Settlement cues in am Atlantic coast population of the ghost shrimp Callichirus major (Crustacea : Decapoda : Thalassinidea)

Larvae of Callichirus major (Say) were first given stimuli upon reaching th e last zoeal stage (ZIV), decapodid stage (D), or first juvenile stage (J1) . Animals were provided with sand and unconditioned water, water conditione d by previous exposure to adults (adult-conditioned water = AW), or both st imuli to determine if these factors could serve as settlement cues. ZIV was found to be important in the settlement process. Animals that received san d and AW at ZIV burrowed more than those that received stimuli at D. Wherea s larvae of the Atlantic population delayed molting to D in the absence of stimuli, they did so differently than previously found in a Gulf of Mexico population. In the Gulf population, duration of ZIV decreased in the presen ce of AW. In contrast, duration of ZIV in the Atlantic population increased significantly in the presence of stimuli at this stage, which led larvae t o molt directly from ZIV to D. The percentage of animals molting directly f rom ZIV to D (bypassing the fifth zoeal stage) was significantly higher in animals that received stimuli at ZIV than it was in animals that did not re ceive stimuli at this stage. When each stimulus was offered alone results w ere not equivalent to those when stimuli were offered in combination. The A tlantic population required presence of both stimuli at ZIV for settlement, while exposure of ZIV larvae to sand alone was sufficient to induce settle ment of postlarvae in the Gulf population.