Taurine found to stabilize the larval state is released upon induction of metamorphosis in the hydrozon Hydractinia.

Abstract

Larvae of Hydractinia echinata, a colonial marine hydroid, can be triggered experimentally to undergo metamorphosis into polyps. The efficiency of induction is density-dependent: a high larval density allows fewer larvae to metamorphose than a low density. Culture medium of metamorphosing larvae was found to contain taurine as the major constituent of its inhibitory activity. The concentration of taurine in larvae is 90 mM which is much higher than that of other free amino acids. Taurine interferes effectively with the onset of metamorphosis if applied externally at a concentration equivalent to 1/1000 of the animal's overall internal concentration. Upon induction of metamorphosis the larva releases three quarters of its taurine into the medium. Taurine may have a function in control of onset of metamorphosis because, applied exogenously in micromolar quantities, it stabilizes the larval state, i.e. the larvae resist metamorphosis-inducing stimuli. The chemically related compounds 4-aminobutyric acid (GABA) and β-alanine are much less effective. There exist other larval state stabilizing compounds in Hydractinia including homarine, trigonelline, betaine and methionine. These compounds are though to act by delivering methyl groups leading to the production of S-adenosylmethionine. Taurine is not able to supply methyl groups. Furthermore, in contrast to the four other compounds taurine does not interefere with the advance of metamorphosis when applied after induction, and of these five substances, only taurine is released upon induction of metamorphosis.