Den sharing by juvenile Caribbean spiny lobsters (Panulirus argus) in nursery habitat: cooperation or coincidence?

Spiny lobsters (Palinuridae) commonly share crevice shelters with conspecifics, a behaviour usually hypothesized to benefit mutually cooperative defenders through reduced predation risk. The group-defence hypothesis predicts a suite of distinct corollary life history traits and ecological correlates including more frequent co-denning than solitary denning, especially where predators or den competitors are numerous and when the lobsters are small, moulting or otherwise vulnerable. Two alternative co-denning hypotheses, the dilution effect (a type of selfish herding) and the guide effect (attraction to a denned conspecific), have different and distinctive sets of predictions. To test among these hypotheses, the den-sharing patterns of newly emerged postalgal juveniles of Panulirus argus and associated ecological conditions in the Florida Bay (USA) nursery habitat were examined. Only half of the juveniles shared dens, and rarely was den sharing in an area greater than that expected by chance. Den-sharing frequency was most highly correlated with conspecific density and scarcity of local dens. The lack of correlation between den sharing and lobster size, moult condition, shelter type and predator density failed to support either the group-defence or the dilution-effect hypothesis. Instead, the data better support the guide-effect hypothesis.