China was probably the first country where people considered sea cucumbers as a culinary delicacy and as a tonic food. Recent scientific research findings show that sea cucumbers and their processed products can cure certain forms of cancer, such as lymphoma and breast cancer. Due to the popularity of these marine organisms, they have been overfished and their natural resources sharply depleted. In order to increase their production, the promotion of their cultivation has become an important part of mariculture activities in China.
The sea cucumber, Stichopus japonicus, is found in the Western Pacific Ocean along the coast of China, Japan and Korea. The vertical distribution of this species ranges from tidal areas down to 20–30 m depth. In China, it is mainly distributed along the coast of Shandong, Liaoning and Hebai Provinces, as well as in some areas of Jiangsu Province.
Stichopus japonicus is a cold water species, its optimal temperature tolerance range being 5–15 °C. At water temperatures below 3 °C or above 17 °C, its feeding activity declines sharply, while as the temperature rises to 24 °C, they enter the aestivating phase.
The optimum salinity range for Stichopus japonicus is 27–35 ppt. It shows better tolerance to higher salinity values than to lower ones. Specimens placed in low salinity seawater (10 ppt), have a LT50 of 8–17 hrs and a LT100 of 9–24 hrs.
Sea cucumbers have a very strong regenerative capacity. When the animal is faced with a predator or other harmful materials, it is able to extrude its viscera through the anus and regenerate it within 2 months. Sea cucumbers will also survive when they are cut into 2–3 pieces, each of which will grow into a new animal.
Sea cucumber is a dioecious animal, but it is hard to differentiate the male from the female with the naked eye. Its genital hole, located on the dorsal side of the head a little posterior to the buccal opening, is very small. Most individuals possess only one genital hole, but a few have 2 or 3 genital holes. All of them have spawning function.
According to the developmental stages of the gonad gland, five reproductive phases are distinguishable, namely spent, proliferative, active, ripe, and spawn.
The spent phase is observed for about 4–5 months during the period June to November, when no germ cells are found in the gonad.
The pre-proliferative and proliferative phases extend from December to March, when the gonad gland weighs about 0.2–2.0 g, and gonadal index is less than 1%. The diameter of oocytes is about 40 μm.
The active phase can be separated into two sub-phases. The first lasts from March to May, when the gonad weighs about 2–5 g and is yellow or orange in colour. The gonadal index being 1–3%. The second phase starts from mid-May, when the gonad develops very quickly and weighs 3–13 g. About 70% of the gonads weigh over 7 g, with some reaching even 43 g. The gonadal index in this phase is 7%. The oocytes range in diameter from 60–90 μm, while the sperms begin to be formed in the males.
The ripe stage extends from late May to early June, when more than 50% of the broodstock gonads weigh over 10 g. The diameter of oocytes is about 120 μm, while the gonadal index is 10%.
The spawning stage begins from early June. The bigger theroodstock, the earlier is the spawning time. The biological minimum size is 110 g. The fecundity of a female spawner ranges 2.5–3.6 millions of eggs.
The gonad of Stichopus japonicus enters the ripe stage when the temperature rises to 15–17 °C. In Qingdao area (Shandong Province), the best season for collecting broodstock is in late May, while it is early June in the Dalian coastal region of Liaoning Province.
It is observed that the relationship between body weight and gonad weight in sea cucumber is not linear. In general, it is advisable to select spawners weighing more than 250 g, in order to ensure the obtaining of quality gametes.
After collection, the sea cucumbers are put in a plastic bag filled with clean sea water. The bag is then placed in a big canvas barrel filled with seawater. The top of the barrel is covered with eel grass. Early morning or late night is the ideal time for transportation. When the barrel arrives at the destination, the sea cucumbers are transferred to a tank, where the water temperature should be the same as in the plastic bags containing the sea cucumbers, as otherwise the temperature shock would cause the sea cucumbers to extrude their viscera.
Artificial induction of spawning is highly imperative in the culture of a new species, as it allows the production of larvae over a longer period than would be possible with natural spontaneous spawning only.
It is possible to distinguish the sexes only when they start to spawn, since there are no secondary sexual characters. As a general rule, the males begin to spawn slightly earlier and require less stimulus to induce them to spawn than in the case of females.
Of the several methods tried for the induced breeding of sea cucumbers, only some have proved successful, while the rest had only limited success.
(1) Stripping. In the very first attempt to artificially obtain viable gametes from sea cucumbers, a Japanese scientist attempted the stripping technique in the early 1930s. But the rate of fertilization was only about 20%, and many of the larvae were malformed. Therefore, this method is no longer used for larvae production.
(2) Thermal shock. Thermal shock is an effective method to induce spawning. When the water temperature is raised by 3–5 °C above the ambient temperature, most of the mature spawners release eggs or sperms. But this method has a drawback in that immature gametes are also released.
(3) Combination of desiccation and flowing water. This method is commonly used to produce sea cucumber larvae on commercial scale. The operation starts at about 17:00 hrs, when the water in the temporary stocking tank is drained away and the spawners exposed to air for 30–60 minutes, after which they are jetted with water for about 5–10 min. After about 1.5–2.0 hrs, the spawners move upwards, become restless and toss their head from side to side frequently. The males begin to spawn first, followed by females about half an hour later. As high as 95% fertilization rate can be obtained in this method.
The embryo hatches out when it reaches the gastrula stage, which is followed by three more larval stages, viz. auricularia (Fig 3.1 a), doliolaria (Fig. 3.1 b) and pentactula (Fig. 3.1 c). The time schedule of various developmental stages from fertilized egg up to pentactula is delimited in Table 3.1.
Table 3.1. Developmental stages of fertilized eggs and larvae of sea cucumber (water temperature: 20–21 °C).
|First polar body||20–30 min|
|Second polar body||30–35 min|
|First cleavage||43–48 min|
|Second cleavage||48–53 min|
|Third cleavage||30–60 min|
|Blastula||03 hr 40 min – 05 hr 40 min|
|Hatching||12 hr 00 min – 14 hr 20 min|
|Early gastrula||14 hr 20 min – 17 hr 40 min|
|Gastrula||17 hr 40 min – 25 hr 20 min|
|Early auricularia||25 hr 20 min – 31 hr 30 min|
|Doliolaria||about 10 days|
|Pentactula||about 11–12 days|
Figure 3.1. Three major embryolocial stages of a sea cucumber: a) auricularia, b) doliolaria, and c) pentactula.
On entering the early auricularia stage, the larvae begin to take food. Experiments have indicated that most of the unicellular algae can be used to feed sea cucumber larvae, although some have been found to be more suitable than others, depending mainly on their size, nutrition value, digestibility, etc. For larvae of Stichopus japonicus, green algae, Dunaliella salina and diatoms such as Phaeodactylum tricornutum are considered as ideal feed, particularly the first one, which is easy to digest because of the lack of cell wall.
The feed requirement differs with the three different stages of auricularia larvae. At a larval density of about 500 per liter, the feed concentration required is 10,000 cells/ml for the first stage, 15,000–20,000 cells/ml for the second stage and 20,000–25,000 cells/ml for the third stage.
The enhancement of natural resources includes three main aspects: (1) protection of broodstock and their larvae; (2) release of larvae and transplantation of broodstock and (3) improvement of ambient environment. These measures have proved successful in the enhancement of sea cucumber productivity.
As early as the 1950s, biologists in China started work on resource enhancement. The results indicated that in overfished grounds, an effective measure was to improve environmental conditions in the sea cucumber habitat and to transplant broodstock from other places. The results of studies carried out from 1976 to 1980 in the Qingdao region are delineated in Table 3.2.
Table 3.2. The effects of attempts at enhancement of sea cucumber resource in an experimental region of Qingdao.
|Sampling date (Day/month/Yr)||Quantum of collection by diver (No./min)||Percentage increase over 1976 status (before enhancement)|
The experimental region had an area of 3 hectares, where prior to enhancement only 30 sea cucumbers could be collected per hour by one diver. After March 1976, 600 tonnes of broken stones and 1200 bundles (about 9000 kg) of tree branches were introduced into this region, followed by the transplantation of 14,100 adult sea cucumbers. In 4 years of enhancement practice, the output increased 16 folds.
Harvesting of sea cucumber, Stichopus japonicus, is conducted in two seasons, spring (April–May) and fall (October–November).
before they are sold in the domestic market, all the harvested sea cucumbers are processed into dried products, known in Chinese ‘Haisheng’, meaning ‘sea ginseng’. The curing process mainly involves cleaning, boiling, salting, reboiling and sun drying.
the collected sea cucumbers are slit open lengthwise on the ventral side and their internal organs removed. Cleaning is best done soon after their collection and while they are still alive, as otherwise removal of their guts becomes difficult.
After cleaning, the animals are boiled in seawater until they achieve a rubbery consistency. The boiling time depends on the size of the animals. In general, it needs 3 hours. The boiled sea cucumbers are then transferred to a container, evenly mixed with 30% common salt to ensure their shelf-life for 1–3 months. After salting, they are reboiled for several hours until their bodies become very hard and salt crystals form on their body surface. Drying is done in sunlight. The dried sea cucumbers are mixed with charcoal ash to prevent the growth of molds or fungi during their long storage over several years.