a starfish or are fused to produce an urchin's globe, but are much smaller platelets in the sea cucumber's mainly soft but leathery coat.
Some sea cucumbers, such as the cotton-spinner, feed by shovelling sand and mud into their mouths as they crawl across the seabed. The small amount of organic material is digested and the remainder is left as a distinctive chain of droppings. In areas where such deposit-feeding sea cucumbers are abundant, most of the surface sediment passes through their guts several time each year.
With no hard spikes or spines like those of their starfish and sea urchin relatives, most sea cucumbers rely on toxins in their body walls to discourage predators - apart from man, that is. Dried sea cucumbers of many types are highly prized ingredients of oriental soup.
The cotton-spinner, that brown or black slug-like cucumber so common in UK waters, also lives as far south as the Azores. In the Mediterranean, small parasitic fish may live inside its body. They enter and leave via the anus.
Many sea cucumbers, like the one pictured, are suspension-feeders, living with their bodies buried in the seabed or tucked into a crevice. Their sticky tentacles are extended into the water to catch food which they eat by rolling up each tentacle in turn and inserting it into the mouth. Watching this evokes a child slowly sucking jam from its fingers.
The cotton-spinner gets its name from its habit of shooting out a mass of white threads if threatened. These are so sticky that they can tangle up a large crab for long enough to allow the cucumber to escape. The threads are part of the animal's internal organs and sometimes the intestines and gonads will be shot out as well. They can all be regenerated, luckily!