The cause and consequence of ontogenetic changes in social aggregation in New Zealand spiny lobsters

Ontogenetic changes in the behavior, spatial distribution, or habitat use o f a species are presumably adaptations to ecological forces that differ in their effect on various life stages. The New Zealand rock lobster Jasus edw ardsii is one of several species of spiny lobster that exhibits dramatic on togenetic shifts in sociality and spatial distribution, and we tested wheth er such changes are adaptive. We first surveyed several natural populations of J. edwardsii to document size-specific differences in aggregation. To d etermine if chemical cues discharged by conspecifics promote aggregation of certain ontogenetic stages, we tested the responsiveness of lobsters of 3 ontogenetic stages (early benthic juvenile, juvenile, and subadult) to the chemical cues produced by conspecifics of different sizes. Finally, we teth ered lobsters of different ontogenetic stages alone and in groups to test t he effect of lobster size and aggregation on mortality. Our results offer c ompelling evidence that pre-reproductive J. edwardsii undergo an ontogeneti c change in sociality that alters their spatial distribution and survival. Our field surveys show that J. edwardsii are solitary as early benthic juve niles and become social and aggregate as they grow larger. We then demonstr ate, using laboratory experiments, that there is a size-specific increase i n the response of pre-reproductive J. edwardsii to the chemical cues of lar ger conspecifics which facilitates these ontogenetic changes in aggregation . Finally, our tethering results confirm that this change in social conditi on is selectively advantageous: aggregation does not increase the survival of small lobsters, but larger lobsters survive better in groups. Thus, in t his study we demonstrate the linkage between ontogenetic changes in the spa tial distribution of a species, the behavioral process that creates the pat tern, and the selective advantage conferred by these developmental changes.