Hard clams can “walk” across the sediment
Hard clams or quahogs (Mercenaria mercenaria) are one of the most popular species of clams along the East Coast of North America. They support important commercial and recreational fisheries as well as a growing aquaculture industry (Figure 1). Hard clams are commonly sold under names based on their size from the larger chowders and cherrystones to top necks and little necks.
Hard clams live in intertidal and subtidal areas of coastal bays and can be found in a variety of sediments but prefer coarse sand (Figure 2). These clams burrow into the sediment using their muscular foot and extend their siphons to filter phytoplankton from the water. It was previously believed that hard clams generally were sessile in nature until members of Dr. Stephen Tettelbach’s laboratory from Long Island University’s Post campus observed more than two dozen clams at the ends of furrows on the bottom of New York’s Gardeners Bay. It was obvious that the clams had traveled horizontally across the surface of the sediment, a behavior that had not yet been documented with hard clams.
Three years later, hard clams were directly observed “walking” across the sediment during a bay scallop (Argopecten irradians) population survey in the Peconic Bays of eastern Long Island, New York (Figure 3). This prompted the researchers to fully document this behavior. To investigate why the clams engaged in walking activity, data was collected from “walking areas” and adjacent areas where clams were not found walking (“non-walking areas”). This included measuring water temperature and chlorophyll a (an indicator of phytoplankton biomass), examining the grain sizes of the sediments, and determining the density of clams. Clams were also dissected to determine their sex and reproductive state.
It was found that hard clam walking activity is influenced by seasonal spawning activity. Hard clam walking mainly occurred in June, July, and October, when the clams were ready to spawn. Both male and female clams were observed walking and they tended to walk towards members of the opposite sex. In the walking areas, clams were more aggregated. When hard clams spawn, they release their sperm and eggs into the water column where fertilization takes place. By aggregating near one another, the clams increase the likelihood that eggs are successfully fertilized.
By examining the environmental variables, the researchers found no differences between the water temperatures, chlorophyll measurements or composition of the sediments between the walking and non-walking areas. The walking behavior appeared to have been triggered by the release of pheromones or perhaps pressure waves produced by adjacent clams. Hard clam walking was not rare, it was documented at nine different sites surveyed in the Peconic Bays. Walking behavior was also influenced by clam density. At one site, the research team inadvertently suppressed the walking behavior by removing over 200 clams for analysis back at the lab. With the decrease in clams, walking was not observed at this site for over three years. This may occur at other locations where clams have been overharvested.
Further studies will be needed to find the extent of hard clam walking throughout their range and the factors that trigger this behavior. Understanding why hard clams walk and how overharvesting can influence their behavior will be beneficial to the management of the species.
Tettelbach, S.T., J.R. Europe, C.R.H. Tettelbach, J. Havelin, B.S. Rodgers, B.T. Furman, and M. Velasquez. 2017. Hard clam walking: Active horizontal locomotion of adult Mercenaria mercenaria at the sediment surface and behavioral suppression after extensive sampling. PLOS One: 0173626.