Caridean prawns show five different types of mating systems and may show specific morphological characters depending on the reproductive strategy, resulting in sexual dimorphism between males and females. The present study aimed to analyse the sexual dimorphism in the freshwater prawns Macrobrachium pantanalense and Macrobrachium iheringi , two species that presumably have different mating systems. Our predictions are that the pattern of sexual dimorphism shows differences between the species, which may be related to its mating system and sexual behavior. Sexual dimorphism was analysed by geometric morphometrics using the carapace (CA), second pleonal pleuron (SPP) and cheliped propodus (CP). The geometric morphometrics analysis showed differences between the sexes in the shape of CA and SPP of M. pantanalense . In M. iheringi , differences between sexes were observed only in the shape of SPP. The sexual dimorphism in the size of CA and CP diverged between the species, while in M. pantanalense the females showed the largest sizes, in M. iheringi the males showed the largest sizes. The patterns observed between size and shape of structures in each species reinforce the presence of relationships between mating system and sexual dimorphism in caridean prawn species. http://zoobank.org/urn:lsid:zoobank.org:pub:03DE0D3E‐598A‐4692‐9771‐79EA449D92EA

The main objective of this study was to estimate the morphological sexual maturity of Macrobrachium brasiliense (Heller, 1862) and to analyze the relative growth of the species. During one year of sampling, from July 2012 to June 2013, 199 specimens were collected in a stream located in a Cerrado biome, in the Triângulo Mineiro region, state of Minas Gerais. Once identified and sexed, the length of the carapace, the length of the segments of the chelipeds (ischium, merus, carpus, propodus and dactyl) and width of the pleura were measured (mm). The maximum and mean sizes of the carapace length measured 20.5 mm (Mean: 9.6 ± 4.2 mm) and 20.1 mm (Mean: 7.7 ± 3.4 mm) for males and females, respectively. The propodus length for males, and the pleura width for females, were used for the classification of the specimens into juvenile and adult, using K-means analysis and discriminant analysis. The relative growth of the species was also evaluated through covariance analysis for all structures (ANCOVA, α = 0.05). In females, the growth of most structures occurred equally between juveniles and adults, with the exception of the merus and pleura structures. On the other hand, the growth of almost all structures of males differed between juveniles and adults. Only the carpus and the pleura growth were similar. We also estimated the size at onset of morphological sexual maturity at 8.64 mm CL for males (CL_50% r = 1.71) and 8.03 mm CL for females (CL_50% r = 0.07). Our results contribute to the understanding of some important questions related to the reproductive biology of M. brasiliense. We noted that males grow differently from females and become the largest individuals in the population, resulting in a sexual dimorphism. Such sexual dimorphism might promote the development of the temporary mating guarding behavior, a reproductive strategy very important for caridean shrimps.

During the ontogenetic development of crustaceans, the relative growth of some structures may change, especially during the transition from juvenile to adult. This study describes the relative growth of body structures of Macrobrachium iheringi, and provides information on its population dynamics, such as structure, fecundity, and morphological sexual maturity. The sampling of M. iheringi was carried out in “Ribeirão da Hortelã”, in Botucatu (SP, Brazil). The length of the carapace (CL), abdomen (AL), and ischium (IL), merus (ML), carpus (CrL), propodus (PpL), and dactyl (DcL) of the second right pereopod were measured. In addition, the width of the second abdominal pleura (PW) and propodus height (PpH) were included in analyses. The relationships that best demonstrated the changes in the allometric coefficient were CL vs PpL in males and females. The CL, in which males and females reach morphological sexual maturity, was estimated as 13.3 mm and 11.1 mm, respectively. The sex-ratio differed from the expected 1:1 and was skewed towards females. Precipitation and temperature influenced the abundance of different demographic classes. Macrobrachium iheringi has few but large eggs, which is expected since this species has an abbreviated larval development. Based on these results, we conclude that the propodus are good indicators of the size at onset of morphological sexual maturity. In addition, important information was obtained on the biology of M. iheringi, including its life cycle pattern, reproduction and influence of abiotic factors.

Animal weapons are generally complex systems composed of more than one structure. A crab’s claw, for instance, is composed of a dactyl (a movable finger) and a propodus (where the muscle resides). Any weapon feature that increases winning probability also increases an individual’s fitness, meaning that all moving parts of a weapon will be under the same selective force—increasing their integration. By comparing weapons to homologous structures not used for fighting, we can test whether fighting increases morphological integration. Here, we tested that hypothesis by comparing the right (non-fighting) and left (fighting) claws of males of Aegla. First, we tested if the shape and size of the propodus and dactyl differed between claws. Then, we compared the degree of morphological integration. We showed that claws differed in their shape and size, but both claws showed similarly high levels of integration. This pattern can occur if selection favours an adequate squeeze on both claws, even though squeezing performance might differ between the claws. Nevertheless, the dactyl is important in determining the integration of a claw but it has been greatly ignored in crustacean studies and its study might increase our knowledge of claw evolution.