The Second Antenna of Caridean Shrimps: An Approach on the Variation of Flagellar Setae among Species with Different Mating Systems

The setae on the antennae of caridean shrimps perform chemoreceptive functions, including the identification of potential mating partners. Variation in the number of setae on the antennal flagellum has been reported as a sexually dimorphic trait in these organisms. This difference is correlated with the distinct mating systems and reproductive behaviors of these animals. This study investigates the variation in the number of antennal flagellar setae in four species of shrimps from the genus Macrobrachium, which are grouped into two distinct mating systems. Two species with a pure search mating system (M. jelskii and M. pantanalense) and two with a neighborhoods of dominance mating system (M. acanthurus and M. olfersii) were analyzed. The setae were quantified using light microscopy, morphologically characterized through scanning electron microscopy, and then compared among the demographic groups of each species. The results show that in the pure search species, males possess a significantly higher number of setae than females (P < 0.01). In the neighborhoods of dominance species, dominant males of M. acanthurus and M. olfersii had more setae than subordinate males and females (P < 0.01). The same types of setae, denticulate and simple, were found in all species. We conclude that the variation in the number of flagellar setae is correlated with the mating system of each species and their specific reproductive behaviors, suggesting a morphological adaptation to optimize chemoreception in different reproductive events. These findings contribute to the understanding of reproductive strategies and sensory adaptations in crustaceans.