Intraspecific group hunting has received considerable attention because of the close links between cooperative behaviour and its cognitive demands. Accordingly, comparisons between species have focused on behaviours that can potentially distinguish between the different levels of cognitive complexity involved, such as "intentional" communication between partners in order to initiate a joint hunt, the adoption of different roles during a joint hunt (whether consistently or alternately), and the level of food sharing following a successful hunt. Here we report field observations from the Red Sea on the highly coordinated and communicative interspecific hunting between the grouper, Plectropomus pessuliferus, and the giant moray eel, Gymnothorax javanicus. We provide evidence of the following: (1) associations are nonrandom, (2) groupers signal to moray eels in order to initiate joint searching and recruit moray eels to prey hiding places, (3) signalling is dependent on grouper hunger level, and (4) both partners benefit from the association. The benefits of joint hunting appear to be due to complementary hunting skills, reflecting the evolved strategies of each species, rather than individual role specialisation during joint hunts. In addition, the partner species that catches a prey item swallows it whole immediately, making aggressive monopolisation of a carcass impossible. We propose that the potential for monopolisation of carcasses by one partner species represents the main constraint on the evolution of interspecific cooperative hunting for most potentially suitable predator combinations.