Morphotypic differentiation of male Malaysian giant prawns, Macrobrachium rosenbergii

Mature male Macrobrachium rosenbergii differentiate into 3 distinct morphotypes differing in behavior and growth rate. We provide an operational definition of these morphotypes using relative growth techniques supplemented by observations on color and spination. Transition from the small male to the orange claw morphotype was gradual and a recognizable intermediate form, the weak orange claw, was identified and characterized. The shift from the orange claw to the blue claw morphotype occurred at a single metamorphic molt. Maturation from a juvenile male, lacking an appendix masculina, to the small male morphotype was not accompanied by an allometric change in the claw segments. Recognition of the morphotypes will be useful for genetic work using controlled matings between blue claw males and appropriate females and will facilitate experimentation on growth, behavior, and the evolutionary significance of morphotypic differentiation in M. rosenbergii. This analysis also implicates the androgenic gland in the control of growth rates of these prawns.