MEPS 121:125-137 (1995)  -  doi:10.3354/meps121125

A 10 mo field experiment in seagrass meadows of the Exuma Cays, Bahamas, indicated that grazing by queen conch Strombus gigas L. had an important effect on the abundance of seagrass detritus (an important conch food) and the structure of macrofauna communities. At 2 sites where conch were abundant, conch exclosures always contained higher (to 97x) quantities of detritus than enclosures of conch at high density (1.2 conch m^-2) typical of values found in the field. No significant effects were observed on living seagrass biomass, shoot count, or growth, nor on epiphyte biomass. Epiphyte loads were relatively low and apparently caused no reduction in seagrass growth as a result of shading. Density of epibenthic macrofauna ranged from 799 to 6610 individuals m^-2 (mostly tanaids, amphipods and ostracods), and conch exclosures always contained higher (to 3.8x) densities than enclosures of conch at high density. A positive correlation was observed between macrofauna density and detritus. At the end of the experiment at one site, exclosures had at least twice the number of species per sample as any of the enclosures. Relative compositional differences among the 211 different species encountered during this study were subtle; however, except for a few species, absolute numerical differences were higher in the exclosures than in the high density enclosures. Largest increases in density associated with increases in seagrass detritus occurred in tube-building tanaidaceans and free-living amphipods and ostracods. Results from 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 invertebrates. Variation in dynamics of seagrass communities may be a function of large herbivores and detritivores.

Seagrass . Herbivory . Strombus gigas . Plant-animal . Macrofauna . Community . Diversity . Bahamas