© 1994 by The Society for Integrative and Comparative Biology
Ecological Consequences of Altering the Timing Mechanism for Metamorphosis in Anural Ascidians1
Department of Biology, Carleton University Ottawa, Ontario, Canada K1S 5B6
Correspondence: 2 Dr. W. R. Bates, Bamfield Marine Station, Bamfield, B.C. VOR 180 Canada
SYNOPSIS. In the present study the timing of metamorphosis in an anural ascidian, Molgula pacifica, was compared to metamorphosis in a urodele species Boltenia villosa. Metamorphosis in M. pacifica was triggered at a fixed time in development (32–36 hours after fertilization), just prior to hatching. In contrast, metamorphosis was triggered in B. villosa after the hatched larvae responded to substrate cues. The timing of metamorphosisin B. villosa was often delayed for up to four days, whereas delays in M. pacifica were not observed. An antibody, termed Epi-3, was found to cross-react exclusively with epidermal cells in both species. The binding of FITC-labelled Epi-3 was very low prior to metamorphosis and then it increased dramatically after metamorphosis was triggered. The cytoplasm of ampulla tip cells and the tunic immediately surrounding each ampulla showed the highest levels of Epi-3 fluorescence. The histological and ultrastructural features of the ampulla cells suggest that Epi-3 antibody recognizes granules localized in the apical cytoplasm.
How the evolution of an internal "clock" mechanism responsible for initiating metamorphosis may be beneficial to anural species is discussed. One possibility is that the anural type of timing mechanism reduces mortality rates during this critical phase of its life cycle.