Sea cucumbers are echinoderms, like sea stars, urchins and feather stars, and are thus part of a very broadly ranging group of animals across the depth and breadth of the oceans of the world. The common sea cucumber, or rori (Stichopus mollis), is abundant in the waters of the Taputeranga Marine Reserve, especially on the sands and muds to the north of the Island and around the boat moorings in Island Bay. It can be readily seen on sands and cobbles around the Snorkel Trail in shallow and deeper waters. The name describes body shape rather than something you might want to slice up for a summer salad, The species ranges from black to almost white, with shades of brown and yellow brown in between. They should not be confused with sea hares, which are mollusca.
Like other echinoderms, sea cucumbers can have an internal skeleton, although usually reduced to microscopic spicules or plates in some species. Rather body structure is given shape by collagen fibres in the body wall, giving the animal an leathery skin and an ability to squeeze through narrow crevices and holes. They can crawl slowly, by contractions in the muscles of the body wall or with small tube feet. Sea cucumbers feed on plankton and the detritus of the sea floor, digging into sand and mud with short tentacles. Some will position themselves in currents so that they can take in food pieces that flow by. Sea cucumbers are very common in large numbers in the detritus beneath mussel farms.
These animals have no eyes nor a true brain. A variety of nerve endings at the body wall provide sensory information. The sea cucumber breathes through its bottom. It uses its respiratory filaments 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.
Common sea cucumber at Taputeranga Marine Reserve Photo M. Francis
In some parts of the world’s oceans sea cucumbers can comprise a majority of marine biomass in deeper waters. The strawberry sea cucumber of Fiordland can exist in masses of up to 1000 individuals per square metre of rocky wall. This a spectacularly bright, scarlet species, with delicate tentacles, living down to 1000 m below sea level. Some cucumbers eject sticky fibres as a defense mechanism, whilst others may eject a good part of their internal organs along with poisons to discourage predation. They may also do this as a means of eliminating waste, and the internal parts will grow back.
Sea cucumbers are an edible resource, regarded as delicacies in tropical climates, where they may be known as bêche-de-mer or trepang. They are included in the NZ Quota Management System. Annual wild catch in New Zealand seas is reported at about 20 tonnes. Cucumbers were traded by Australian aborigines with Asian peoples to the north, centuries ago. Preparations from sea cucumbers are also used in folk medicines and some species are being investigated as sources of modern pharmaceuticals. There is potential for aquaculture of sea cucumbers.