Holothurian, commonly called as sea cucumbers, is a group of economically important echinoderms. Which are consumed either fresh or boiled or prepared as beache- de-mer (trepang) by people of Japan, Korea, China etc. Body wall, ovaries, intestine and respiratory trees are considered delicious food for Japanese. Holothurians exhibit a world wide distribution. About 1200 species are recorded as belonging to the class Holothuridea. There are around 40 edible species. Along the Indian coast Holothuria scabra commonly called ‘sand fish’, is the predominant species. Whereas in Japan and Korea Stichopus japonicus predominates. Some species live on hard substrates like rocks, coral reef etc and vary in their habitats from foreshore to deep water zones. The other Holothurians of commercial value are Holothuria nobilis (black teat fish), H. Fusogilia (white teat fish), Actinopyga echinites (deep water red fish), A.milaris (black fish), Thelonata ananas (prickly red fish), Holothuria ata (lolly fish), H. Fuscopuncata (elephants trunk fish) etc.
Both sandfish ( Holothuria scabra) and golden sandfish (H. lessoni) are considered to have the best potential for aquaculture because they have many attributes that make them suitable for hatchery production
BIOLOGY
The internal organs of the cucumber lay within the tube-like body chamber surrounded by the skin and a layer of longitudinal muscle bands. It is the skin and muscle bands that are the edible part of the animal. The internal organs consist of the digestive system for food processing, the gonad for reproduction, the respiratory trees for the removal of oxygen from the water, and a nerve ring that directs the operation of the muscles and tentacles. The cucumber is capable of eviscerating (casting off) its internal body organs during times of stress which can later be regenerated.
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| Evisceration |
The sea cucumber is a non-selective suspension feeder, taking its food indiscriminately from the surrounding water. The small bits of detritus and microscopic organisms that are floating just above the bottom are trapped by the cucumber's tentacles.There are ten tentacles that are covered with a sticky mucous and are extended in the water until they are filled with food particles. The sexes are separate, but microscopic examination of the gonad is the only reliable way to distinguish males from females. The gonad is located along one side of the body cavity and begins to produce eggs or sperm in the fall for the next spring's spawning season. Spawning occurs from about late March to mid-April and coincides with the spring plankton bloom.
At this time the eggs and sperm are released into the water column where the eggs are fertilized and develop into a brief larval stage, at which time the animals are bright red, planktonic and called a "pentacula". By the end of May the larvae have evolved into juveniles and settled to the bottom.
LIFE HISTORY
Holothurians usually spawn in the late afternoon or evening or during night. During spawning, the males release the spermatozoa first and the females release the eggs.The males first lift the anterior end and perform swaying movements for some time after which they start releasing sperm and it continues for 1-2 hours. Ripe females, if any are present nearby, exhibit responsive behaviour. The anterior region of the female gets bulged and eggs are released through the gonopore forcefully in a continuous jet.
The mass of eggs are released appear light yellow and mucus like. The fertilization is external. Taking place in water. One adult female release about 1 million eggs. Eggs are spherical, about 180-200 microns in size. The auricularia larva hatches out after 48 hours. The auricularia larva is transparent, pelagic, it is slipper shaped and performs locomotion by the movement of flagella of the ectodermal cells that forms ridges or bands. The larva has a digestive tract consisting of mouth, pharings, stomach, and anus. There are three coelomic sacs- hydrocoel, and right and left somatocoels.
REPRODUCTION
Holothuria scabra attains a length of 400 mm and weight of 500g and lives on sandy muddy bottom and become sexually mature at 18 months. The size at first maturity is 210 mm. the sexes are separate. There is no distinct sexual dimorphism. The ovary in females and testes in male are in the form of a tuft of tubules attached to the dorsal mesentery, through which the gonoduct passes terminating in gonopore situated on the dorsal side near the oral region.
The gonadal development is distinguishable into five stages such as Immature, resting, growing, mature, and post spawning phases. During the immature and resting stage, the ovarian tubules are transparent, short and thin distal end of tubules are club shaped. During grawing phase the tubules are having opaque spherical oocytes 20-120 microns in diameter. During the mature phase, the tubules are swollen containing ripe oocytes of 150-200 . It has been reported to breed twice in a year, first spawning season in from March to May and second during October to December in Gulf of Mannar.
Culture of sea cucumber
Technique for pond culture.
The ponds should be located in a area with free tidal current, The size of one pond is between 30-300mu (2-20 hectares) ，water depth is above 1.8-2 m, salinity is above 26 ppt year-round. Water circulation is very important factor. Rocks and concrete are to be applied to strengthen the inner layer of pond wall. The best substrate of pond bottom would be solid mud-sand or sand- mud; the substrate will affect the growth rate and survival tremendously. The sea cucumber culture purposes including: mud removal, disinfection and reef building. No feeding required in most of ponds. Sea cucumbers mainly depend on natural food resources. Routine monitoring of salinity, temperature, pH, alkalinity, water colour, transparency and adjust if required.
In addition mixed culture predominated by Chaetoceros also shall be required.
Selection and construction of the culture ponds:
The farm sites should provide suitable conditions for the growth of sea cucumbers. A supply of clean and unpolluted seawater should be easily accessible. Salinity levels should range between 25 and 35, with optimal values around 27-32. Ponds with muddy and sandy bottoms and of 2-3 hectares in size are preferred; however some operators use ponds as large as 7 hectares. If necessary, stones or other artificial materials are placed in the pond to provide an adequate substrate for sea cucumbers to aestivate and live through cold winters. Hard substrates should cover 50-70 % of the bottom. The depth of water should be between 1.5 to 2 m and the seawater temperature maintained between 0-30 °C.
Dam Pond Culture
Dam pond can be build by concrete based on the local geomorphological characters in a shallow intertidal area, or a bay with rocky substrate, or by small reef islands area, where there is natural distribution of sea cucumber. The artificial reefs are built by rocks inside the pond. The height of the dam is based on the high tide line, usually is higher than 2.0m above the ground. There are water intake and release holes underneath the dam, so the water exchange can be driven by tidal action. Seeding density:30 seeds/m2 for 1- 2cm seed, 10 seeds/m2 For 5cm seed.
Longline culture in open sea
In an area there is less wave action, free tidal current, the existing long line system can be used for sea cucumber farming. The mesh size of the lantern nets is around 1.0cm, open and close by zipper, so it is easy to do the routine feeding operation. Scallop net, abalone net and modified plastic bucket can all be used for sea cucumber culture, and most facilities can do the poly-culture with abalone. Seeding density is around 200- 300seeds/m2 for 5 cm seed, placed 5-8m below the water surface, The density has to be reduced as the sea cucumbers grow. This type of technique can be used as poly-culture with kelp as well.
Seabed cage culture
In a bay area, round or rectangle cages (2ｍ ×1.5ｍ×1ｍ) were built with steel bars, covered with 1.0cm polyethylene mesh netting outside the cage and hold rocks inside the cage. A 5cm space is needed to keep the rock from the frame of the cage to prevent netting damage due to friction. Seeding density: 3-5cm seed at 200-300 seeds/m2. Routine management includes monitoring for mesh damage and reducing the sea cucumber’s density through the grow-out period.
Artificial breeding techniques
Holothurians hatchery should have a Brood stock maintenance unit, Spawning unit, Larval rearing unit and an Algal culture unit.
Brood stock maintenance unit
The Brood stock is usually collected from wild as well as from commercial catches and induced to spawn immediately or held in land based tanks and conditioned in captivity.
Sand fish live in high nutrient environment at densities of 100/ha. Tropical sea cucumber can be difficult to hold in captivity and reduced feeding, weight loss, and poor gonad development. The large and healthy specimens which are not injured or eviscerated during capture are chosen for breeding. Collection of breeders is done during the breeding season i.e., March to May or October to December.
Preparation of brood stock tank:
FRP tanks of 1 ton capacity provided with 6 cm thick sand at the bottom are used for keeping the breeders brought from natural ground. They are stocked at the rate of 20-30 adults in one tank.
The sand is also brought from the natural beds. These animals usually live buried in the sand and hence the sandy bottom is recommended. The tank is filled with filtered, clear sea water of about 32-35ppt.
The water has to be changed every day, and sand is changed once in a fortnight. Feeding is done with fresh algae brought from the sea and ground to a fine paste which is given in the tank once in a week. Excess food may cause water fouling. In case of any water fouling, the sea cucumbers eviscerate and become useless for breeding. As algal paste settles to the bottom, the sea cucumber ingests the same with sand. If the feeding is not proper the animal gets shrunken and are the specimen not fit for spawning purpose. The Brood stock rearing tanks are kept in an air-conditioned room to maintain a low temperature of 18-20 degree c.
Spawning unit
Spawning is carried out in rectangular FRP tanks of about 100 litre capacity. The provision for an immersion heater with thermostat, thermometer and aerator are provided in the tank for thermal stimulation of spawners. . After introduction the spawners into the tank having filtered, clear and clean sea water, the temperature of the water is raised by 3c-5c by using the immersion heater. This thermal stimulation induces the sea cucumbers to spawn. This is the most widely used and most reliable method to induce the holothurians to spawn.
Apart from the thermal stimulation there are three other ways to do the breeding of holothurians. These are (1) Natural Spawning (2) Stripping and (3) stimulation through drying and powerful jet of water.
1. Natural spawning
The male and female may release the gametes into the surrounding water without any artificial stimulation.
2. Stripping
This is done mainly on an experimental scale only. The animals are cut open from cloaca to mouth through the dorsal side. The ovary which is translucent is taken out from the female and the same is slightly dried in a shade. It is then placed in sea water in a petridish and punctured with the scissors to release the eggs into the sea water. In the same way, the testis is taken out and cut into pieces. When the sperm move out in the water it is mixed with the eggs kept in a beaker with sea water. Mild aeration fecilates higher rate of fertilization.
3. Stimulation through Drying and powerful jet of sea water
The breeders conditioned for more than a weak in the hatchery are utilized for this purpose . First of all water in the brood stock tank is removed and specimen are dried in a shade for about half an hour. After this a powerful jet of water is sprayed on the specimens for a few minutes. Then the animals are put back into the tanks with sea water.
After 1-2 hours, the animals move up the tank wall and exhibit swaying movement indicating the release of gametes. First the males release the sperm and after one hour the females release the eggs.
Larval rearing unit
After the spawn and eggs are released, the breeders are removed from the tank carefully. The fertilized eggs are removed to the rearing tanks. The auricularia lavae hatch out after 48 hours. The healthy larva occupies the surface layer of water whereas dead or deformed one settles at the lower layer of water column or at the bottom of the tank. Those settled at the bottom are siphoned out. The healthy larvae are collected in a sieve and counted using a plankton counting chamber. Then the larva are released into the rearing tank containing clean, clean filtered sea water at density of 300 to 700 numbers per liter. The larvae are taken out once in 3 days to clean the tank to avoid infestation of other organisms.The larva is fed on micro algae Isochrysis galbana, two times a day.. This may increased or decreased depending on the stage of larvae. After 4-5 days, the larvae may be fed with mixed culture of phytoplankton mainly having Chaetoceros.
Under the above conditions of rearing, the auricularia develops to Doliolaria larva between 2-3days. The Doliolaria larva transforms into the pentacula larva within 10days. The pentacula is the creeping stage. Late pentacula larvae settle on hard surface provided suitable substratum is provided in the tank. Hence, Artificial settling bases (settlers) are provided for them to settle.
Two types of settling bases are tried.
(1) polythene sheets are taken and kept in a tank kept outdoors having enough sunlight. For 4-5 days , filtered sea water is circulated continuously. Benthic Diatoms and other algae settle on these sheets. These are kept suspended in the water having late Doliolaria which are about to settle. The larvae settle on them as it gets food and hard surfaces.
(2) In another type of settlers used, the polytheen sheets are kept in a tank having sea water. In this, filtered algal extract (50 mu filter) is added. Usually species of algae sargassum are used to make the extract. Algal extracts shall stick to the sheets. Fresh extract is put daily and the water is also changed daily. After 4-5 days. when the sheet is covered with algal extract it is given as settling base for the larvae. Juveniles that settle to the hard substrate has very weak motility. Hence, algal extract is again given daily twice morning and evening. Which is filtered through 40 micros sieve, After one month, 80 micron sieve can be used, and large sized juveniles of 15-20 mm size are separated and put in tank with very fine sand. There are also fed on algal extract. Optimum density of larvae should be adjusted to 200-500individuals per square meter.
Nursing of juvenile sea cucumbers:
As the juveniles grow, the water quality and dissolved oxygen must be maintained at the optimal level.
Increasing aeration and water exchange rates becomes necessary. The oxygen level has to be maintained above 5 mg/L. It is also important to use formulated feed that can be digested and absorbed easily. Experimental results have shown that the growth rate of juveniles fed on the formulated feed is at least two times higher than that of individuals fed on traditional feed during the 20 to 30 day period. As the accumulation of excess food and faeces increase, harmful germs tend to multiply rapidly and can cause very serious disease outbreaks among the juvenile sea cucumbers, including what is known as the ‘stomach ulcer’. Another disease is ‘white muscle syndrome’ which causes muscle tissues to turn white and rigid.
Transfer of young sea cucumbers to the pond:
It has been demonstrated that the release of young sea cucumbers measuring 2-3 cm in body length produce the best farming results. These will attain commercial size after 1.5 years. For an optimal growth the culture density should not exceed 10 individuals/m2.
Environmental Factors Affecting Larval Rearing
The ideal temperature is reported to be 27C-29C, Aeration is carried out in larval rearing tanks to maintain oxygen at saturation level (above 5mg/L). pH of the water may be between 6 and 9. Normal sea water has a pH of 7.5 to 8.6 which is suitable. Normal sea water salinity (32 to 35ppt) is favourable. If salinity falls lower than 12.9 ppt. the larvae shall die. Most favourable salinity has been shown to be between 26 to 32.7 ppt. Ammoniacal nitrogen should not exceed 500 mg per cubic meters. It tolerates a range of 70-430 mg/m3.
Larve stages
Larve stages
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1000,000 juveniles / kg
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6000 juveniles / kg
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600 juveniles / kg
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200 juveniles / kg
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Management:
Sea cucumbers can be farmed with shrimp and certain species of finfish, although they are commonly reared alone. Prior to stocking the ponds with the hatchery-reared sea cucumber juveniles, It is necessary to clean and sterilize the ponds as well as inoculate the seawater with benthic Diatoms. These measures will provide an appropriate culture environment and ensure high survival rates. The addition of formulated feed will also enhance growth particularly during spring and autumn. Some field tests have shown that the growth rate of sea cucumbers fed on formulated feed is as high as two times that of non-fed individuals.
Nutritional value of seed cucumber
It is a healthy food. It consists of 21.5% proteins, Mucopolysaachraide, minerals and other biological active substance. Recent medical research proved that the muscle aging is related to reduction of acid Mucopolysaccharide. It has cancer resistant effect, it involved in the enhancement of immune system, and anti blood clotting. It can be used to cure or additional therapeutic method for some desease: such as tuberculosis (TB), stress, erection problems, stomach, duodenum ulceration, diabetes, aplastic anemia.
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