Predation by xanthid crabs on early post-settlement gastropods: the role of prey size, prey density, and habitat complexity

Small predators in marine benthic communities create a hazardous environmen t for newly settled invertebrates, especially for the smallest individuals. To explore the effects of predation on a newly settled gastropod, queen co nch (Strombus gigas Linnaeus), by a xanthid crab (Micropanope sp.), prey si ze, prey density, and habitat complexity were manipulated in five laborator y experiments. All crabs > 3.1 mm CW killed all conch < 2 mm SL when indivi dual crabs (< 14 mm carapace width (CW)) were offered individual conch that were 2-35 days old after metamorphosis (1.2-8.8 mm shell length (SL)). Onl y 10% of the crabs > 5.0 mm CW, however, killed conch that were > 5.0 mm SL , suggesting that conch may reach a size refuge from xanthid crabs at 5 mm SL. Furthermore, when given a choice, crabs (4.8 mm CW) preferred smaller c onch (2.0 mm SL) to larger (3.7 mm SL), suggesting that 1 week of additiona l growth in shell length is advantageous to survivorship. Proportional mort ality decreased as conch density increased when crabs were offered conch at seven different densities (two to 96 individuals). Crabs proved to be effe ctive predators regardless of the amount of seagrass structure provided in a microcosm experiment, and could consume two conch in 10 s. The high densi ties of xanthid crabs that occur in the wild, their effectiveness as predat ors, and their large appetites point to the important role that small preda tors may potentially play in structuring the population dynamics of their s mall prey immediately after settlement. (C) 1999 Elsevier Science BN. All r ights reserved.