Small Decapod Crustaceans Research Articles

Epifaunal invertebrates as predators of juvenile bay scallops (Argopecten irradians)

Predation strongly influences populations of numerous benthic invertebrates, although many predation studies to date have focused on macroscopic individuals, ignoring critical early life stages. Juveniles of the bay scallop, Argopecten irradians, settle and grow on the blades of eelgrass, Zostera marina, then migrate to the sediment surface when their mobility and size provide a refuge from benthic predators. During their time in the eelgrass canopy, scallops co-occur with a diverse array of small invertebrates, including peracarid and small decapod crustaceans, whose role as predators is largely unexplored. We measured consumption by amphipods, isopods, and a shrimp on recently settled bay scallops ranging in size from 0.5 to >1.5mm in a series of 24-hour experimental laboratory assays. These invertebrate predators, which were common concurrent epifaunal surveys of restored eelgrass beds in the mid-Atlantic, consumed up to 63% day−1 of juvenile scallops when the scallops were <1mm, but predation impacts decreased as scallops exceeded this size. Our data have implications for current restoration of both bay scallops and their eelgrass habitat, suggesting that previously unrecognized consumers may significantly affect scallop population dynamics at early life stages.

Settlement behaviour and early post-settlement predation of the sea urchin Strongylocentrotus droebachiensis

We examined behaviour of competent larvae of the green sea urchin Strongylocentrotus droebachiensis provided with pebbles encrusted with coralline red algae, a strong settlement inducer, in laboratory experiments. Larvae settled at greater frequencies on upward-facing coralline surfaces and in small gaps between coralline and glass surfaces than expected by a random distribution of settlement. These patterns may be explained by encounter rate with inductive cues. There was no change in settler distributions within ~ 1 week of settlement, indicating no net movement between adjacent microhabitats. In flow, live and recently killed larvae settled or were passively entrapped at greater frequencies on high- than low-rugosity coralline crusts. Recent settlers (0.5-1 mm test diameter) were consumed by small decapod crustaceans and bulldozed by periwinkles. Juveniles became less vulnerable to predation by hermit crabs with increasing size, and reached a growth refuge at ~ 10 mm test diameter. Our laboratory findings suggest that the cryptic distribution of recent settlers is probably not due to microhabitat selection by settling larvae or early post-settlement migration, at least not in response to physical cues such as light or surface texture. Differential rates predation of young juveniles between exposed and cryptic habitats cannot be ruled out as an important determinant of this pattern.

Characterizing temperate rocky shores from the perspective of an early juvenile snail: the main threats to survival of newly hatched Nucella emarginata

Mortality factors most likely to constitute substantial selective pressures for early juvenile gas- tropods on temperate rocky shores were identified by examining the vulnerability of hatchlings of an inter- tidal snail, Nucella emarginata, to heat stress, desicca- tion, and predation in 1992 and 1993. The highest temperature of substrata measured at tidal heights col- onized by N. emarginata in Barkley Sound, British Columbia, Canada, was 28.5 ~ This temperature was not lethal to hatchlings in laboratory tests. In labora- tory and field desiccation experiments, all hatchlings died within 6 h of emersion. Early juveniles could not survive direct exposure to even moderate drying condi- tions for the duration of a low tide. Hence, intertidal microhabitats which dry up even for short periods during low tides would prove lethal. Of 45 intertidal animal species to which hatchlings were exposed in the laboratory, small decapod crustaceans were the only organisms to cause substantial hatchling mortality. Of these, Pagurus hirsutiusculus and Hemigrapsus nudus were by far the most abundant in the field, and are probably the only important predators of early juvenile N. emarginata at most sites. Total predator densities in the field were as high as 438 individuals m-z, sugges- ting that predation pressure may be intense. Desicca- tion and predation by decapod crustaceans appear to be the most significant threats to early juvenile N. emarginata. These factors commonly occur on most temperate rocky shores and undoubtedly constitute major selective agents influencing population param- eters and shaping life-history strategies and early juvenile traits of intertidal invertebrates.