Life cycle and development of Asterias.

Life cycle and development of Asterias.

The life cycle in Asterias occurs Fertilisation, Embryogeny and Larval development A short description about this is discussed below.

Fertilization:

Fertilisation takes place in the spring season. Among all species of Asterias have the breeding season in a year. At the breeding season, both types of mature sexes shed their sex cell in the sea and union of male and the female sex cell i,e sperm and the ova occurs in the sea water during fertilisation. Sea star is highly prolific. Females lay about 1.25 million eggs per hour, providing in all, about 200 million eggs in a season. The male produces many times that number of sperm . The fertilisation occurs in Asterias is external.

Embryogeny:

The embryological development take place in sea-star is indirect and thus include various larval stages. The fertilised eggs or zygote of sea-sta is spherical and occupied with a little amount of yolk. They are half a millimetre in diameter.The cleavage in Asterias is rapid, holoblastic practically equal, radial and intermediate and it converts the unicellular zygote into a single layered, hollow, spherical called coeloblastula. It is typically a spherical yellow one layered and ciliated embryo which can swim freely.The blastula then undergoes embolic invagination and become two layered, cup-like structure known as gastrula. Its outer and inner layer are called endoderm and ectoderm. New cavity gastrula formed by the invagination and lined by ectoderm is called archenteron or primitive gut. It opens through the exterior by a wide opening called blastopore.

The narrow proximal part of the sea star communities to the exterior part by the blastopore and in later stages form the stomach, and intestine while the wider part of Asterias completed archenteron expands and cuts off on each side into a hydro enterocoel.

During the gastrulation stages the advancing end of archenteron buds off cells, into a blastocoel, which forms the mesenchymal or mesoderm.Their position to the right and he left sides of the archenteron and develop into coelomic pouches. These later give to coelom, its mesodermal lining and the water vascular system. The embryo at this stage become a free-swimming larva.

Larval development in Asterias.

The larval development takes place in this stages are Bipinnaria larva, Brachiolaria larva .

Bipinnaria larva:

In one week the zygote develops into bipinnaria larva. They are bilaterally symmetrical larva which forms three lateral lobes and possess a preoral and a postoral ciliated band, and a preoral lobe with the preoral loop of the ciliated band. The various projections emerging out of its body corresponding to the arm.The alimentary canal and the coelomic apparatus appear inside the body. It feeds mainly on diatoms, etc. by creating food-bearing currents by ciliary tracts in the stomodael wall. Its swims and feed freely by forwarding its anterior end, with a clockwise rotation, and after some weeks transform into the next larval stages, the brachiolaria larva.

Brachiolaria larva:

In the brachiolaria larva the lobes develop into, slender, ciliated, contractile structure, called larval arms. The preoral arm moves and contract. From the preoral lobe arises three, short and non-ciliated appendages, each bearing in a sucker or adhesive disc. These appendages are called brachiolaria arms or fixing processes. The arms of brachiolaria larva also possess tips of adhesive cells and coelomic prolongation. Larva is now are called brachiolaria. It also bilaterally and sims and feeds actively. The base of the arm of brachiolaria larva surrounded the elevated, adhesive, glandular area performing the function of a sucker and also fixation disc by which the larva become attached at the time of metamorphosis.

Metamorphosis:

The brachiolaria larva settles on the bottom or on some solid object in 6 or 7 days and is fixed with that by its adhesive arms. Now the stalk later degenerate and becomes completely absorbed. Now the bilaterally symmetrical and rounded posterior end of larval metamorphosis or develop into a radially symmetrical adult. The larval mouth, anus and ciliated bands close or disappear.New mouth and anus are formed on the left side and a new anus is developed on the right side.The general body is thus subsequently differentiated into oral and aboral surfaces of the adult in a left and right side. In around the oral-aboral axis, five lobes called arm rudiments grow out.

In later stages, the skeletal elements appear on the arm rudiments and the radial canals grow into them.In each arm two pairs of outgrowths from the coelom from the tube feet and serve for attachment. Further, that recognition changes result in the formation of adult sea star.

Regeneration and Autotomy:

Seastar possesses the considerable great power of regeneration and they are capable of regenerating its any host part of the body at any time. Moreover, if an arm is held, injured or unduly stimulated, the Asterias usually casts it off near the base at the fourth or fifth ambulacral ossicle. This process is termed autonomy. The opening in the left side of the central disc by the broken off arm is immediately closed by the concentration of the adjacent body wall musculature for the protection of internal body organs and regeneration of new arm starts at that place.Lost parts regenerate slowly, resulting in strange forms. A disc deprived of all its arms regenerate. In Asterina Vulgaris, a single arm with a portion of disc regenerate an entire animal. But in Linckia, an arm totally devoid of a disc can also regenerate complete animal. Specimens with small regenerating arms at the base of the large original arm are popularly called comets.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

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