Nuclear following associations between Caranx crysos and two rays, Dasyatis pastinaca and Aetomylaeus bovinus, observed in Malta, Central Mediterranean

Abstract

An association of Blue runner, Caranx crysos (Carangidae), with two species of rays, Common stingray Dasyatis pastinaca (Dasyatidae) and Bull ray Aetomylaeus bovinus (Myliobatidae), in which an individual C. crysos (the follower fish) follows a foraging ray (the nuclear species), is described, based on a sample of 61 video clips. Follower fish show three types of behaviour: swimming under the pectoral fin of the nuclear ray, swimming above the mid-dorsal line of the nuclear ray and very close to it, or swimming above the ray and darting from side to side. The first two of these behaviours seem to be species-specific as the first is only shown when the nuclear ray is A. bovinus and the second when the nuclear ray is D. pastinaca. Following fish do not allow conspecifics to approach the nuclear ray but are mostly indifferent to non-conspecifics doing so. The following fish benefit from the association by feeding on prey disturbed by the foraging activities of the nuclear ray but the nuclear ray is unreactive to the following fish, suggesting that this is a case of trophic commensalism.