Evidence of self-organization in a gregarious land-dwelling crustacean (Isopoda: Oniscidea)

How individuals modulate their behavior according to social context is a major issue in the understanding of group initiation, group stability and the distribution of individuals. Herein, we investigated the mechanisms of aggregation behavior in Porcellio scaber, a terrestrial isopod member of the Oniscidea, a unique and common group of terrestrial crustaceans. We performed binary choice tests using shelters with a wide range of population densities (from 10 to 150 individuals). First, the observed collective choices of shelters strengthen the demonstration of a social inter-attraction in terrestrial isopods; especially, in less than 10 min, the aggregation reaches its maximal value, and in less than 100 s, the collective choice is made, i.e., one shelter is selected. In addition, the distribution of individuals shows the existence of (1) quorum rules, by which an aggregate cannot emerge under a threshold value of individuals, and (2) a maximum population size, which leads to a splitting of the populations. These collective results are in agreement with the individual's probability of joining and leaving an aggregate attesting to a greater attractiveness of the group to migrants and greater retention of conspecifics with group size. In this respect, we show that the emergence of aggregation in terrestrial isopods is based on amplification mechanisms. And lastly, our results indicate how local cues about the spatial organization of individuals may favor this emergence and how individuals spatiotemporally reorganize toward a compact form reducing the exchange with the environment. This study provides the first evidence of self-organization in a gregarious crustacean, similar as has been widely emphasized in gregarious insects and eusocial insects.