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Eelgrass (Zostera marina) is an ecologically valuable seagrass which is exposed to a wide range of stressors and has declined worldwide. The proliferation of epifaunal fouling organisms such as the sponge Halichondria panicea, colonial tunicate Botryllus schlosseri, and solitary tunicate Molgula manhattensis represents additional stress for 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 Northwest Atlantic off southern Massachusetts, USA, green crabs Carcinus maenas and spider crabs Libinia dubia were examined as potential predators of sponges and tunicates attached to eelgrass. Crabs were somewhat starved for one week and then, in the lab, 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 did occur. After 24 hours, C. meanas had still not consumed any tunicates or sponges, while L. 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 on eelgrass at Martha’s Vineyard (up to 6,700 per m), thus we sought to determine a M. manhattensis consumption rate for L. dubia in the lab. A single L. dubia can consume at least 30 M. manhattensis in a 24-hour period. Because of the recent declines in eelgrass acreage, understanding the ecological mechanisms that minimize impacts to eelgrass can be advantageous to both the individual pant and the meadow. Natural predation by L. dubia in eelgrass meadows heavily fouled by tunicates and sponges is an important ecosystem function that may contribute to controlling the epifauna, and thus maintaining healthy eelgrass and eelgrass habitat. However, the extent to which crabs control eelgrass epifauna in the field is unknown. Our results were determined in a laboratory setting and further work should be done to confirm that similar results will be found in the field.

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