Shellfish Breeding and Hatchery Management (2+1)

22.3.1 Spawning and early development in holothurian lifecycle

Spawning

Spawning occurs mostly from June to August by releasing of ova and sperm in the water where fertilization takes place. During spawning, sea cucumbers adopt a "cobralike" posture, with the front end lifted off the bottom.

Fertilization

The fertilization is external and takes place in the water column. The oocytes are fertilized quickly as they make contact with the spermatozoa. After the oocytes and sperm are released in the water, the sea cucumbers are removed from the tank. The eggs are washed in fresh seawater several times to remove the excess spermatozoa. Excess spermatozoa can reduce the rate of fertilization and cause the development of deformed embryos.

Early development

Large females can release about one million oocytes. About 0.7 million eggs were stocked in 750 litres of water. The eggs were spherical, white and visible to the naked eye and were found floating in the water. The diameter of the oocytes ranged from 180 to 200 µm. After fertilization the eggs underwent cleavage and developed into the dipleurula stage with eggs ranging in length from 190 to 250 µm. The dipleurula transformed into early auricularia larva after 24 hours. They are 430 µm long and 280 µm wide at this stage.
The early auricularia larvae have a buccal cavity, ciliary bands, a cloaca and an anus and they actively feed. They were fed on the microalgae, Isochrysis galbana, and a mixed culture dominated by species of Chaetoceros spp. and Skeletonema spp. As days passed the auricularia became more and more transparent and the lateral projections also became prominent. On each side of the late auricularia larva, four lateral projections were seen and at the end of each projection there was a hyaline sphere. The oesophagus and the pear-shaped stomach were well demarcated. Right and left stomatocoel were clearly seen. The ciliary bands showed a number of pigment spots. The length of the late auricularia larvae varied from 660 to 1050 µm (with an average of 860 µm) and the width was 240-690 µm (with an average of 500 µm). Some of the auricularia larvae remained small.
A few of the late auriculariae transformed into doliolariae on the tenth day. The doliolariae were barrel-shaped with five hyaline spheres on each side. Later, the first two tentacles developed at the anterior end. Their length varied from 420 to 570 µm (with an average of 485 µm) and 240-390 µm in breadth (average of 295 µm).
On the thirteenth day, some of the doliolariae transformed into pentactula larvae . The body of the pentactula was tubular with five tentacles at the anterior end and with one short stumpy tube foot at the posterior end which helps in locomotion. The anal opening was distinct. The length varied from 330 to 750 µm (average of 307 µm). By the eighteenth day the tube feet and tentacles became distinct. Two long tube feet developed at the posterior end.

At this stage the length of the juveniles was 550-720 µm (average of 656 µm) and the width varied from 210 to 320 µm (average of 262 µm). The pentactulae had the habit of moving to the edge of the tank, remaining just below the surface of the water. They soon settle down on the bottom of the tank.