Toru Moriyama

Emergent Runaway into an Avoidance Area in a Swarm of Soldier Crabs

Emergent behavior that arises from a mass effect is one of the most striking aspects of collective animal groups. Investigating such behavior would be important in order to understand how individuals interact with their neighbors. Although there are many experiments that have used collective animals to investigate social learning or conflict between individuals and society such as that between a fish and a school, reports on mass effects are rare. In this study, we show that a swarm of soldier crabs could spontaneously enter a water pool, which are usually avoided, by forming densely populated part of a swarm at the edge of the water pool. Moreover, we show that the observed behavior can be explained by the model of collective behavior based on inherent noise that is individuals’ different velocities in a directed group. Our results suggest that inherent noise, which is widely seen in collective animals, can contribute to formation and/or maintenance of a swarm and that the dense swarm can enter the pool by means of enhanced inherent noise.

Information transfer in a swarm of soldier crabs

Collective behavior is broadly observed in animal groups such as insect swarm, bird flock, and fish school. Both theoretical studies and field observations have investigated possible underlying principles based on local interaction among individuals in a group without global information via conductors or leaders. Information transferred among individuals would play a key role to understand it. In this study, to investigate how individual in a swarm uses information of its own past behavior or swarm mates’ behavior, we analyzed behavior of soldier crabs Mictyris guinotae in terms of local active information storage and local transfer entropy.

Self-corrective behavior for turn alternation in pill bugs (Armadillidium vulgare).

Pill bugs (Armadillidium vulgare) demonstrate a behavior called turn alternation that keeps their overall direction of movement straight after obstacles in experimental settings force them to deviate from a course. For example, this behavior is seen when they alternate their path choice on successive trials of the T-maze test. However, sometimes pill bugs stop after turning and change their direction (directional change). The function of this directional change has not been investigated because such individuals are usually omitted from the data. The present paper shows that pill bugs use directional changes to prevent them from turning in the same direction on two successive turns, a behavior called turn repetition. We examined the behavior of 36 pill bugs that each completed 130 successive T-maze trials. Directional changes appeared more frequently when individuals had begun a turn repetition than when they had begun a turn alternation. Furthermore, after correcting for turn repetition, turn alternations increased. These results suggest that pill bugs have an inherent mechanism that acts to maintain turn-alternating behavior.

Behavioral pattern of pill bugs revealed in virtually infinite multiple T-maze

A behavior called turn alternation has been studied extensively in terrestrial isopods. This behavior is seen when they alternate their path choice on successive trials of the T-maze test. We made the multiple T-maze device which consists of two turntables with a T-maze mounted on each and examined the behavior of 36 pill bugs (Armadillidium vulgare) that each completed 130 successive T-maze trials. As a result, in addition to turn alternation, turn repetition (turning in the same direction on two successive turns) appeared at a rate of 20%. In the turn sequences, we observed segments consisting of successive turn alternations and defined the number of turn alternations in a segment as the length of it. Cumulative frequency distribution of segment lengths obeyed power law with exponent of 1.76. This result suggests that pill bugs in the multiple T-maze device behaved as Lévy walkers which forage in an environment, where resources are unpredictably distributed.

Circatidal activity rhythms in the soldier crab Mictyris guinotae

Abstract Mictyris guinotae is endemic to the Ryukyu Islands, Japan. During low tide, the crabs emerge onto the tidal flat to feed, and then burrow into the sand before the incoming tide. They feed in droves during daytime, but separately at night. Under constant conditions without sand sediment, crabs exhibited a bimodal daily activity pattern, with a free-running period of ~12.8 h, comprising an active phase of ~11 h alternating with a resting phase of ~1 h, and a lag of ~3 h between the activity peak and low tide. Crabs were more active during the notional night-time than during the notional daytime. In crabs placed in an arena with sand sediment, a free-running period of ~12.8 h comprised a surface-active phase of ~3 h and a subsurface resting phase of ~9 h, with a lag of 1.5 h. In contrast to the non-sand condition, more crabs were active during daytime than during night-time. Thus, M. guinotae possesses circatidal and circadian locomotor rhythms that are modified by the sediment.

Visual image of neighbors to elicit wandering behavior in the soldier crab

The soldier crab appears in great numbers and feeds while wandering during daytime low tide. When they see an approaching object, they screw themselves into the sand. The mechanism of formation of mass wandering has not been clarified. In this study, to investigate if the soldier crabs use visual images of neighbors as a stimulus for wandering, dummy crabs were presented to crabs. In the experiments, one, two, four, or eight dummies were placed in a circle on a sand arena. Each crab was placed in the center of the arena and observed whether it burrowed into the sand or wandered. The proportions of wandering individuals in each experimental treatment were compared with the expected value. Significantly more crabs were wanderers when only two and four dummies were present. This result suggests that soldier crabs chose burrowing or wandering depending on visual image of the distribution of the neighbors.

Can Hermit Crabs Perceive Affordance for Aperture Crossing

An animal’s perception of its body size is modified when it adapts its body to changes in a complex environment. This ability is essential for animals that use tools or cross apertures. Here, we show that terrestrial hermit crabs, Coenobita rugosus, which frequently change shells, can perceive the width of the aperture to be crossed, dependent on the shape of their shells. Hermit crabs walked in a corridor that had two different size apertures; most of the crabs with a large shell did not cross the narrow aperture, indicating an awareness of aperture width. Moreover, most of the crabs with a small shell with an attachment did not select the narrow aperture, either. These results are the first demonstration of animals perceiving affordance while carrying objects.