Control of larval metamorphosis and recruitment in sympatric agariciid corals

Abstract

The larvae of three sympatric shallow-water agariciid corals, Agaricia tenuifolia Dana, A. agaricites humilis Verrill, and A. agaricites dana Milne Edwards et Haime, are shown to be induced to metamorphose by crustose coralline red algae (CCA). These corals display different degrees of stringency and specificity in their requirements for CCA to induce metamorphosis, and different responses to light in the control of the distribution of newly metamorphosed individuals. The morphogenetic inducer from CCA has been fractionated by ultrafiltration, and shown to be a water-insoluble, ether-insoluble, and acetone-insoluble unstable biochemical. This inducer of agariciid metamorphosis is different from the water-soluble peptide inducer of gastropod metamorphosis previously isolated from CCA. Transduction of the morphogenetic signal in the agariciid larvae also is apparently controlled by an internal pathway that is different from the signal-transduction pathway found to control metamorphosis in several other species. Analysis of the distribution of recruits of two of the agariciid species indicates that larval requirements and specificities for metamorphosis may contribute significantly to determine the distribution of recruits in the natural environment. Our evidence suggests that differences in larval requirements for metamorphosis thus may contribute to the maintenance of niche diversification among the sympatric shallow-water agariciids. The competence and CCA requirement of the larvae of these species persist for at least 1–2 wk in the plankton, thereby promoting dispersal and site-specific metamorphosis. For these sympatric shallow-water agariciid corals, then, larval metamorphosis and recruitment are not wholly stochastic lottery-like processes, but instead appear to be determined, in part, by larval recognition of, and responses to, environmental and biochemical factors that can be experimentally resolved and identified.