Settlement cues in successive developmental stages of the ghost shrimps Callichirus major and C-islagrande (Crustacea : Decapoda : Thalassinidea)

Larvae of Callichirus major (Say) and C. islagrande (Schmitt) were given un conditioned water with sand, water conditioned by previous exposure to adul ts (AW) without sand, or the combination of both AW and sand to determine i f these factors served as settlement cues. Shrimp were first given stimuli upon reaching the last zoeal stage (ZIV), the decapodid stage (D), or the f irst juvenile stage (J1). Once initiated, exposure of shrimp to stimuli was maintained until termination of the experiment. Stage duration, burrowing activity of decapodids and early juveniles and location of exuviae were obs erved daily. Exposure to sand and AW did not decrease duration of the decap odid stage in either species. However duration of ZIV in C. major appeared to decrease when these stimuli were offered at this stage; this suggests th at molting to Stage D may be delayed without such cues. When stimuli were e xamined separately, AW was found to have a significant effect on the durati on of ZIV, while the effect of sand with unconditioned water and effects of simultaneously offered stimuli were not significant. Shrimp that received stimuli at ZIV subsequently burrowed more at D and J1 than individuals that did not receive stimuli at ZIV. Location of exuviae was recorded as on top of sand (not burrowed during molt) or within sand (burrowed during molt) f or each shrimp's molt from Stages D to J1 (D molt), and from J1 to J2 (J1 m olt). In C. major, the percentage of D exuviae and J1 exuviae found within sand as opposed to on top of sand was significantly higher in individuals w hich received stimuli at ZIV than for those that first received them at D o r J1. When stimuli were examined separately in C. major, the effect of sand on burrowing activity was highly significant, while AW and the stimuli in combination had weaker effects. For C. major, AW appeared to accelerate dev elopment to D, while sand triggered settlement. Neither stimulus altered st age duration or burrowing activity of C. islagrande.