RECRUIT MORTALITY BY PREDATION IN BRACHYURAN CRABS: EFFECTS OF PREY SIZE AND HANDLING METHODS OF PREDATOR.

LUPPI TOMAS

Atenas

Congreso; The Crustacean Society Summer Meeting; 2012

The Crustacean Society

Luppi*1, T. 1) Instituto de Investigaciones Marinas y Costeras (IIMyC), Comisión Nacional de Investigaciones Científicas y Técnicas (CONICET), Universidad Nacional de Mar Del Plata, Casilla de Correo 1245, 7600 Mar del Plata, Argentina. taluppi@mdp.edu.ar Introduction Predation and their intra-specific form, cannibalism, are a key factor in the regulation and abundance of several species, through all zoological taxa. In Decapod crustacean mortality of recruits and juveniles due to predation and cannibalism may explain cyclic fluctuation in population or have a major role in regulating recruitment. Intraguild predation and intercohort cannibalism is associated with an asymmetry between predator and prey, often in size, in that the victim is smaller and more vulnerable than predator. The predator-prey system between crabs and hard preys is function of size of prey, predator-prey size rate, handling methods, handling times and morphology and mechanics of predator’s chelae, but relationship among size of predators and preys appears to prevail. The objectives of this study are three-fold: (1) to compare the morphology and mechanics of four first (R) instars and juveniles (J) of Neohelice granulata and Cyrtograpsus angulatus (2) to investigate vulnerability of R instars with homogeneous age, moult stages and hunger levels of preys and predators (3) to describe and compare the handling methods and handling time. Material and methods Four first R (C1 to C4) of both species were obtained rearing in laboratory megalopae collected from recruitment habitats at Mar Chiquita coastal lagoon, Argentina (37º45’S, 57º19’W). J of two size class (carapace width “CW” 5 and 7mm) were collected from same habitats. Measurement of R carapace and J chelae were made by a stereomicroscope. To evaluate the effect of size of R on vulnerability to cannibalism and predation by J, handling times and handling methods, we performed a laboratory experiment where (n=20) R of each instar were added to J’ predators of both species and size class. Through 30 min of observation we recorded: R killed by predators, time to first attacks, numbers of failure attacks, handling methods and times. Results Chelae of JNeo7 showed the maximum and significant biggest values at all measurements and chelae of JCyr5 showed the minimum and significant smallest values. N. granulata as prey: Proportional mortality (PM) was dependent on R and J sizes. At all R, JCyr7 predator produced the highest values of PM. For JNeo7 high values of PM were observed in C1 and C2 and only in C1 for JNeo5. C. angulatus as prey: PM was dependent on R and J sizes. At all R, JCyr7 produced the highest values of PM, while moderate values were observed in C1 and C2 for JCyr5. Handling methods of predators: N. granulata: R was always grasped from cefalopereion with the chelae in the first attack, then R was manipulated and reoriented perpendicularly to J cefalopereion and began bite and eat. C. angulatus: grasped all R instars with the chelae by the legs. Handling times: N. granulata: Lineal regressions of recruit volume (RV) with handling time showed high adjustment coefficient (R2 > 0.85). C. angulatus: Potential regressions of RV with handling time showed high adjustment coefficient (R2 > 0.93). Discussion Recruit mortality by cannibalism and predation is size dependent; however at similar predator-prey size rate the effect of two predators could be so different and seems to be related to morphometrics limitations associated to handling methods.