EFFECTS OF LIMB AUTOTOMY AND TETHERING ON JUVENILE BLUE-CRAB SURVIVAL FROM CANNIBALISM

High frequencies of limb loss (18 to 39%) in blue crab Callinectes sapidus Rathbun populations over broad temporal and spatial scales suggest that the autotomy response is an important escape mechanism. Limb loss, however, may increase vulnerability of prey in future encounters with predators. If individual survival is reduced significantly and injury frequency in the population is density-dependent, such nonlethal injury could affect population size. Annual frequencies of limb loss were positively correlated to blue crab abundances in the Rhode River, Maryland, USA, between 1986 and 1989, but results of open-field tethering experiments indicated that, overall, missing limbs did not increase juvenile vulnerability to predators. Limitations imposed by the tether on normal escape behavior, however, may have masked real survival differences among limb-loss treatments. To test for interactive effects of limb loss and tethering on survival from predation, I conducted a set of field experiments in 10 m(2) enclosures, using adult blue crabs as predators and intact and injured (missing 1 or 4 limbs), tethered and untethered juvenile conspecifics as prey. A second experiment, conducted in small wading pools, tested the impact of limb loss on escape speed and direction of juvenile blue crabs. Results of enclosure experiments demonstrated that: (1) under typical field conditions and crab densities, larger conspecifics do inflict lethal and nonlethal injury on juveniles; and (2) in encounters with predators, prior limb loss does not handicap crabs if escape is possible (untethered treatments), but does impose a defensive cost if escape is restricted (tethered treatments). In addition, survivorship patterns suggest that prey missing multiple limbs altered their activity patterns to decrease vulnerability. In wading pools, limb loss altered escape speed and direction, although effects varied depending on the type and number of missing Limbs. Together, these experiments indicate that prior Limb loss can have complex effects on escape effectiveness, defensive ability, and anti-predator behavior. They also suggest that, despite density-dependence, prior limb loss does not reduce blue crab fitness sufficiently to regulate population size.