Pre-juvenile and first juvenile stages of the freshwater crab Fredius reflexifrons (Ortmann, 1897) (Crustacea: Decapoda: Pseudothelphusidae)

Morphological descriptions of juveniles in Crustacea serve as a guide for identifying species during this stage, thus increasing our understanding of life cycle strategies and potential adaptations to the environment. In addition, these studies can facilitate the elucidation of phylogenetic relationships within taxa. Pseudothelphusidae Ortmann, 1893 is the family with the largest number of species among the neotropical freshwater Brachyura, but descriptions of juvenile forms in freshwater crabs are still rare.

Notably, only a few species within the Trichodactylidae, such as Dilocarcinus pagei Stimpson, 1861, Dilocarcinus septemdentatus (Herbst, 1783) and Trichodactylus sp., have been documented with descriptions of their juvenile forms. In this study, we present a comprehensive description of the pre-juvenile and first juvenile stages of the pseudothelphusid crab Fredius reflexifrons (Ortmann, 1897), offering a new contribution to the field.

We also describe the characteristics that allow us to differentiate F. reflexifrons juveniles from other species of freshwater crab juveniles. Pre-juveniles are characterized by the absence of calcification and the presence of nonfunctional appendages. The distinctive feature of pseudothelphusid crabs, a reduced exopodite of the third maxilliped, is already evident in the first juvenile stage.