Displaying items by tag: holothurians
Growing demand and attractive prices are accelerating overuse
This article was featured in EUROFISH Magazine 4 / 2020.
Sea cucumbers, also called holothurians, are the most species-rich group of echinoderms (which also include sea urchins and starfish). On the seabed these worm-like creatures are of similar ecological importance as earthworms in the garden. Sea cucumbers take up sediments, digest the organic matter they contain and excrete the sand in a purified state… so act like “marine vacuum cleaners”.
Most of the time sea cucumbers seem to lie motionless on the seabed and with their worm-like, cylinder-shaped bodies some of them actually resemble the vegetable whose name they bear. Other forms are often compared to fat caterpillars, thick prickly sausages, or even the little heaps that dogs leave behind on pavements. Indeed these strange animals exist in a huge variety of shapes. They live in the benthic habitats of all seas from shallow tidal zones to the dark bottom of the deep sea. Among the more than 1,700 known species there are tiny representatives such as the Rhabdomolgus species, which measure only a few millimetres, but also giants such as Synapta maculata, which can grow up to two and a half metres in size. However, the majority of species measure only 10 to 30 centimetres in length and live for about 5 to 10 years. Sea cucumbers play an important role at the bottom of the oceans. They feed on the seabed, digesting sedimented plankton (detritus) as well as organisms that live in the seabed, or the remains of dead organisms that have sunk to the bottom of the sea, and then excrete the sand again – now freed from organic pollution. This “sanitary service” that the sea cucumbers perform is of great importance for the health of the seabed: the cylindrical animals clean the ocean floor, removing dead organic matter, thereby preventing excessive oxygen depletion in the depths. At the same time, on the light-flooded surface they release nutrients that enable renewed the growth of microalgae with which the marine food chains begin. Sea cucumbers have a special function in tropical coral reefs: they “recycle” calcium which many marine animals need to build their shells or skeletons. A high calcium carbonate content also increases the alkalinity of the water and acts as a buffer against local acidification. In some areas of marine life, especially in the deep sea, sea cucumbers are the dominant life form. In fact they sometimes account for almost 90 per cent of the biomass present there. Common to all sea cucumbers is the soft but muscular tubular body with its thick leather-like skin. The mouth opening at the front end of the trunk, which is often surrounded by tentacles, bears a ring of calcareous plates that surrounds the oesophagus and serves as an attachment for the often very complex tentacles and longitudinal muscles that extend backwards in the body. In contrast to starfish and other echinoderms, sea cucumbers do not have an exoskeleton but have spine-like hard formations, so-called sclerites, on their skin. These are attached flexibly to the body, can be moved with special muscles and are used for digging in the sediment and for protection against predators. The five-beam radial symmetry characteristic of all echinoderms (many starfish have five arms, the chewing apparatus of sea urchins has five teeth) can only be recognized externally in sea cucumbers by the five rows of ambulacral feet. These are small movable feet with tiny suckers, hundreds of which are arranged on the body in long rows. With their help sea cucumbers can move forward, but only very slowly. In adaptation to life on the ground, some holothurians have developed flattened bodies where a sole (for crawling) and a back can be seen. However, this bilateral symmetry is a secondary development and does not replace the five-beam radial symmetry. Some sea cucumbers can also swim and in doing so detach themselves briefly from the seabed.