Density‐Dependent Settler‐Recruit‐Juvenile Relationships in Blue Crabs

Abstract

Current theory on the population dynamics of marine species with complex life history patterns posits that a suite of physical and biotic forces (e.g., habitat structure and density—dependent predation or emigration) control survival and abundance in early life history, particularly after settlement. We have conducted a long—term sampling effort accompanied by a series of field and laboratory experiments examining the joint effects of habitat type, body size, and population density upon abundance and survival of early juveniles of the blue crab, Callinectes sapidus. In addition, the chance occurrence of a tropical storm during one set of experiments provided an opportunity to assess the impact of a physical disturbance upon newly settled blue crab survival and abundance. In the 10—yr sampling effort, we quantified relationships between sequential life history stages (juvenile crab instars) in seagrass beds, the initial nursery habitat for blue crabs in the lower Chesapeake Bay. Inter—instar relationships were defined as the densities of larger instars as dependent on the densities of smaller instars. Inter—instar relationships for the youngest instars are described by hyperbolic functions until crabs begin to emigrate to unvegetated habitats at approximately the fifth instar. Inter—instar relationships between crabs larger than the fifth instar and smaller crabs become either parabolic or linear functions and decay as the number of instars between sequential life history stages increases. While both the hyperbolic and parabolic functions are indicative of populations regulated by density—dependent processes, either predation or emigration, the decay in the functions describing the inter—instar relationships for crabs larger than the fifth instar indicates that the suite of processes regulating this segment of the population changes qualitatively. In laboratory and field experiments, the effects of vegetated and unvegetated habitats and size—specific predation on newly settled juveniles were tested. Tethering was used to quantify relative rates of predation, and a laboratory study was conducted to determine if tethering induced treatment—specific bias. We found no statistically significant interactions between the tethering treatment and the factor treatments of crab size and habitat during the laboratory study, indicating that tethering did not produce treatment—specific bias. Thus, tethering provided a relative measure of predation that allowed comparisons between treatments of habitat and crab size on crab survival. In both laboratory and field experiments, survival was significantly higher in vegetated habitats and with increasing size until the ninth instar, when survival did not differ by habitat. This difference explains the dispersal from vegetated to unvegetated habitats that occurred between the fifth and seventh instars. In addition, survival of all crabs was significantly increased both during and after Tropical Storm Danielle compared to pre—storm conditions. A model is developed that describes juvenil...

Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

“…conspecific adults Luppi, 1999 . Similar pattern of cannibalism-mediated habitat shift have been described for C. sapidus; crabs between 5 and 15 mm have an ontogenetic Ž shift in habitat use from seagrass to unvegetated mud and sand Moksnes et al, 1997;. Pile et al, 1996 . C. granulata megalopae settle in or near the adult burrows, where various juvenile Ž .…”

The geography and bathymetry of American lobster benthic recruitment as measured by diver-based suction sampling and passive collectors

“…Developing sampling methods to quantify newly settled cohorts in benthic population studies can be a powerful way of understanding processes operating before and after larval settlement that drive subsequent recruitment and population dynamics (Roughgarden et al 1988;Pineda 2000;Balch & Scheibling 2000;Cowen et al 2000;Navarrete et al 2005). Studies of lobsters and crabs that track the fate of early benthic cohorts have provided critical insights into larval transport leading to benthic recruitment and the strength of inter-and intraspecific interactions influencing the fate of cohorts after settlement (Eggleston & Armstrong 1995;Pile et al 1996;Incze et al 1997;Moksnes & Wennhage 2001;Gardner et al 2001;BrionesFourzan et al 2008). The western rock lobster, Panulirus cygnus George 1962, is a noteworthy example of a species for which annual monitoring of postlarval settlement has been an integral part of fishery management for more than four decades (Caputi et al 1995).…”

Role of two contrasting ecosystem engineers (Zostera noltii and Cymodocea nodosa) on the food intake rate of Cerastoderma edule

“…blue crab; Pile et al 1996). These biodiversity eVects have been ascribed to a number of diVerent possible indirect mechanisms: (1) altered plant-animal interactions (settlement of new recruitments ;Eckman 1983;Boström and BonsdorV 2000), (2) changes on the intensity of biological interactions (mainly predation-prey interactions; Nelson and BonsdorV 1990;Irlandi and Peterson 1991), (3) decrease on the Xow (Fonseca et al 1982;Gambi et al 1990) causing (3a) an increase on the sediment stability (Fonseca and Fisher 1986;Irlandi 1996) and (3b) alterations on the food supply to the organisms (Irlandi and Peterson 1991;Allen and Williams 2003).…”