Production and supply of larvae as determinants of zonation in a brooding tropical coral

Production and settlement of planktonic larvae of the coral Favia fragum (Esper) were studied. The species is restricted to shallow back- and fore-reef habitats throughout the Caribbean Sea. Adults are in their greatest abundance on the reef-crest and shallow reef slope (< 3 m) at Tague Bay, St. Croix, in the US Virgin Islands. Because F. fragum broods larvae that are capable of immediate settlement, this distribution pattern may be due to variation in fecundity among depths. Corals were collected from shallow (1.0 m) and deep depths (10-13 m) and cultured in individual containers exposed to shaded ambient light. Corals from shallow depths had greater fecundity (polyp(-1) lunar cycle(-1)) and were larger than deep corals. To test the hypothesis that fecundity was related to successful fertilization, corals were kept in different densities in an area with sea-grass, where there were no natural adults. Production of larvae 6 months later was not affected by density of adults, possibly due to self-fertilization. Larval choice of habitat was also examined. In the laboratory, twice as many larvae settled on coral rubble fragments collected from depths where adults were common (1.5 and 3 m) than on those from depths where adults were rare (10 m). Larval supply may establish the vertical distribution of adults on St. Croix. (C) 2002 Elsevier Science B.V. All rights reserved.