Retrogressive Metamorphosis in Urochordates
Dr. Tapashi Gupta, Associate professor (Retd), Department of Zoology (HOD), Lumding College, Assam
Metamorphosis is a change from the juvenile to adult stage in which the development passes through different larval stage, which is quite different from the adult stage. Metamorphosis of the ascidian larva is unique and begins almost explosively. It involves transformation of an active non-feeding, pelagic, lecithotrophic (i.e., that feeds on its own yolk reserves) and tailed larva having many advanced features such as axial notochord, dorsal neural tube and special sense organs, into an inert, sedentary or sessile, simple (primitive) and plankotrophic filter feeding adult with only a phaynx with stigmata and endostyle, both indicating the chordate features of adult ascidian. 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. Retrogressive Metamorphosis is a characteristics of Ascidian or Tunicates of Urochordate. Ascidians are sac-like marine invertebrate filter feeders which composed of a tough outer "tunic" made of the polysaccharide cellulose, so they are also called Tunicates. Urochordate adults, being sedentary show degenerative characters while the free swimming tadpole larva shows advanced chordate characters which are lost during metamorphosis.
Metamorphosis in ascidian tadpole larva involves three types of changes :- 1) Retrogressive and 2)Progressive changes and 3)Molecular changes
- Retrogressive Changes: It involves degeneration or destruction of larval tissues and some structures such as follows:
a) The long tail with caudal fin starts reducing and finally disappear.
b) Caudal muscles, nerve cord and notochord disappear as they break down and are consumed by phagocytes.
c) Larval sense organs (the ocellus and the otolith) are lost and sensory vesicle is transformed into an adult cerebral ganglion.
d) Adhesive papillae and ectodermal ampullae disappear completely.
e) Anterior region between point of attachment (adhesive papillae) and mouth shows rapid growth, while original dorsal side with atriopore stops growth. This causes shifting of mouth through 90°. Therefore, the final branchial and atrial apertures in the adult represent the original anterior and dorsal sides of the larva.
2) Progressive Changes:
Some larval structures necessary for survival become more elaborated and specialised in each adult, such as:
a) Due to loss of tail, the trunk becomes pear-shaped and four larger ectodermal ampullae grow out of its four corners. These ampullae firmly anchor the metamorphosing tadpole to the substratum and also serve for respiration as a blood-like fluid keeps circulating through them. Soon two more smaller ectodermal ampullae appear dorso-laterally.
b) Anterior region between point of attachment (adhesive papillae) and mouth exhibits rapid growth, while original dorsal side with atriopore stops growth. This causes shifting of mouth through 90°. The body too rotates so that general form of the adult sessile organism is assumed.
c) Adult neural glands and nerve or cerebral ganglion are formed by the neural tube and trunk ganglion come to lie mid-dorsally between mouth and atriopore. The trunk ganglion itself persists as visceral nerve.
d) With the absorption of its test covering, the mouth becomes functional and filter-mode of feeding by incoming ciliary water currents.
e) Pharynx greatly enlarges, develops blood vessels and stigmata multiply rapidly, forming the branchial sac.
f) Stomach enlarges, intestine elongates and gets curved and liver develops.
g) Atrial cavity becomes more extensive.
h) Circulatory system with heart and pericardium develops.
i) Gonads and gonoducts develop from larval mesodermal cells.
j) Test or tunic spreads to cover entire animal, becomes thick, tough and vascular and attaches the animal by forming a foot if necessary.
Thus, foregoing metamorphic changes mark the beginning of a sedentary, actively feeding, sexual adult life which soon starts producing gametes, first ova and later sperms.
3. iii) Molecular Changes:
Manket and Cowden (1965) studied the molecular changes which take place during metamorphosis. They studied the metabolism of protein and nucleic acid and pointed out that some protein synthesis occurs throughout the development but with the outset of metamorphosis, extensive degradation of proteins begins followed by rapid synthesis of new proteins.
Conclusion of Retrogressive Metamorphosis :
As a result of retrogressive metamorphosis the free swimming photo-positive and geo-negative ascidian tadpole larva changes in to fixed, inactive geo-positive and photo-negative adult. The chordate characters of larva like notochord, nerve cord and sense organs are completely lost in adult.
Reference:
Modern Text Book of Zoology Vertebrates - R. L. Kotpal.
Chordate Zoology - Zordan and Verma.
Chordate Zoology - T. C. Majupuria
