Secret Life of Sea Cucumbers
Sea cucumbers are quiet and unassuming dwellers in our seas. But they are diverse, specialized, and critical to the world’s oceans,
Found in every marine environment throughout the 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; sea cucumbers have been found swimming a staggering depth of 2000m, and they are evolutionary ancients, with the oldest recorded dated at 400 million years. Ranging in size from a diminutive 1 cm to the enormous Synapta maculata at 5m, most will grow to around 20cm.
An estimated 1,250 species have been recorded worldwide with the highest number in the Asia Pacific realm. Related to starfish and sea urchins, sea cucumbers perform the same job as earthworms on land, breaking down detritus and organic matter for bacteria, thus recycling nutrients back into the world’s seas. They have a key role in the ocean ecosystem and have evolved a glittering diversity with many in a bizarre array of colors and designs. And far from being sedentary, some sea cucumbers are able to swim through the water column while others will live their entire lives as plankton traveling the world’s ocean currents.
Not only critical to the health of our oceans, but they have also recently been able to shed light on tissue regeneration in humans. Because of their astonishing powers of repair, scientists are using sea cucumbers to study tissue regeneration in humans.
And they are further helping scientists 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 armor hardening its skin when threatened and 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 and then supple once inside the body.
Not many people see the unassuming sea cucumber sitting on the reef as a master of defense, but it has a capable armory up its knobbly sleeve. Famed for expelling its innards when threatened, the so-called cuverian tubules, part of the respiratory system, are used as defensive structures and are 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, holothurin, lethal to any unlucky creature in the general vicinity.
They are far from a pushover to other reef inhabitants. Other sea cucumbers simply curl up and roll away from danger, while some species live in groups adopting the safety in numbers strategy; there is more chance of your neighbour being eaten than you. And of course, simply hiding or camouflage is always a popular method. Sea cucumbers are also capable of perfect mimicry. The juvenile sea cucumber, Bohadschia graeffei,mimics the toxic Phyllidia nudibranch. Perhaps one of the most surprising abilities though, is in loosening the collagen in their skin enabling them to squeeze through small crevices for safety. They are a hidden Houdini of the sea.
When it comes to sea cucumber sex, they aren’t known for their courtship. Many sea cucumbers don’t favour the haphazard squirting of potential young into the big blue in the hope of survival and they diligently carry the eggs and then the young in a pouch. The young are then hatched into the body cavity where they develop and are “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.
But life isn’t all rosy for sea cucumbers. Around the world sea cucumbers are collected for the beche de mer markets of Asia. Considered a delicacy and a status symbol, dried and salted sea cucumbers are an expensive and a lucrative business, and sea cucumber populations around the world are being systematically decimated 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 data show that the traditional fishing grounds of Asia and the Pacific are either depleted or under intense fishing pressure and new grounds are opening around the world as far as Russia and the Galapagos.
“Sea cucumbers are in dire straits in many parts of the world” concludes a recent Food and Agriculture Organisation (FAO) report. This alarming assessment is from the world’s foremost experts and includes sites such as the Great Barrier Reef in Australia and the Galapagos Islands, both high profile World Heritage Sites and Protected Areas. The collapse of their sea cucumber populations is well documented and there appears no recovery as yet. Caribbean and Latin America are further at risk of collapse; most countries in Africa show evidence of overfished stocks, stocks in Asia and the Pacific are under intense pressure with many already depleted.
Aquaculture and restocking programs are being developed in some countries such as Australia and The Philippines. However, restocking is an expensive solution to overfishing and poor management of existing stocks. Further stress from pollution, habitat damage and climate change impacts impede any chance of recovery in many areas. Most alarmingly, demand for beche de mer is growing not declining.
The sea cucumber has a vast evolutionary history, its diversity is nothing short of astounding, and its range of life histories; remarkable. Its place in the oceans ecosystem is critical. They have survived 400 million years of evolution and have spread across the globe to all ocean realms; they are now under the biggest threat to their survival. And their quiet existence may well be their downfall.
Sources:
- James.D.B (2009) An annotated Bibliography of Sea Cucumbers
- Toral-Granda & Lovatelli A (Eds) (2009), “Sea Cucumbers A Global Review of Fisheries and Trade.” FAO Fisheries and Aquaculture Technical Paper. No. 516. Pub.FAO