Sand as a stimulus for settlement in the ghost shrimp Callichirus major (Say) and C-islagrande (Schmitt) (Crustacea : Decapoda : Thalassinidea)

Larvae and postlarvae of Callichirus major (Say) and C. islagrande (Schmitt ) were offered heat-treated sand from which organic compounds had been comb usted (= combusted sand) to determine if organic constituents of the substr ate provided cues for settlement. Whereas settlement of C. major is known t o be triggered by the presence of untreated sand at the fourth zoeal stage (ZIV), postlarvae of C. islagrande require no previous contact with sand in order to settle at the decapodid stage (D). When larvae received natural o r combusted sand at ZIV or D, C. major burrowed significantly less in combu sted sand than natural sand and postlarvae of C. islagrande burrowed in bot h sand types. To further define the characteristic of sand to which ZN larv ae of C. major respond, larvae received either natural sand, no sand, combu sted sand, combusted sand conditioned by seawater, combusted sand condition ed by seawater and adults, or autoclaved sand. All animals received natural sand after the molt to D. Although decapodids burrowed significantly less commonly in combusted sand, the combusted sand did provide a stimulus to ZI V larvae. Exposure to combusted sand at ZIV was sufficient to trigger decap odids to burrow in natural sand. This finding suggests that ZIV larvae dete ct a mechanical or inorganic cue from sand, whereas decapodids respond to t he presence of organic compounds. These organic compounds were not necessar ily adult-derived because combusted sand conditioned by seawater aging alon e induced burrowing by decapodids. (C) 1999 Elsevier Science B.V. All right s reserved.