The secret lives of Sea Cucumbers
NOT MUCH TO LOOK AT,” said a diver in our group. “The highlight of the dive was a sea cucumber!” It raised a few smiles.
I felt defensive, and wondered briefly if I had the time before our next dive to tell him, and his newly founded anti-sea cucumber club, how diverse, colourful, evolutionarily adapted and well, just damned amazing sea cucumbers were.
Could I make them see the dazzling light of the sea cucumber before the day was out
Found in every marine environment throughout the world’s oceans, from the Equator to the Poles to the deep ocean trenches, sea cucumbers, or Holothurians, live mainly in shallow seas and are most abundant in tropical oceans.
Few families in nature span such a diverse range of depths and habitats. And they are evolutionary ancients, with the oldest recorded form dated at 400 million years.
They range in size from 1cm to the enormous Synapta maculata at 5m, though most grow to around 20cm.
During their long history, sea cucumbers have evolved a glittering diversity of around 1400 species, many in a bizarre array of colours and designs.
And far from being sedentary, some can swim through the water column and some live their entire lives as plankton travelling the currents.
Remarkably, sea cucumbers have been found swimming at depths of 2000m.
A deep-sea survey found some at 1500m, swimming “so gracefully above the mud bottom that they were confused with jellyfish”, according to Bob Carney of Louisiana State University.
They can make up a phenomenal 90% of the biomass of animal life on the deep sea floor.
SEA CUCUMBERS FEED on decaying organic matter on the seabed, clearing detritus and essentially cleaning the oceans. An astounding 54 tonnes of sand passes through a Holothurian body every year. They also provide an important food source for other organisms – and they are now proving valuable in other ways.
Western science has discovered that the toxins produced in some species can act as anti-inflammatory and anti-microbial agents. Some can even be used to prevent blood-clotting, thanks to their strong anti-coagulant properties.
Traditional cultures have long known that sea cucumbers possess special powers of healing. Pacific islanders use the cuverian tubules in the sea cucumber’s respiratory system to place over wounds, like bandages, to stem bleeding.
Sea cucumbers seem to have astonishing powers of repair, and scientists are using this to study tissue regeneration in humans with kidney failure. They are further helping scientists to develop a material that may one day be used for brain implants to cure Parkinson’s disease.
THE CURIOUS ABILITY of the sea cucumber to activate its body armour, hardening its skin when threatened and then becoming flexible once it relaxes, has sparked the development of a revolutionary material.
Scientists have mimicked the skin of sea cucumbers to make brain electrodes that are rigid when implanted but supple once inside the body.
All this from a formless creature sitting quietly on the seabed.
Many divers who see a sea cucumber sitting on the reef might not recognise it as a master of defence, but it has quite an armoury. Famed for expelling its innards when threatened, the so-called cuverian tubules, part of the respiratory system, are used as defensive structures.
Expelled through the anus, becoming sticky and expanding dramatically, they are designed to entangle predators such as crabs and other would-be attackers, rendering them immobile as the sea cucumber makes good its escape.
The release of the tubules may also cause the ejection of a potent toxin called holothurin, a soapy substance that can kill any unlucky creature in the vicinity. Sea cucumbers are far from a pushover to other reef inhabitants.
Some simply curl up and roll away from danger, while other species live in groups to find safety in numbers, on the assumption that there is more chance of your neighbour being eaten than you.
Concealment or camouflage is always a popular method of defence, but sea cucumbers are also capable of perfect mimicry. The juvenile Bohadschia graeffei mimics the toxic Phyllidia nudibranch. But perhaps most surprising is the sea cucumber’s ability to loosen the collagen in its skin, enabling it to squeeze through small crevices for safety. Holothurians are a hidden Houdini of the sea.
Sea cucumbers aren’t known for their courtship. Both sexes release sperm and ova into the ocean, although some are hermaphrodite, causing all sorts of confusion.
SOME SEA CUCUMBERS don’t favour the haphazard squirting of their potential young into the big blue, and carry their eggs and then their young in a pouch. The young are hatched into the body cavity, where they develop. They are then “born” through a body wall rupture.
Emerging onto a beautiful rocky and muddy seabed, they either find a spot and start their lives or float around in the currents. It’s enough to bring a tear to your eye.
Largely solitary, sea cucumbers don’t mix well, though a few have decided that it’s good to socialise.
Pearly fish live in symbiotic relationships, or more accurately commensalism; where two species coexist, one benefitting with no effect on the other species. The tiny fish live in the cloaca (or anus) of the sea cucumber, using it for protection from predators, as a food source and to grow and develop into adults.
Some worms, crabs and shrimps have also taken to using the cloaca for protection and for hitching a ride. It is unclear what, if any, benefit the sea cucumber gets, but there doesn’t seem to be any negative impact on it.
LIFE ISN’T ALL ROSY for our sophisticated sea-dwellers. Around the world, sea cucumbers are collected for Asia’s beche de mer markets. Considered a delicacy and a status symbol, dried and salted sea cucumbers are an expensive and a lucrative business. Populations around the world are being depleted systematically to supply the market.
Sea cucumbers are far from anyone’s idea of cute, cuddly or charismatic, and they are often ignored in research and conservation projects. But what data does exist shows that the traditional fishing grounds of Asia and the Pacific are either depleted or under intense pressure, and new grounds are opening around the world, as far apart as Russia and the Galapagos.
The anti-sea cucumber club was on the boat waxing lyrical about a gorgeous yellow and black spotted nudibranch. My moment had come: “Actually, it’s a sea cucumber,” I said, with none of the smugness I felt they rightly deserved.
Eyebrows were raised. I flipped through an identification book and pointed out its tubular feet and its lack of exterior gills. Sea cucumbers are commonly confused with nudibranchs.
The anti-sea cucumber club were soon huddled around the book, eagerly ticking off which ones they had seen and which they had misidentified.
“Wow,” said the president, “they can shoot out sticky threads to catch their attackers, just like Spiderman!” I smiled: they would never again look at a sea cucumber in the same dim light.
Sea cucumbers have survived 400 million years of evolution and spread across the globe. They are now under the biggest threat to their survival – and their quiet existence and colourful secret lives may well be their downfall.