FERTILIZATION AND DEVELOPMENT IN STAR FISH

Published in Zoology
Monday, 05 June 2017 17:18
Star fish is unisexual. Male and female animals are separate. Sexual dimorphism is not seen. Only during the breeding season they can be recognized.
 
Gonads: Rve pairs of gonads are present in star fish. One pair is present at the base of each arm. The gonads of male are called testis. The gonads of female are called ovaries. They are present between the pyloric caecae and the ampullae. The ducts and other structures are absent usually.
 
Each gonad is a branched structure. It looks like bunch of grapes. It is surrounded by blood sinus. The side of gonads differ in different seasons. In breeding season they increase in size, and may reach upto the tip of the arm.
 
Each gonad gives a small ciliated duct which is termed as gono duct This duct opens through a small gonopore on the aboral surface at the angle of two adjacent arms, Sperms and ova are produced in gonads.

Fertilisation: Fertilisation is external. It takes place in water-sperm fertilises an ovum and a zygote is formed.
 
Development: The fertilised egg is 0.5 mm in diameter. The egg is homolecethal. Cleavage is holoblastic. A ciliated coeloblastula is formed. This swims in water. It undergoes invagination and gives two layered gastrula. The outer layer is ectoderm and inner layer is endoderm. A narrow archenteron is formed in the endoderm. The archentron opens to the outside by blastopore. The blastopore gives a larval anus. Gastrula develops into a larva called Bipinnaria.
 
BIPINNARIA LARVA:
 
The gastrula elongates in length and it gives rise to Bipinnaria larva.
 
  1. It is a bilaterally symmetrical, free swimming pelagic larva.
  2. The pre oral region is elongated. Post oral region is broad. The anterior end forms pre oral lobe.
 
The ciliated band at the pre oral fobe forms into 2 separate bands.
 
  1. Pre oral band of cilia,
  2. Post oral band of cilia.
 
These 2 bands of cilia are drawn into many arms. They are nothing to do with the arms of the starfish. They are
 
  1. Ventro median arm
  2. A pair of pre oral arm
  3. Median dorsal arm
  4. A pair of antero-dorsal arm
  5. A pair of posterio-dorsal arm
  6. A pair of posierio-lateral arm
  7. A pair of post-oral arm
  8. The digestive system is developed with mouth and arms.
 
This larva slowly grows into the next larval form called Brachiolaria larva.
 
BRACHIOLARIA LARVA:
 
Bipinnaria larva swims for few weeks in the sea water. It finally transforms into next larval stage called Brachiolaria larva.
 
  1. It is bilaterally symmetrical.
  2. It is pelagic larval form, ft shows 3 brachiolar arms with suckers. They are one median and two lateral in position.
  3. At the tip of brachiolar arms adhesive disc will make their appearance and they are for attachment.
  4. The larva shows all the arms that are seen in the Bipinnaria, but these arms are very long and hanging. These ciliated arms will be helpful for swimming in the water.
  5. The digestive system is completely developed with definite stomach and intestine.
 
This larva after swimming for few weeks settle on a solid object and gets attached to it by its adhesive arms.Posterior end of the larva enlarges and pushed to the right side. From this rudiments of 5 arms will arise. Thus slowly the larva metamorphosis into adult.
 
Metamorphosis:
 
  1. The brachiolaria larva is bilaterally symmetrical.
  2. After swimming for 7 or 8 weeks it settles down on some substratum and attached by its sucker and adhesive arms to it.
  3. The posterior end of the larva enlarges and bends to the right side.
  4. Five projections are formed. They develop into five arms of the adult.
  5. The larva undergoes clock wise rotation of 90° because of which right side of the posterior part becomes the aboral surface and the left side becomes the oral surface.
  6. The ciliated bands and the larval arms disappear.
  7. Water vascular system and haemel system will develope. It slowly transforms into adult, it can creep on the substratum with the help of tube feet.
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