Sand-bubbler crabs distinguish fiddler crab signals to predict intruders

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. Significance statement By using field experiments, I demonstrated that sand-bubbler crabs distinguished the courtship and aggressive displays of fiddler crabs and showed evasive behaviours more frequently when they see the aggressive displays. The courtship displays of the fiddler crabs are more intensive than its aggressive displays; therefore, the reactions of the sand-bubbler crabs did not simply increase with the intensity of the performers’ displays. The displays of fiddler crabs were performed to other crabs (male or female fiddler crab, or a predatory crab) but not to the sand-bubbler crab; therefore, the sand bubbler crabs may have eavesdropped on heterospecific signals and utilised the heterospecific signals to predict the risk level of approaching intruders. To the best of my knowledge, d