Morphological variations in relation to sex and habitat in southern Mexico populations of the longarm river prawn <i>Macrobrachium tenellum</i> (Smith, 1871) (Decapoda: Caridea: Palaemonidae)

Abstract The longarm river prawn, Macrobrachium tenellum (Smith, 1871), is an amphidromous species inhabiting rivers, estuaries, and coastal lagoons along the Pacific slope of the Western Hemisphere, having morphological differences depending on habitat. The species has an important ecological role, playing a crucial position in coastal-river equilibrium. There is nevertheless scarce information on morphological variations among specimens, which are influenced by factors such as sex and habitat. We investigated whether it is possible to detect such differences in the carapace and second pleonal pleuron between sexes and among three habitats (river, estuary, and lagoon) by using geometric morphometrics. Our results suggest that the size and shape of the carapace and pleura can effectively be distinguished between sexes based on morphological criteria. Significant differences in carapace size and shape were also detected across habitats, with less evident variations in the pleura. The findings suggest that rivers provide a suitable habitat for adult females, having a significantly larger carapace and second pleonal pleuron in this habitat. We identified three distinct ecophenotypes in females corresponding to the varying habitats: 1) river specimens possessing a robust carapace with a short, marked upcurved rostrum; 2) estuarine specimens exhibiting broader dorsal and antero-inferior regions with a thicker rostrum; 3) lagoon specimens having a slender carapace with a long, thin, upward-sloping rostrum. These variations are likely a result of indeterminate growth and adaptations to the specific hydrodynamics and substrates present in each habitat. This information is useful to understand how the environment has a significant effect on the shape and size of M. tenellum and provide useful insights for future research on the species of Macrobrachium.