Summer Series 7: Sea cucumber - the slug that dispels its digestive organs through its bottom!
It looks like a slimy worm –- but it lives in the sea! The common sea cucumber is a sluggish creature, brown and blotchy, designed to blend in with its habitat: rocky reefs and sandy bottoms.
It is sub tidal and can be found at depths up to 100 metres, all around the coast of New Zealand.
"They look like a worm crossed with a sausage, and the adults can grow to 20 cm and live for five years," says NIWA aquaculture scientist Jeanie Stenton-Dozey.
Sea cucumbers are closely related to starfish. "Imagine the 5 arms of a starfish being stuck together and elongated and then laid on its side with the mouth one end and the anus the other. On the underside there are tiny tube feet and on the back are protuberances that function to look like armour. The blotchy tough outer skin helps with camouflage," says Dr Stenton-Dozey.
More than 100 other species of sea cucumber are found in New Zealand waters.
Sea cucumbers are most active at night when it's safer to crawl around grazing like a little vacuum cleaner on detritus on rocks and sea bed sediments. They move using their tiny tube feet, and the rhythmic contraction of their body wall. They are very important 'cleaners' of marine ecosystems where they turn complex organic waste into important minerals, like nitrogen, that can be used by marine plants for growth.
They have no eyes, although there are various nerve endings scattered through the skin giving a sense of touch and a sensitivity to the presence of light. "Sea cucumbers have no true brain. A ring of nerve tissue surrounds the oral cavity, and from here nerves extend to the tiny tentacles around the mouth, and down the length of the body. If the nerve ring is cut off, the sea cucumber can still function and move about! So it does not need nerves to co-ordinate its body," says Dr Stenton-Dozey.
The sea cucumber breathes through its bottom. It uses its 'respiratory trees' to take oxygen from the water. These 'trees' branch out inside the animal in the region of the anus so when they 'breathe' the water in, it enters and then is expelled through the anus.
The sea cucumber's camouflage tends to save it from predators. But, if need be the sea cucumber has a surprising defence mechanism. Its major defensive action is to expel its digestive organs through its bottom. An action that would frighten anyone! While the would-be predator is distracted by the delicious looking bits and pieces floating about, the sea cucumber makes a hasty retreat. Fortunately, the sea cucumber can regenerate its internal organs within a few weeks.
The natural predators of sea cucumbers are large fish that can eat them whole and starfish. Many are taken as by-catch in bottom trawling.
Sea cucumbers have unique 'elastic' body walls. Collagen in the walls can be stretched or tightened to make the animal long and floppy or short and compacted. This means that the sea cucumber can squeeze through tiny gaps, and once safe in a crevice, it can firm up. This is another way to hide from the jaws of predators!
There is a fishery catch quota for this species of sea cucumber which is sold locally after processing. This processing involves removing the insides and drying the body wall which is the part that is so expensive fetching up to 500 dollars per kilogram. Outside New Zealand, processed dried sea cucumbers are generally traded under the name of bêche-de-mer, representing an important fishery in South Pacific and Asian countries.
Species Fact File: Sea Cucumber
Common names: Sea cucumber
Scientific name: Australostichopus mollis, Phylum Echinodermata
Size: Up to 20 cms
Lifespan: They live up to five years.
Diet: They can be found where there is organic food matter.
Reproduction: They reach reproductive maturity at two years. The complete larvae stage takes approximately 21 days. After fertilisation there are seven stages of larval metamorphosis. They have an annual reproductive cycle, spawning between October to March. The sexes are separate and develop synchronously.
They are broadcast spawners, so eggs and sperm are released into the water column, and following fertilisation, they undergo a three-to-four week larval phase before settlement. Populations from sheltered areas such as fiords and sheltered bays may be largely 'self seeding', while larvae released on open coasts may disperse more widely.
Things you need to know: They don't bite. They often go limp and slimy if stressed by handling.
Something strange: Sea cucumbers have leathery skin.
About Jeanie Stenton-Dozey
Jeanie Stenton-Dozey is an aquaculture scientist, currently researching the potential to culture Australostichopus mollis and to grow them on sea-based farms with other species like finfish and mussels. Since sea cucumbers are excellent ecosystem cleaners they can play an important role in eating organic farm waste while at the same time increasing culture productivity per farmed hectare.