Effects of a large herbivorous gastropod on macrofauna communities in tropical seagrass meadows

Abstract

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 communit~es. 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 . 8~) densities than enclosures of conch at high density. A positive correlation was observed between macrofauna density and detntus. 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 21 1 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 freeliving 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.

A groundwater-fed coastal inlet as habitat for the Caribbean queen conch Lobatus gigas—an acoustic telemetry and space use analysis

“…Animals often leave a patch before completely depleting it, usually when its quality drops below the average quality of the whole site (Charnov 1976, Searle et al 2005. Stoner (1989b) , comparable to densities found in Xel-Há), Stoner et al (1995) found that zones where queen conches were excluded contained 3.8 times more detrital matter compared to zones containing ani mals. This suggests that queen conches can deplete much of their food resources even at low densities.…”

The relative importance of nutrient enrichment and herbivory on macroalgal communities near Norman's Pond Cay, Exumas Cays, Bahamas: a “natural” enrichment experiment

“…1). The biology and ecology of queen conch, S. gigas, in proximity to NPC is particularly well known as a result of extensive research conducted at the nearby CMRC laboratory (Wicklund et al, 1988;Stoner, 1989;Stoner andWaite, 1990, 1991;Stoner et al, 1995).…”

Seasonal variation of epiphytic hydroids (Cnidaria: Hydrozoa) associated to a subtropical Sargassum cymosum (Phaeophyta: Fucales) bed

“…DAHMS et al 2004). Other members of the macrofauna may also affect epiphytic species abundance indirectly, by grazing on macrophytes (STONER et al 1995). Herbivore grazing pressure greatly affects the macrophyte community structure, reducing parts of the plants frequently occupied by epiphytic hydroids (GARCÍA-RUBIES 1987, ELGER et al 2009).…”