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.
Sea cucumbers reproduce in different ways
In terms of colour, sea cucumbers are extremely variable, with black, red, yellow and even multicoloured species. Their blood vessel system is relatively highly developed and consists of a ring vessel running around the mouth opening, from which five blind ending vessels branch off and run towards the back of the body. Two vessels which run above and below the intestine are linked together by pulsating “hearts”. For respiratory organs sea cucumbers have water lungs. These are tree-like formations that are formed by protrusions of the rectum. Sea cucumbers “breathe” by drawing water in through the anus and then expelling it. The water lungs of some species of sea cucumber often contain small parasitic fish that feed on the intestines of their host animals. The pearl fish species Carapus acus and Encheliophis boraborensis, for example, pursue this rather peculiar way of life. While most animals would probably not survive the consumption of their entrails this presents no great problem to sea cucumbers because they have an extremely high regenerative capacity and have even developed a very special protection and defence technique from it. When they are attacked, holothurians expel part of their intestines by sudden contraction of their skin muscle tube and spray it in the form of sticky and often even poisonous threads of mucous (“Cuvier’s tubes”) at their predator. This completely unexpected behaviour of the supposedly defenceless prey not only aggravates the attacker but can incapacitate or even kill it, because the viscous and sticky secretion of the mucous threads contains poisons (holothurin, holotoxin) which attack the sensitive epithelia of a fish’s gills, block the fish’s oxygen uptake and lead to suffocation. Although this defensive tactic of sea cucumbers is highly unusual it is not kamikaze behaviour since the intestines grow again in a surprisingly short time. The mucus that some sea cucumbers secrete for defence purposes is so sticky that researchers are now looking for medical applications, for example for the efficient and longterm attachment of electrodes to patients. Sea cucumbers have separate sexes and spawn simultaneously within limited areas without direct physical contact. It is still completely unknown how holothurians synchronise the release of their gametes. The starting signal for reproduction is always given by the males. To do this, they straighten their front end vertically and eject a whitish sperm cloud from their mouth opening. This bizarre optical image has given them the trivial name ‘cazzo di mare’, (‘marine penis’). When the males have ejected their sperm the females release their yellowish eggs which are then fertilized by the sperm in free water. The bilaterally symmetrical larvae hatch after two days and initially live as plankton before settling permanently on the bottom after about three weeks (pentacula stage). Sea cucumbers can also reproduce asexually by transversal division. Both the front and the back of the halved organism completely regenerate the missing half within three to seven months. This possibility is also used in the aquaculture of some species by simply placing a rubber band in the middle of the body which triggers the division within about two weeks.
Insufficient management of the sea cucumber fishery
With the exception of a few waters in temperate areas of the northern hemisphere sea cucumber stocks and populations are under cons
iderable pressure in large parts of the oceans, even in well-known World Heritage nature reserves such as the Great Barrier Reef Marine Park or the Galapagos Islands National Park. Echinoderms have disappeared from many areas, especially from coastal ecosystems. Seven of the 70 or so commercially used species are already threatened with extinction and are therefore on the International Union for Conservation of Nature’s (IUCN) Red List, nine others are classified as endangered. Worldwide, 38 of the sea cucumber fisheries are considered overfished (average catch size has decreased by one third). In addition, there is illegal fishing (IUU), the extent of which is difficult to estimate. Worldwide, more than two dozen sea cucumber fisheries have already been closed due to overfishing. The driving force behind the global run on sea cucumbers is the growing demand in Asia, especially in the Chinese market, where exorbitant prices are paid for the sea cucumber product trepang, also known as bêche-demer. The combination of strong demand and the relatively easy methods of catching sea cucumbers has caused stocks to shrink considerably, especially since in many places the removal of sea cucumbers is barely regulated and rarely controlled (“open access”). When stocks off their own coasts are depleted, fishermen simply move a little further afield and plunder another area. The idea that sea cucumber fishing must be regulated and monitored in the same way as traditional fisheries is maturing far too slowly. Common management tools such as closed seasons, minimum sizes or catch quotas would be helpful, but are difficult to enforce in many of the main fishing areas. Sea cucumbers have been fished for centuries but at the turn of the millennium the quantities harvested were five to six times higher than in the 1950s and 60s. In the meantime, catch figures amount to almost 500,000 tonnes per year. Fishing is carried out using both artisanal and industrial methods and the method used varies according to the region and the species of sea cucumber. In many coastal communities it makes an important contribution to the regional economy – driven by attractive export prices – and offers worthwhile incomegenerating opportunities for the poorer population. The technical effort is manageable because sea cucumbers are often simply collected by hand in coral reefs and shallow lagoons or brought up from greater depths by divers. It is estimated that around 3 million fishermen worldwide earn their living mainly by collecting sea cucumbers. In Pacific island states sea cucumbers are often even the main source of income for some coastal villages. As a rule, it is not only the fishermen who benefit but also those members of the village community who process the catch to trepang or bêche-de-mer.
Trepang is the most important and most lucrative sea cucumber product
Trepang (derived from the Malay word ‘teripang’ for sea cucumber) or Bêche-de-mer (from the Portuguese ‘bicho do mar’, literally ‘sea worm’) is a preparation form for sea cucumbers that is very popular in many Asian countries, especially in China, Japan and the Philippines. For this purpose, the animals are freed from their entrails, cleaned and the remaining skin muscle tube is salted, dried in the sun, steamed two or three times in between and additionally often smoked, so that a durable and protein-rich product (41-63 protein depending on the type of sea cucumber) is obtained. Before preparation and consumption the outer layer of skin is removed by rubbing with pieces of coral to get rid of the annoying sclerites (the small calcareous bodies). Already since the 18th century, trading trepang has been of great economic importance for the remote islands in the Pacific and Indies. Trepang is nutritious, rich in vitamins (A, B1, B2, B3), and contains a lot of calcium, magnesium, iodine, iron, zinc and antioxidants which have anti-inflammatory and antimicrobial effects. Chinese health therapists say that sea cucumber preparations have numerous effects. Sea cucumber is said to delay the emptying of the stomach and stabilize the blood sugar level, to help fight cancer diseases, to heal wounds, retard coagulation of the blood and lower blood pressure, and to improve bone density. Of course trepang is also said to have potency-enhancing effects, as is hardly to be expected otherwise given the phallic shape of the echinoderms. The product range includes creams, tinctures, oils and cosmetics enriched with sea cucumber extracts. In addition, Chinese cuisine has countless dishes based on the preparation of trepang. Mostly soups with vegetables and noodles or the famous “Buddha’s temptation” (“Buddha jumps over the wall”), which is traditionally made from shark fins, but is now just as popular prepared with trepang. While uninformed Europeans often attribute to trepang the culinary sophistication of a boiled bath sponge, many Asians consider it a delicacy on an equal footing with truffles and caviar, which the Japanese venerate in poetry and which the Chinese call “marine ginseng”. In Japan, konowata, a dish prepared from pickled offal of a sea cucumber species, is even considered a real delicacy. For a lot of Chinese people a decent New Year’s celebration without sea cucumbers would probably be unthinkable. The prices paid for good trepang are accordingly outrageous. Depending on the type and quality of the sea cucumber, high-quality goods cost between 120 and 1,700 dollars per kilogram dry weight. For coveted products such as the Japanese prickly sea cucumber Apostichopus japonicus the price can sometimes be as high as 3,000 USD. Prior to preparation, trepang is marinated for two to three days to make it nice and soft. When cooked, its consistency is said to resemble tender crab meat, its taste is described as mild with a slight iodine note. For a pleasant aroma strongly spiced meat, seafood or vegetable sauces are usually poured over trepang pieces to add flavour and then served with Chinese cabbage, winter melon and shiitake mushrooms. Sea cucumbers are not only eaten as a dry product but also fresh and raw, deep-fried, steamed, roasted or smoked. Although the per capita consumption of sea cucumbers in China is lower than in some other countries (e.g. Japan) the large population results in a huge market with corresponding demand. Between 1996 and 2011 the number of countries supplying holothurian products to China’s seafood market rose from 35 to 83. Only a small proportion of imports currently originates from verifiably sustainable sources since it is difficult to guarantee the necessary traceability in the complex supply chains.
Sea cucumber aquaculture contributes to market supply
Due to the ongoing strong demand and attractive prices, sea cucumber aquaculture has advanced considerably since the 1980s. The Chinese and Japanese were among the first to develop successful hatching technologies and rearing protocols for echinoderms. At the moment, farming is mainly focused on the two species Apostichopus japonicus and Holothuria scabra for which market demand is high. Sea cucumbers are now produced in several countries, including Australia, Indonesia, New Caledonia, the Maldives, the Solomon Islands and Vietnam. China mainly breeds the much sought-after Japanese prickly sea cucumber and already covers around a third of its needs from aquaculture. The sea cucumbers are bred in pond and net cultures, but above all through “sea ranching”, also known as “free soil culture”, which currently accounts for about three quarters of total production. Sea ranching is the preferred method because only a few conditions are required for successful farming. Suitable locations must have the right sediment qualities with high organic contents for adequate nutrition of the sea cucumbers and the right salinity, temperature and current. Since holothurians are loyal to their habitat, rarely leaving the area once they have been deposited there, barriers such as nets or fences are not necessary. However, large stones and othe
r coarse structures are an advantage, as they offer the sea cucumber population some protection against typhoons, strong waves and predators. Another breeding method that is becoming increasingly popular is the polyculture of sea cucumbers with shrimps in culture ponds. The holothurians feed in the ponds on waste from the crustaceans, keep the bottom clean and thereby improve the sediment quality. Polyculture in ponds stabilizes the oxygen regime and this also has a beneficial effect on shrimp growth. The possibility of artificial breeding has greatly contributed to the development of aquaculture of sea cucumbers. Initially, the stock was taken from natural waters, but today commercial hatcheries have specialised in this task. Usually the spawning process is triggered by a short temperature shock (reduction of water temperature by 3 to 5 degrees for about 5 minutes). Alternatively, the spawners can be taken out of the water for half an hour for stimulation, but this method leads to higher losses. In good hatcheries, 15 to 20 of the larvae survive until the sea cucumbers are ready for stocking.
MK