Give an illustrated account of the reproductive system, life history and development of Asterias.

Reproductive System of Asterias

Asterias exhibits a largely diffuse gonadal system. Unlike many animals with concentrated gonads, the reproductive organs are distributed throughout the pyloric region of the central disc, extending into the arms. The reproductive system is unisexual, meaning individuals are either male or female, though some species can exhibit hermaphroditism.

• Gonads: Paired gonads are present in each arm, extending from the pyloric stomach into the ambulacral regions. They appear as orange or yellowish colored branched structures.
• Duct System: Each gonad connects to a short duct that opens to the exterior through gonadal pores located on the aboral surface.
• Gametes: Asterias produces small, numerous gametes (eggs and sperm).

Life History and Development

Gametogenesis

Gametogenesis, the process of gamete formation, occurs within the gonads.

• Spermatogenesis: Spermatogonia undergo meiosis to produce spermatozoa. Sperm cells are small and motile, possessing a flagellum for locomotion.
• Oogenesis: Oogonia undergo meiosis to produce ova (eggs). Eggs are relatively large and contain a yolk reserve to nourish the developing embryo.

Fertilization and Early Development

Fertilization in Asterias is external. Both eggs and sperm are released into the water column, and fertilization occurs when they meet.

• Fertilization: The sperm penetrates the egg, leading to the fusion of nuclei and the formation of a zygote.
• Cleavage: The zygote undergoes radial and holoblastic cleavage, resulting in a blastula.

Larval Stages

The development of Asterias involves a complex larval phase with three distinct stages:

Bipinnaria Larva

The blastula develops into a bilaterally symmetrical, free-swimming larva called the bipinnaria.

• Structure: The bipinnaria larva is characterized by bilateral symmetry, ciliated bands for locomotion, and a gut. It lacks an anus.
• Feeding: It feeds on phytoplankton and other microscopic organisms.

Brachiolaria Larva

The bipinnaria larva transforms into the brachiolaria larva. This stage is distinguished by the development of adhesive arms (brachiolaria arms) used for attachment to a suitable substrate.

• Structure: The brachiolaria larva retains the ciliated bands of the bipinnaria but develops three adhesive arms bearing suckers at their tips.
• Attachment: The brachiolaria larva uses its adhesive arms to attach to rocks, seaweed, or other surfaces.

Pluteus Larva

After attachment, the brachiolaria larva undergoes metamorphosis into the pluteus larva. This is a distinctly different larval form.

• Structure: The pluteus larva is characterized by a post-oral arm, a pre-oral arm, and a hydrocoel (water vascular system). It is also bilaterally symmetrical.
• Development: The pluteus larva continues to develop internally, undergoing significant reorganization of tissues and organs.

Metamorphosis to Adult

The pluteus larva eventually undergoes metamorphosis into the juvenile Asterias. This is a dramatic transformation involving:

• Rudiment Formation: A bud-like structure, the rudiment, develops from the left side of the pluteus larva.
• Organogenesis: The rudiment develops into the five arms and the central disc of the juvenile starfish.
• Resorption: The larval structures (post-oral arm, pre-oral arm, hydrocoel) are gradually resorbed as the adult form develops.
• Settlement: The juvenile starfish settles on the substrate and begins to grow into the adult form.

Illustrative Representation (Verbal Description)

Imagine the bipinnaria as a flattened, oval-shaped larva with ciliated bands running along its body. The brachiolaria adds three arms extending from its body, like tiny grappling hooks. The pluteus is more elongated, with a distinct post-oral arm extending backwards and a pre-oral arm near the mouth. The metamorphosis involves the gradual development of the five arms from a bud on the pluteus, eventually replacing the larval form.