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Breeding and culture of the sea cucumber, Apostichopus japonicus, Liao

Wang Renbo and Cheng Yuan

Dalian Bang Chuidao Sea Cucumber Development Co. Ltd., Dalian, China

Abstract

Apostichopus japonicus Liao is the most important and valuable commercial sea cucumber species in China. The life cycle of A. japonicus includes the following stages: auricularia, doliolaria, pentactula, juvenile sea cucumber, young sea cucumber and adult. This paper outlines several spawning induction methods and artificial rearing techniques. The specific means of cultivation during the different stages of development, the control of chemical and physical factors in the seawater and the prevention and cure of diseases and harmful organisms will be discussed. The work compares different methods of culture and their respective merits and limitations.

At present, artificial breeding and culture of sea cucumbers is still a work in progress, but the scale of the production is increasing and a number of questions related to the culture techniques are being raised, calling for further studies on the commercial aspects of A. japonicus aquaculture.

Keywords: China, release, diseases, growth, survival rate

Introduction

The history of sea cucumber fishery dates back for more than 1 000 years. Over the last century, and especially the past 20 years, Chinese research projects have focused on the breeding, artificial culture and processing of sea cucumber.

There are over 1 000 species of sea cucumbers known worldwide. More than 100 species can be found in China, amongst which more than 20 species are considered edible. Most of them, e.g. Thelenota ananas, Stichopus chloronotus, S. variegatus and Apostichopus japonicus, are distributed in the southern sea of China.

Apostichopus japonicus belongs to Echinodermata, Holothuroidea, Aspidochirota, Stichopodidae; it is generally referred to simply as sea cucumber. It can be found in Russia, Korea Rep., in the Chinese provinces of Liaoning, Hebei and Shandong as well as in northern Japan. Because of its high nutritional value, it is considered an important seafood. In addition, more recently medical research has discovered that sea cucumbers are rich in acidic amylose, suggesting that they could possess medicinal properties.

According to several sources of information, the output of Apostichopus japonicus in Dalian was 906 tonnes in 1955. With the improvement of breeding technology, the quantity of farmed sea cucumbers has increased significantly. In 1999, the farming area was estimated at around 32 000 hectares and the output at 2 000 tonnes; the following year, in 2000, the farming area covered 48 000 hectares, while the output was 3 000 tonnes valued at 200 million Yuan. In 2002, the production reached 8 000 tonnes.

Developmental biology of A. japonicus

The development of A. japonicus includes six major phases: fertilized oocytes, early development (including cleavage, blastula and gastrula), auricularia (including early auricularia, middle auricularia and late auricularia), metamorphosis (including doliolaria and pentactula), juvenile, young sea cucumber and adult (Figure 1).

Kinetics of development

After the oocytes have been fertilized, they slowly sink and start their development. When the water temperature ranges from 21 to 24 °C the fertilized eggs require about 45 minutes to begin to divide themselves and finally develop into a blastula (Table 1). The blastula is ciliated and rotates actively. After about 2 to 3 h, the hatched embryo gradually elongates and moves to the surface of the water column. During this ascent the embryo begins to invaginate and gradually develops into gastrula. A fully developed gastrula appears at the surface of the culture tanks 24 to 28 h after fertilization. Twelve to twenty hours later the gastrula gradually elongates and begins to fold into the auricularia stage.

Table 1. Development of the sea cucumber Apostichopus japonicus1.

Stages of Development

Time to each developmental stage

Japan2

Marine Fisheries Research Institute, Liaoning Province

Marine Fisheries Research Institute, Hebei Province

Yantai and Changdao2

First polar body

15-20min


43-48min


Second polar body

45min


48-53min

30min

2-cell stage

1h 50min

1h 10-20min

1 h 3-30min


4-cell stage

2h15min

1h 58min to 2h 15min

1h30min-2h 15min


8-cell stage

2h41min

1 58min-2h 15min



16-cell stage

3h 3min

2 30min-2h 50min



32-cell stage

3h 25min

2h 53min



64-cell stage

3h 50min

3h 24min



128-cell stage

4h 10min

3h 40min



Blastula

7h 5min

10-12h

3h40min~5h 40min

11h

Gastrula

28h 35min

24-26h

17h-20h

24h

Early auricularia

40~48h

46~48h

25h20min-31h

45-46h

Middle auricularia

4-5d

5-7d


4-5d

Later auricularia

6-7d

7-1 0d


11-13d

Doliolaria

8-9d

10-16d

5-7d

13-15d

Pentactula

10-11d

13-17d

9d

15-17d

Juvenile sea cucumbers

11-13d

13-17d

13d

15-23d

Juvenile sea cucumbers
(2~4 tube-feet)

30d




1. Water temperature maintained between 21.5 and 23 °C.
2. Results obtained in research facilities in Yantai and Changdao (China) and Japan.

Figure 1. The development of A. japonicus. 1. Gastrula; 2. Early auricularia; 3. Mid auricularia; 4. Late auricularia; 5. Early doliolaria; 6. Doliolaria; 7. Early pentactula; 8. Pentactula (bar = 100 mm).

The auricularia stage

The auricularia stage lasts from day 7 to day 10 at a water temperature ranging from 18 to 26 °C. Early auricularia larvae have a symmetrical bilateral and a length varying from 320 to 600 mm. As the larva grows, the stomach increases in size. When the auricularia reaches 600 to 750 mm, its left cavity, which adjoins the stomach and gullet, begins to grow and gradually takes on a semi circular shape. At this point, the auricularia has reached the middle phase of its development. Later, the cilia develop into five pairs of symmetrical circles. At the same time, the primary tentacles begin to develop as the larva reaches the late auricularia stage. Typically, the ball shaped body of the larva at this stage is the early sign of the upcoming doliolaria stage.

Doliolaria stage

In the late auricularia phase, the larvae begin to shrink; the five lipid spheres finally connect while the tentacles enlarge and move to the centre of the body. At this stage the auriculariae are still swimming in the water column; they move from the surface to the bottom. The settlement plates should be placed in the tanks at this time.

Pentactula

The doliolaria phase usually lasts for 1 or 2 days at the end of which five tentacles reach out of the body. At this moment, the larvae begin to creep along the bottom for an additional 1 or 2 days. The number of cilia on the body surface of the larvae progressively decreases and x-shaped ossicles gradually begin to appear. The body becomes rounder and the first tube-like foot emerges on the left posterior section of the body. The presence of podia is a significant precursor to the juvenile stage.

Juvenile sea cucumber

The primary change in the juvenile stage is the appearance of tube-like feet (which enables the organism to attach to a substrate) and the ability to ingest food. The gut, mouth and papillae become clearly visible. In the next couple of months the juvenile sea cucumbers grow to about 1 cm in length assuming the appearance of an adult individual. The body pigmentation changes gradually from a transparent whitish appearance to a red, grey or light green colouration (Figure 2). At this point the individual has reached the young sea cucumber stage.

Figure 2. Sea cucumber juveniles (Photo: A. Lovatelli).

Young and adult sea cucumber

There is almost no morphological difference between a young and an adult sea cucumber. In a hatchery facility the movement and feeding activity of young sea cucumbers sharply decreases when the seawater temperature exceeds 23 °C. The young specimens move from the surface of the water to the bottom of the tank. For this reason it is important to maintain an optimal temperature level in the culture tanks to ensure adequate growth and reduce the length of this relatively fragile developmental stage.

Artificial breeding techniques

Facilities

A large sea cucumber hatchery in northern China typically has the following facilities: a larvae culture volume of 2 600 m3 divided into tanks of 10 to 20 m3 with a depth of 1.4 m; a juvenile culture volume of 4 000 m3 divided into tanks of 30 m3 with a depth of 1.7 m (Figure 3); a phytoplankton production unit with a total tank volume of 500 m3; 4 filter tanks of 200 m3/h filtering capacity; and 5 overhead troughs of 200 m3 each. The facility is also fitted with a high quality water supply and drainage system as well as a heater to raise the water temperature when necessary.

The devices used for larval settlement include metal frames each fitted with 5-10 polyethylene screens measuring 50 x 50 cm and used as the settlement substrate (Figure 4).

Figure 3. Sea cucumber settlement device (Photo: A. Lovatelli).

Figure 4. Sea cucumber hatchery in the vicinity of Dalian, Liaoning Province. Juvenile culture tanks (Photo: A. Lovatelli).

Physical and chemical factors

The optimum water temperature in a hatchery is between 23 and 25 °C, even though sea cucumbers can adapt to a range between 10 and 26 °C. The best salinity level is 31.6, but individuals can tolerate salinities from 26 to 33. The optimal pH is 7.8, but it can range from 7.5 to 8.2. The dissolved oxygen should be maintained at >5 mg/litre.

Apostichopus japonicus appears to be more tolerant to environmental fluctuations than many other species of sea cucumbers. However, the larval stages remain rather sensitive and less tolerant to variations in their environment.

Broodstock collection and maintenance

Broodstock harvesting. Individuals are generally collected from the 20th June to the 20th July in the Dalian area. At that time, the seawater temperature varies from 15 to 17 °C. A few individuals are usually sacrificed and dissected in order to adequately determine the maturation stage of the gonads. Specimens weighing around 300 g and measuring around 20 cm in length are generally preferred.

Considerable care is required during the collection and transportation of the sea cucumber broodstock. It is important to: 1) select each specimen individually; 2) keep the selected sea cucumber away from any source of pollution (e.g. oil spills from the boat engine); 3) avoid exposing the sea cucumbers to high temperatures or violent movements during transport; and 4) avoid simultaneous collection of the sea cucumbers and other marine organisms (e.g. bivalves) in order to avoid damaging the broodstock.

Maintaining broodstock. Following collection, the sea cucumbers should be placed in a tank filled with seawater at ambient temperature. The tanks should be clean and the individuals kept at a density of 30 individuals/m3 for 2-3 days. No feed is supplied. At this point the water temperature is gradually raised to 19 °C at a rate of 1 °C per day. After 7 to 10 days the broodstock can be induced to spawn. During this phase it is necessary to maintain the quality of the water in the culture tanks. Water exchange should be carried out rapidly, when required.

Spawning induction

When the seawater temperature has been raised to 19 °C, the sea cucumber activity should be carefully monitored particularly during the night hours. The bottom of the tanks should be examined for the presence of oocytes as this is an indication that the sea cucumbers are likely to start mass release of gametes. There are two ways to induce spawning: the first method consists of drying the sea cucumber in the shade followed by a jet of violent water; the second method uses a temperature shock by raising and lowering the water temperature by several degrees.

With A. japonicus, spawning often occurs between 2100 h and 0200 h. It is important to maintain a quiet environment and keep the tanks in the dark. Male sea cucumbers usually are the first to spawn, which in turn induce the females to start releasing the eggs. At this point the number of spawning males should be reduced, only retaining a few strong spawners. Following the spawning activity the tanks are drained and all the sea cucumbers removed. The eggs are gently washed and the tanks refilled with clean seawater at a temperature of 22 °C.

Hatchery techniques and larval selection

The clean seawater in the tanks should be stirred gently every 30 minutes ensuring that whirlpools do not form. EDTA is added at a concentration of 3-4 ppm while the larval density is kept at about 5 larvae/ml. The hatching success may exceed 90 %.

Selection of the larvae begins before the auricularia’s tube-like foot is formed, or shortly after. An hour prior to the selection process, stirring is stopped in order to allow the auriculariae to gather at the surface of the tanks. Malformed larvae are unable to swim and will sink to the bottom of the tanks. The larvae gathered at the surface are collected from the uppermost 0.5 m of the water column with the use of a fine mesh. These are then transferred to a new tank at a density of 300 larvae/litre.

Larval rearing

Feeding

After the appearance of the mouth in the auricularia larvae, feeding should commence immediately. Early in the growth of the auricularia, the feeding regime should be in the range of 5 000 to 10 000 microalgae cells/ml. At the intermediate stage, feeding needs to be increased to about 20 000 cells/ml. Monitoring the presence of algae in the stomach of the larvae helps the adjustment of the feeding regime. Dunaliella is the primary algal species used and is generally mixed with diatoms and Chaetoceros sp. The three micro-organisms are added in a proportion of 4:1:1. The food mixture should be supplied in small quantities in order to maintain optimal water quality.

Water exchange

Rearing during the auricularia stage should be carried out in still water. Water should be added to the rearing tank at a rate of 20 cm per day until the tank is filled. This usually takes approximately 3 days. Once the tank is filled, one third to a half of the water should be exchanged daily using a siphon. As the larvae grow the water exchange rate should gradually increased. Water exchange should be carried out gently in order to avoid injuries or loss of larvae.

Control of chemical and physical factors

The culture of auricularia larvae requires the proper control of all major chemical and physical factors. Generally speaking, the pH and salinity values should remain within the range tolerated by the larvae. Stirring the water or adding new seawater will control dissolved oxygen levels. It is important to maintain the temperature at around 23 °C. If the larvae are reared in natural seawater, the temperature will rise gradually with the seasonal fluctuations. Appropriate measures must be taken to maintain it within the acceptable range.

Control of diseases and harmful organisms

During the larval culture, the primary harmful organisms are copepods; however their presence can be contained through a proper water exchange protocol and the use of EDTA (3 to 4 ppm), if necessary.

The primary symptoms of disease are a slow development rate of the larvae and the “rotten stomach” phenomenon observed in late auriculariae. This can be caused by food deficiency or sub-optimal larval densities. The use of quality feeds and maintaining low larvae densities can minimise the outbreak of diseases.

Larval settlement

After the larvae have developed for 7 to 10 days, the five lipid spheres on each side of the larvae appear and the hydrocoel begins to develop. The larvae are now transforming into doliolaria. At this stage, the settlement substrates (see Figure 4) should be placed in the rearing tanks. The density of the larvae settled on substrate should be maintained between 1 to 2 individuals/cm2.

Rearing of juvenile sea cucumbers

The rearing techniques at this point of the development of the sea cucumber juveniles are very important and should be carefully applied in order to ensure a high rate of survival.

Water exchange

The rearing tanks are fitted with a water flow through system. The presence of food particles and the relatively high water temperature are responsible for the proliferation of undesirable and harmful organisms, such as copepods and bacteria. The water in the tanks should be clean with a daily exchange rate of up to 200 %.

Feeding

Feeding quantity and feed quality are key parameters to the survival of the larvae. Insufficient food may not only reduce the growth of the larvae but may cause the entire sea cucumber population in a tank to perish. On the other hand, overfeeding may seriously pollute the water in the tanks. In the early developmental stage, diatoms are used as the main food source along with a soup of powdered Sargassum thunbergic. At a later stage, S. thunbergic becomes the primary food item and is complemented using artificial feed. Fresh S. thunbergic is used which is ground to suit the size of the juveniles that are being fed. In addition, 1 to 5 ppm of yeast is added. A mixed food diet is preferred over a single diet when rearing sea cucumber.

Sorting juveniles sea cucumber

Juvenile sea cucumbers grow at different rates and it is therefore important to sort them according to their size. This is done using sieves fitted with different mesh sizes. While sorting, care should be taken not to damage or kill the juvenile sea cucumbers.

Prevention and treatment of diseases and harmful organisms

A variety of diseases affect juvenile sea cucumbers, often causing ulcer-like lesions on their body surface. Additional problems are cause by harmful copepods that compete for the food present in the tanks. Bacteria cause decay of the body tissues, but the mechanism is still not clearly understood. Bacteria are treated with antibiotics (1-3 ppm). Furthermore high numbers of Harpacticoida can also be harmful to the juveniles. These tiny crustaceans are eliminated with the use of a pesticide (e.g. trichlorphon) at a concentration of 1-2 ppm for 8-12 hours, followed by a complete water change.

Growout techniques

Farming methods

Today, in northern China, the sea cucumber Apostichopus japonicus is commonly farmed in earth ponds either in an extensive or semi-intensive system. Sea rafts are sometimes used, but are not very popular.

Extensive pond farming: This farming system has developed and expanded rapidly as a result of the low investment costs involved, easy management of the ponds and remarkable revenues. Either old and abandoned shrimp ponds or new intertidal ponds are used. The sea cucumbers are stocked at a density between 30-100 individuals/m2 depending on their initial body size. Very little or no additional food is needed when abandoned shrimp ponds are used as incoming seawater is usually rich in food particles and algae. Rocks are placed on the bottom and aerators are added to compensate for the low dissolved oxygen levels. Under these conditions the sea cucumbers can grow 1-1.5 times faster than in the wild. Finally, the introduction of pathogenic and other undesirable micro-organisms can be adequately controlled if an appropriate water supply is identified.

Raft culture. Wooden rafts are usually located in sea areas not exposed to strong winds and tidal action. The sea cucumber cages are usually hung under the rafts or placed directly on the sea floor. During the rearing period, sea cucumbers are fed with Sargassum sp. and other macroalgae. This farming method is not very popular due to the high costs involved.

Intensive farming: Intensive sea cucumber aquaculture has only recently developed. Large quantities of rocks or other suitable substrates (e.g. roof tiles) are placed in the culture ponds to increase the surface area for the juvenile sea cucumbers and to provide shelter from predators. A refrigeration system or underground water supply is used to lower the water temperature in the ponds in the summer months when high temperatures can reduce growth and feeding rates. These pond facilities are expensive and therefore intensive farming is only carried out by large and financially strong companies.

Selection of culture areas in the sea

The most suitable sea locations are rocky sites with an abundance of macrophytes and with muddy and sandy bottoms. Areas protected from strong winds and tidal actions are usually favoured.

Stocking method

Juveniles over 1 cm in length are used when stocking directly in the sea. The larger the initial size of the juveniles, the higher the survival rate. When stocking, the juveniles are placed in bags with a mesh size of 0.25-0.5 cm. A diver then places the bags on the sea bottom in the vicinity of existing rocks or artificially prepared rock piles. Subsequently the mesh bags are opened and the juveniles are simply left to crawl away. Larger juveniles (>5 cm) can be released into the sea directly.

Harvest

Generally, two years after stocking, the sea cucumbers have reached a marketable size and can be collected. Harvesting is often done in the spring (mid-April to early June) and autumn (October to December) (Figure 5).

Figure 5. Sea cucumber harvested off the coast of Dalian, China (Photo: A. Lovatelli).

Advantages and disadvantages of sea ranching

Sea ranching takes full advantage of the natural environment and this method is considered to be ecologically acceptable and therefore worth developing. However, due to a number of seasonal limitations and resource use conflict (presence of industrial activities), this growout method can only be carried out in locations which have the right conditions.

Reference

Liao, Y.L. 1997. Fauna Sinica, Phylum Echinodermata, Class Holothuroidea. Science Press, Beijing. 334pp.

Sui, X.L. & Liao, Y.L. 1988. Sea cucumber culture and its enhancement. Agriculture Publishing House, Beijing. 288pp.

Zhang, F.Y. 1958. The preliminary report of aquaculture and sea ranching of sea cucumber (Apostichopusjaponicus). Journal of Zoology, 2(2):65-73.

Zhang, Q.L. & Liu, Y.H. 1998. The techniques of sea cucumber culture and its enhancement. Qingdao Ocean University Publishing House, Qingdao. 157pp.


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