Morphology and Morphotypes of the Hawaiian River Shrimp, Macrobrachium grandimanus

The morphology and morphotypes of the Hawaiian river shrimp, Macrobrachium grandimanus Randall, 1840 (Caridea: Palaemonidae), are examined in this study. This species has a dense aggregation of setae (termed "setal patch") located on the major second cheliped and the function of this setal patch is unknown. Because previous research documented that this setal patch is not associated with grooming behaviors, a morphological study with detailed examination of the morphology of the second chelipeds was conducted using SEM and a dissecting microscope. Using morphometrics, three ontogenetic morphotypes within M. grandimanus were identified: females, small symmetrical males, and large asymmetrical males. There are morphometric differences and setation differences between small and large individuals as well as differences between the sexes. The setal patch is associated with large males, so while the function still remains unknown we present hypotheses about its function in behavioral and life history traits of these large males.