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 combusted (=combusted sand) to determine if organic constituents of the substrate provided cues for settlement. Whereas settlement of C. major is known to 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 or combusted sand at ZIV or D, C. major burrowed significantly less in combusted sand than natural sand and postlarvae of C. islagrande burrowed in both sand types. To further define the characteristic of sand to which ZIV larvae of C. major respond, larvae received either natural sand, no sand, combusted sand, combusted sand conditioned by seawater, combusted sand conditioned 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 ZIV larvae. Exposure to combusted sand at ZIV was sufficient to trigger decapodids to burrow in natural sand. This finding suggests that ZIV larvae detect a mechanical or inorganic cue from sand, whereas decapodids respond to the presence of organic compounds. These organic compounds were not necessarily adult-derived because combusted sand conditioned by seawater aging alone induced burrowing by decapodids.