Effect of body size on cannibalism in early juvenile mud crab Scylla paramamosain (Decapoda: Brachyura: Portunidae) under laboratory conditions

Abstract

Cannibalism is a major cause of mortality in nursery cultures of Scylla paramamosain, and size grading is recommended as an effective measure to reduce cannibalism. However, little is known about the relationship between body size differences and the occurrence of cannibalism in early juvenile stages. We conducted one-to-one match experiments (60 trials) using hatchery-raised juveniles with carapace widths between 5.1 and 10.9 mm. Two crabs were placed in an experimental arena for 24 h, and survival was subsequently observed. Cannibalism occurred in 48 % of the trials, and larger crabs always preyed on smaller crabs. The probability of cannibalistic behavior significantly increased with increasing relative size difference [RSD = 1 − (size of smaller crab)/(size of larger crab)]. The RSD threshold at which 50 % of the smaller crabs are cannibalized by larger crabs was estimated at 0.21.