Allometric sexual dimorphism in the river crab Potamon fluviatile (Herbst, 1785) (Brachyura: Potamidae)

We studied the ontogenetic allometry pattern in the freshwater brachyuran crab Potamon fluviatile (Herbst, 1785) to describe some new sexual dimorphic features on the ontogenic trajectories of the chelae. We surveyed carapace width, chela length and width, dactylus length, and propodus length of both chelae of 93 females and 114 males to the nearest 0.05 mm using a vernier caliper. Our main result was the identification of morphological biometry showing a variability of the chela linked to growth allometry, with some different between-sex ontogenic trajectories. Specifically, we emphasized how the morphometric features of the chelae change during body-size growth with different increasing rates. Particularly, the observed significant shape changes are mainly due to positive allometries in both sexes, although negative allometries (the latter visible only in the propodus length of females) and isometries were observed as well. Our study confirms that a form of sexual dimorphism exists in the ontogenetic allometric trajectories of P. fluviatile in both the large and small chelae, these trajectories being related to mechanical aspects in predation, food manipulation, mate acquisition, and between-sex differences in aggressiveness during antagonistic fights.