Retrogressive Metamorphosis in Urochordates
Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite different from the adult stage. In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters. Urochordate adults, being sedentary show degenerative characters while the free swimming tadpole larva shows advanced chordate characters which are lost during metamorphosis. Parasitic crustaceans, like Sacculina and copepod parasites and stylopids and scale insects (Insecta) also show retrogressive metamorphosis.
Retrogressive Metamorphosis in Herdmania
The tadpole larva of Herdmania is only 1-2 mm long when it hatches out of the egg. It does not feed and hence has only 3 hours of survival during which it has to swim about in search of a suitable substratum for attachment. The larva needs advanced features for its free swimming existence, which is so necessary for dispersal of the population to distant places which the sedentary adult has no means to do.
The advanced Chordate characters of the larva
- There is a rod-like notochord in the tail to which are attached muscle bands for swimming.
- There is a dorsal hollow nerve cord which is enlarged to form brain at the anterior end. A photoreceptor ocellus and a balancing organ, the statocyst are attached to the brain.
- There are only two pairs of gill slits in pharynx but the mouth is closed by a membrane and intestine is rudimentary.
- Endostyle on the ventral side of pharynx is very well developed which functions like thyroid gland and helps in metamorphosis.
- Heart is on the ventral side of gut but is non-functional.
- The larva possesses on the anterior end three ectodermal adhesive papillae which help in firm attachment on the substratum.
Changes during metamorphosis
- Larva attaches to the substratum with the help of chin warts, head downward and tail up.
- Rapid growth takes place between the chin warts (adhesive papillae) and mouth and almost no growth on the opposite side of body.
- Due to rapid growth on one side, body starts rotating in such a way that mouth gradually migrates to the upper side.
- Meanwhile pharynx enlarges and stigmata increase in numbers.
- Intestine becomes functional and atrial opening is formed on the opposite side of oral aperture.
- Both tail and notochord are gradually absorbed in the body during metamorphosis.
- The hollow nerve cord is reduced into a solid nerve ganglion on the dorsal side.
- Sense organs, namely ocellus and statocyst are lost.
When the metamorphosis is over, Herdmania is transformed into a bag-like sedentary animal attached to the rock by a foot and having branchial and atrial openings for inlet and outlet of water respectively. Pharynx becomes enormously enlarged with a large number of stigmata for filter feeding and digestive system becomes well developed. However, other advanced chordate characters of the larva are degenerated into simple structures, due to which it is called retrogressive metamorphosis.