Male morphotypes and alternative mating tactics in freshwater prawns of the genus Macrobrachium: a review

The discontinuous variation in male mating behaviour is termed alternative mating tactics. A distinction is commonly made between alternative mating tactics that are fixed for life or flexible with simultaneous or sequential switches between tactics over the sexually mature life. Freshwater species of the genus Macrobrachium demonstrate flexible tactics with sequential (i.e. developmental) switch between tactics. Sexually mature populations of the commercially important giant freshwater prawn M. rosenbergii consist of three morphologically and behaviourally distinct male morphotypes: small males, orange claw and blue claw males, representing (respectively) successive stages in the developmental pathway of sexually mature males. The dominant blue claw males secure mating by guarding receptive soft-shelled females with their long chelipeds while the subordinate SM use an alternative tactic of sneak mating to fertilize females. Current knowledge of the mating behaviour and reproductive success of these male morphotypes, including male competition over females, female mate choice and the involvement of pheromones, is reviewed. So far, the occurrence of distinct sexually mature male morphotypes has been described in 24 species within the genus Macrobrachium. The prevalence of distinct male morphotypes and alternative mating tactics among Macrobrachium species is discussed in view of the phylogeny of the genus, based on molecular markers. The environmental and genetic determination of the male morphotypes is discussed, as well as the evolution of the developmental alternative mating tactics in the genus Macrobrachium. Possible implications for prawns' aquaculture and future research are suggested.