Sand-bubbler crabs distinguish fiddler crab signals to predict intruders

Abstract

There is growing evidence that animals gain information from heterospecifics, but utilising other species’ signals has rarely been reported in invertebrates. Herein, I conducted field experiments in a mixed colony of two crab species, Scopimera globosa and Austruca lactea, to test whether S. globosa discriminate several different displays of neighbouring male A. lactea. Three types of ‘intruders’ (male or female A. lactea, or a predatory crab, Helicana japonica) were exhibited to the ‘performer’ (male A. lactea) to elicit four different actions (courtship, aggressive, or defensive displays, or evasive behaviours). The actions of the performer, but not the presence of the intruder, were exhibited to the ‘audience’ (male A. lactea or S. globosa); the behaviours of the audience were recorded to analyse whether the audience could take advantage of the actions of neighbours. The evasive behaviour of performers elicited similar rates of evasive behaviours from conspecific and heterospecific audiences. Furthermore, courtship and aggressive displays of performers elicited significantly different rates of evasive behaviours from heterospecifics. The courtship displays of male A. lactea are more intensive than its aggressive displays; therefore, the reactions of S. globosa did not simply increase with the intensity of the performers’ displays. Presumably, S. globosa recognises that courtship displays by A. lactea do not indicate undesirable circumstances. Altogether, S. globosa were able to distinguish two types of displays as well as evasive behaviours of male A. lactea, an ability that may contribute to their estimation of the risk level of approaching intruders.