Bay Scallops and Eelgrass

Although other substrates (including stones, shells and algae) can be used as attachment sites for juvenile bay scallops Argopecten irradians, eelgrass Zostera marina has long been documented as their preferred habitat (Gustell 1930; Thayer and Stewart 1974). As juveniles, bay scallops use byssal threads to attach themselves onto elevated portions of eelgrass blades, staying out of reach from crabs, whelks, and other benthic (sea floor) predators.

The "spat" or settled scallops that attach to the upper canopy of eelgrass (20-35 cm above bottom) have a significantly greater chance of surviving compared to those at the base of the eelgrass shoots or on unvegetated bottom. Also, there is a direct relationship between eelgrass density and scallop survival (Pohle et al. 1991), which means that the more dense the eelgrass, the higher the chances of the scallop surviving.

Adult scallops are also protected by eelgrass. For example, large, above-water predators like sea gulls have a hard time pinpointing an adult scallop within an eelgrass meadow opposed to one on a sandy bottom. The presence of a continuous eelgrass meadow can also help prevent adult scallops from being washed up on shore during a storm event.

Besides reduced predation pressure, another benefit that scallops receive from residing in eelgrass is the increased amount of particulate food which is settled out of the water column by eelgrass blades (Peterson et al. 1984). This allows scallops to grow faster, reaching a size where they are less vulnerable to predation at a faster rate.

Not surprisingly, when epidemics such as the ‘Wasting Disease’ of the early 1930’s and the brown tide blooms in the 1980’s occurred here on Long Island, resulting in large-scale die offs of eelgrass, bay scallop populations also plunged. "In 1982, the harvest of 500,000 lbs of bay scallops from the Peconic Estuary accounted for 28% of all U.S. commercial landings and had a dockside value of $1.8 million.  After the appearance of the Brown Tide in 1985, the bay scallop population was virtually eliminated " (Peconic Estuary Program).

As with terrestrial biologists, marine scientists often consider habitat restoration before attempting to restore a declining species, as habitat loss is often the underlying cause of their decline. Our eelgrass program is taking this approach with some of our restoration projects, including projects in the South Shore Estuary and in Huntington Harbor. Together with the CCE Aquaculture Program, these projects involve planting eelgrass in the early phases and then seeding scallops into these restored meadows in later phases (learn more...).

To learn more about scallop biology and ecology as well as the Cornell Cooperative Extension Bay Scallop Restoration Project, see our Spring 2008 issue of our SEAGRASS.LI newsletter.

References

Gustell, J. (1930). "Natural history of the bay scallop." US Bureau of Fisheries Bulletin 46: 569-632

Eckman, J. E. (1987). The role of hydrodynamics in recruitment, growth, and survival of Argopecten irradians (L.) and Anomia simplex (D'Orbigny) within eelgrass meadows. J. exp. mar. Biol. Ecol. 106. 165-191

Peterson, C.H., H.C. Summerson, and P.B. Duncan. 1984. The influence of seagrass cover on population structure and individual growth rate of a suspension feeding bivalve, Mercenaria mercenaria. J. Mar. Res. 42: 123-138.

Pohle, D.G., Bricelj, V.M., and Garcia-Esquivel, Z. 1991. The eelgrass canopy: an above-bottom refuge from benthic predators for juvenile bay scallops Argopecten irradians. Mar. Ecol. Prog. Ser. 74: 47-59.

Thayer, G. W., and H. H. Stuart. 1974. The bay scallop makes its bed of seagrass. Mar. Fish. Rev. 36(7):27-30.