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Retrogressive metamorphosis of Ascidia sp. - definition - classification - development stages

Submitted by Mousumi Sepai, Last Modified on 2019-11-18

Ascidians definition: Ascidians are sac-like marine invertebrate filter feeders which composed of a tough outer "tunic" made of the polysaccharide cellulose.

Scientific classification and overview of Ascidiacea

Kingdom => Animalia

Phylum => Chordata

Subphylum-Eurochordata => Tunicata (Body covered by an external covering called tunic . It is so called because it has a protein)

Class:Ascidiacea (Blainville, 1824)

Vertibrate => As notochord present at any stage of life and vertibrate coloumn replace the notochord.

Ascidians example: Polycarpa aurata, Ciona intestinalis

Diagram/Image/Picture of Adult Solitary Ascidian (Sea squirts, Tunicates)

Diagram of Adult Solitary Ascidian

Metamorphosis:- Metamorphosis is a process by which the larva undergo a several changes to be transform into a adult is called metamorphosis. Retrogressive metamorphosis is characteristic of ascidians (Urochordata or Tuncicata). It is given this name because here a progressive, active and alert larva metamorphoses into a retrograde and sedentary adult. Ascidians are marine, sedentary, bag-like creatures that remain attached to a rocky substratum all their lives and use a current of water passing through their large and perforated pharynx to collect microorganisms on which they fads. The larva, on the other hand, is active, tadpole-like in shape and has all the three chordate features, gill clefts, notochord and dorsal tubular nerve cord. Most of these characters are lost or become degenerate as the active larva metamorphosises. Hence, the term retrogressive metamorphosis.

Development of solitary ascidian tadpole larva :-

  1. Tunicates are hermaphrodite.
  2. The ovaries and testis are sack like and lying close to the intestine and open to the exterior by duct.
  3. Fertilization is external.
  4. Eggs are small and pelagic.
  5. Cleavage is total.
  6. Gastrolation occurs by invagination.

Free swimming solitary tadpole larva's characters (What is Ascidian larva?) :-

  1. The tadpole larva is about 1 m.m long.
  2. It has an ovoid head and along laterally compressed tapering tail.
  3. The tail is supported by a notochord formed by cells derived from the archenteric wall.
  4. Forty of these cell make up the entire rod, become vacuolated and elongated by swelling.
  5. On either side of the notochord run three rows of muscle cells derived from mesoderm.
  6. Other cells of these tissue migrate ventrally to make the pericardium, heart and mesenchyme.
  7. The muscle cells contain cross- straight moiofrib.
  8. Nervous system contains of a hollow dorsal nerve chord extending to the tail and enlarge in front into the cerebral vesicle.
  9. Cerebral vesicle contains lens cells and visual cells, and a single statocyst cell containing an otolith.
  10. The heart is present in the pericardial region.
  11. Gut is not well developed.
  12. Pharynx is usually with a single pair of gill slits opening into an atrium.
  13. Enterior end of the larva carries 3 adhesive organs, pappilae, which are formed as the process of ectoderm.
  14. The larva is positively phototropic and negatively geotropic.
  15. The larva takes no foods and is highly mostile.
  16. The whole process of development occupied only one or two days and the free swimming larva is set free.
  17. Being positively phototropic it proceeds to the sea surface.
  18. Within a day or two the tropisim becomes reserved.

Diagram of Solitary Ascidian tadpole Larva

Diagram of Solitary Ascidian Larva

Fixation and metamorphosis of tadpole larva:-

The free swimming tadpole larva after a short period of free existence become positively geotrophic. The larva turns to any dark place and finds out a suitable substratum with the help of sense organs. It fixes itself a rocky places or other kind of substratum by the adhesive pappillae are beings to metamorphosis.

Diagram of Fixation and metamorphosis of tadpole larva or evolution of Ascidian Development

Diagram of Fixation and metamorphosis of tadpole larva

The main metamorphosis changes or the events of metamorphosis of Ascidian Larvae are (Formation of adult organs through metamorphosis in ascidians):-

  1. The diminution and eventually the complete disappearance of the tail with its contained notochord.
  2. Caudal part of the nerve chord also decreases or disappearance.
  3. Disappearance of the visual organ and statocyst.
  4. The central part of the nervous system gives rise to the adult ganglion an to the neural gland.
  5. Muscles in the caudal region degenerate.
  6. The trunk becomes broader.
  7. Mouth is shifted 90^0 from the point of the attachment.
  8. Increase in number of gill slits.
  9. The part pharyngeal portion of the gut gets regionated into different parts.
  10. The atrium becomes more extensive.
  11. Gonads and gonoducts develop from mesoderm.
  12. Change from an active free swimming habit to a fixed or sedentary life.


Thus from an active free swimming larva with complex organ of special sense and with a well developed notochord and nervous system. There is a retrogression to a fixed, inter-adult in which all the chordates features have become aborted accepting the gill slits.

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