Date Published: October 5, 2018
Publisher: Public Library of Science
Author(s): Cassandra N. Glaspie, Rochelle D. Seitz, Judi Hewitt.
In Chesapeake Bay, the soft-shell clam Mya arenaria (thin-shelled, deep-burrowing) exhibits population declines when predators are active, and it persists at low densities. In contrast, the hard clam Mercenaria mercenaria (thick-shelled, shallow-burrowing) has a stable population and age distribution. We examined the potential for habitat and predators to control densities and distributions of bivalves in a field caging experiment (Mya only) and laboratory mesocosm experiments (both species). In the field, clams exposed to predators experienced 76.3% greater mortality as compared to caged individuals, and blue crabs were likely responsible for most of the mortality of juvenile Mya. In mesocosm experiments, Mya had lower survival in sand and seagrass than in shell hash or oyster shell habitats. However, crabs often missed one or more prey items in seagrass, shell, and oyster shell habitats. Predator search times and encounter rates declined when prey were at low densities, likely due to the added cost of inefficient foraging; however, this effect was more pronounced for Mya than for Mercenaria. Mercenaria had higher survival than Mya in mesocosm experiments, likely because predators feeding on Mercenaria spent less time foraging than those feeding on Mya. Mya may retain a low-density refuge from predation even with the loss of structurally complex habitats, though a loss of habitat refuge may result in clam densities that are not sustainable. A better understanding of density-dependent predator-prey interactions is necessary to prevent loss of food-web integrity and to conserve marine resources.
Predators exhibit top-down control on communities, influencing the abundance, size structure, and distribution of prey by restricting their survival or activity in time and space [1–3]. Predators also influence community function by preying upon dominant species [4–6]. To understand the structure and function of a community, it is important to consider the impact of the predators. Prey populations experience the effects of predation differently depending on how abundant the prey species is and, for actively foraging predators, how quickly the predator can find and consume prey . The degree to which a predator can reduce prey abundance is a function of the probability of encountering a prey item, and the probability that the prey item will be eaten, given that it has been encountered. Both factors depend on the characteristics of the prey, the predator, and other environmental factors .
Virginia Institute of Marine Science is statutorily mandated as Virginia’s scientific advisor on marine- and coastal-related natural resources and exempt from having to obtain a scientific collection permit for non-protected species in Virginia’s waters.
Blue crabs were the main predators of Mya in all habitats we examined, with no significant difference between stockades and uncaged plots and high incidence of crushed shells, which is evidence of crab predation rather than another source of mortality . This was in line with our hypothesis that crab predation would be important. Despite evidence in the literature that schooling rays can result in mass mortality of bivalves , and evidence from gut content analysis that cownose rays consume Mya , we did not observe evidence that cownose rays increased predation in uncaged plots relative to stockade plots during the time frame of our field experiment (May). These results were contrary to our hypothesis regarding ray predation and indicate that over the time and spatial scale of this study, rays were not a major source of mortality for Mya.