Running title: TAXONOMIC DISCRIMINATION OF Litopenaeus SPECIES Academy Section: ECOSYSTEMS

The shape of the secondary sexual characters is, traditionally, used to discriminate the marine shrimps at the species level. However, the qualitative evaluation of structures that are morphologically variable in the taxonomy of species can favor misunderstandings and misidentifications. These possibilities of taxonomic inaccuracies are especially alarming when there is a need to evaluate the invasion of introduced species. The present study used geometric morphometric analyses to identify differences in the cephalothorax shape that would help discriminate the native and non-native species of Litopenaeus of the South American coast. The comparative morphology analysis was conducted using the right profile of adult males’ cephalothorax of L. schmitti, captured in the natural environment, and L. vannamei captured in the natural environment or grown in shrimp farms. In intraspecific evaluation, it was not possible to distinguish the specimens of L. vannamei that were grown in shrimp farms from those acclimated to the natural environment. However, significant interspecific differences in shape were found in the shape of this body structure. Additionally, the base position of the first rostral spine to the tip of the hepatic spine is indicated as a characteristic that can be used to distinguish these two species by eye in the field.