No abstract

The importance of positive interspecific interactions within physically stressful habitats has received increased attention from community ecologists. The exposed sandy beach is an example of a physically rigorous environment where biological interactions have long been considered insignificant. We examined the interaction between the infaunal clam, Donax variabilis, and the hydroid, Lovenella gracilis, on exposed sandy beaches in North Carolina. Epibiotic occupation of Donax by hydroids has been repeatedly observed on ocean beaches but rarely investigated. By providing a stable substrate for attachment, the clam facilitates the persistence of the hydroid in the intertidal beach; however, benefits or costs experienced by the host as a result of this association are unknown. By exposing clams with and without hydroid colonies to multiple types of clam predators, we tested the effectiveness of the hydroid, which possesses stinging nematocysts, in defending its host. The hydroid defended the clam against one common predator, the Florida pompano ( Trachinotus carolinus). Against speckled crabs ( Arenaeus cribrarius) and ghost crabs ( Ocypode quadrata), however, the hydroid offered no protection for its host and instead facilitated predation. The epibiotic hydroid, which projects above the surface of the sand, allowed the crabs to more readily detect clams below the surface. In the field, we evaluated the effect of the hydroid on the tidally synchronized migrations and burrowing speed of the clam. The hydroid, which can form large colonies on the posterior end of the clam, had no effect on Donax burrowing speed, but did reduce the speed of transport of clams by wave swash. Depending on relative predation pressure, the occupation of D. variabilis by L. gracilis can alternately be characterized as beneficial or detrimental to the host.

Novel wave-tank mesocosms allowed hydrodynamically realistic tests of how sediment modifications affect feeding by a surf fish, Florida pompano, Trachinotus carolinus (Linnaeus, 1766). Pompano demonstrated visually based selection, preferentially preying on bean clams, Donax variabilis Say, 1822, colored in contrast with background. Pompano often took shell into their mouths instead of live bean clams. Sediments of nourished Bogue Banks (North Carolina) beaches exhibited >2.5-yr persistence of elevated coarse shell content at levels that suppressed pompano feeding in wave tanks. Elevated turbidity 4-8 mo after nourishment indicated that wave-induced erosion of buried fine sediments in sacrificial beach fill resulted in repeated turbidity events, reaching levels that in wave tanks reduced pompano feeding on Donax and mole crabs, Emerita talpoida (Say, 1817). Burial speeds of these prey in the beach swash zone were progressively reduced by increasing shell concentrations. Reduced burial rates reduce feeding opportunity and expose these mobile invertebrates to greater risk of wave transport out of the swash-zone habitat. Sampling six beaches revealed that density of Donax decreased linearly with increasing sediment size and shell concentration. Beyond the immediate mass mortality of invertebrate prey caused by >1 m of sediment deposition during beach filling, coarse shell fragments and other large particles persist as a press disturbance for years after the nourishment ends, and elevated silts/clays can become resuspended by erosive wind events in repeated pulse disturbances for at least months afterward, in each case reflecting demonstrable long-term degradation of sandy-beach foraging habitat for surf fish.

Disposal onto a sandy beach of sediments dredged from a navigation channel suppresses abundances of most intertidal invertebrates and increases surf-zone turbidity (insets).