A preliminary assessment of crab predation on epifaunal fouling organisms attached to eelgrass at Martha’s Vineyard, Massachusetts, USA

摘要

Eelgrass (Zostera marina) is an ecologically valuable seagrass which is exposed toa wide range of stressors and has declined worldwide. The proliferation of epifaunalfouling organisms such as the sponge Halichondria panicea, colonial tunicate Botryllusschlosseri, and solitary tunicate Molgula manhattensis represents additional stressfor eelgrass. Predation of this epifauna that would otherwise cause harm to eelgrass,will likely reduce their impact. On Martha’s Vineyard, an island in the NorthwestAtlantic off southern Massachusetts, USA, green crabs Carcinus maenas and spidercrabs Libinia dubia were examined as potential predators of sponges and tunicatesattached to eelgrass. Crabs were somewhat starved for one week and then, in thelab, put in enclosures with three eelgrass shoots and tunicate/sponge epifauna.Consumption of prey items and crab survival were measured at one and 24 hours.After one hour, C. meanas did not consume any prey, while predation by L. dubia didoccur. After 24 hours, C. meanas had still not consumed any tunicates or sponges, whileL. dubia consumed eelgrass with H. panicea (100%); B. schlosseri (40% completely,40% partially, and 20% unconsumed); and M. manhattensis (60% completely, 20%partially, and 20% unconsumed). High densities of M. manhattensis often occurs oneelgrass at Martha’s Vineyard (up to 6,700 per m2), thus we sought to determine aM. manhattensis consumption rate for L. dubia in the lab. A single L. dubia canconsume at least 30 M. manhattensis in a 24-hour period. Because of the recentdeclines in eelgrass acreage, understanding the ecological mechanisms thatminimize impacts to eelgrass can be advantageous to both the individual pant andthe meadow. Natural predation by L. dubia in eelgrass meadows heavily fouled bytunicates and sponges is an important ecosystem function that may contribute tocontrolling the epifauna, and thus maintaining healthy eelgrass and eelgrass habitat.However, the extent to which crabs control eelgrass epifauna in the field isunknown. Our results were determined in a laboratory setting and further workshould be done to confirm that similar results will be found in the field.