Taurine in the marine hydrozoan Hydractinia echinata: stabilizer of the larval state?

Taurine (beta-aminoethane sulfonic acid) is present in high concentrations in tissue of planula larvae of the marine hydrozoan Hydractinia echinata. It has been proposed to function as a stabilizer of the larval state mainly because of the previous findings that larvae induced to undergo metamorphosis appeared to lose most of their taurine, and taurine added to the medium antagonizes metamorphosis. Release of taurine was assumed to be a necessary prerequisite for the onset of metamorphosis. The primary aim of the present study was to confirm this by determination of taurine release accompanying metamorphosis induction by inducers other than CsCl. However, a decrease of the larval tissue taurine content was not found, irrespective of schedule of treatment and the inducer applied. The cause for this difference from the preceding study could not be clarified. Taurine in the medium, even at low concentration, causes elevated tissue concentrations high enough to cause general adverse effects on cell physiology. In order to ascribe an alternative function to taurine in H. echinata variations of the free amino acid pool under osmotic stress were examined. The tissue concentration of beta-alanine strongly correlates with the salinity of the medium. Large amounts of gamma-aminobutyric acid (GABA) are present in animals adapted to high salinity. Taurine content appears not to depend on osmolarity of the medium. Nevertheless, taurine may constitute the foundation of the cellular organic osmolyte system of the H. echinata larva.