EFFECTS OF A LARGE HERBIVOROUS GASTROPOD ON MACROFAUNA COMMUNITIES INTROPICAL SEAGRASS MEADOWS

A 10 mo field experiment in seagrass meadows of the Exuma Cays, Bahama s, indicated that grazing by queen conch Strombus gigas L. had an impo rtant effect on the abundance of seagrass detritus (an important conch food) and the structure of macrofauna communities. At 2 sites where c onch were abundant, conch exclosures always contained higher (to 97x) quantities of detritus than enclosures of conch at high density (1.2 c onch m(-2)) typical of values found in the field. No significant effec ts were observed on living seagrass biomass, shoot count, or growth, n or on epiphyte biomass. Epiphyte loads were relatively low and apparen tly caused no reduction in seagrass growth as a result of shading. Den sity of epibenthic macrofauna ranged from 799 to 6610 individuals m(-2 ) (mostly tanaids, amphipods and ostracods), and conch exclosures alwa ys contained higher (to 3.8x) densities than enclosures of conch at hi gh density. A positive correlation was observed between macrofauna den sity and detritus. At the end of the experiment at one site, exclosure s had at least twice the number of species per sample as any of the en closures. Relative compositional differences among the 211 different s pecies encountered during this study were subtle; however, except for a few species, absolute numerical differences were higher in the exclo sures than in the high density enclosures. Largest increases in densit y associated with increases in seagrass detritus occurred in tube-buil ding tanaidaceans and free-living amphipods and ostracods. Results fro m this study suggest that conch strongly affect community structure by consuming detritus, an important component of the overall vegetative structure that provides food and shelter for other benthic invertebrat es. Variation in dynamics of seagrass communities may be a function of large herbivores and detritivores.