Ecological Consequences of Altering the Timing Mechanism for Metamorphosis in Anural Ascidians

SYNOPSIS. In the present study the timing of metamorphosis inan anural ascidian, Molgula pacifica, was compared to metamorphosisin a urodele species Boltenia villosa. Metamorphosis in M. pacificawas triggered at a fixed time in development (32–36 hoursafter fertilization), just prior to hatching. In contrast, metamorphosiswas triggered in B. villosa after the hatched larvae respondedto substrate cues. The timing of metamorphosisin B. villosawas often delayed for up to four days, whereas delays in M.pacifica were not observed. An antibody, termed Epi-3, was foundto cross-react exclusively with epidermal cells in both species.The binding of FITC-labelled Epi-3 was very low prior to metamorphosisand then it increased dramatically after metamorphosis was triggered.The cytoplasm of ampulla tip cells and the tunic immediatelysurrounding each ampulla showed the highest levels of Epi-3fluorescence. The histological and ultrastructural featuresof the ampulla cells suggest that Epi-3 antibody recognizesgranules localized in the apical cytoplasm. How the evolution of an internal "clock" mechanism responsiblefor initiating metamorphosis may be beneficial to anural speciesis discussed. One possibility is that the anural type of timingmechanism reduces mortality rates during this critical phaseof its life cycle.