Collective behavior of soldier crab swarm in both ring- and round-shaped arenas

In study of collective behavior, though collective foraging behavior against surroundings is one of the most vivid examples about how animal group responses to external environment, relative experiment is rare. In this paper, we show an experiment with respect to collective foraging using a swarm of soldier crabs Mictyris guinotae, which live in the tideland and can form large swarms. We recorded the behaviors of soldier crab swarms with 10, 20, 30, 40 individuals in experimental arenas. Thanks to markers attached to crabs’ shells and image-processing software, we obtained time series of individuals’ position during 30 min. As a consequence, we found the soldier crabs form densely swarm to some extent in the ring-shaped experimental arena. Moreover, when we calculated the time between direction changes of the swarm, we found that it followed power-law distribution, containing very long moves. Such movement pattern is frequently found in foraging behavior of single animals. Our results, therefore, suggest that collective swarms show a type of foraging behavior as a single group.