Sexual shape and size dimorphism in Norway lobster (Nephrops norvegicus) from the Adriatic Sea

Norway lobster (Nephrops norvegicus) is a highly valuable crustacean, distributed in the eastern Atlantic and the Mediterranean Sea. Most stocks are currently overexploited and have declined over the last ten years. While important parameters for stock assessment are well established, research into the morphology of Nephrops is still scarce, despite its relevance to foraging, behaviour and other life history traits. This study addresses this gap by investigating the phenotypic traits of Nephrops in the Adriatic Sea, focusing on sexual shape and size dimorphism of the cephalothorax and median tooth. Geometric morphometrics analysis revealed that males exhibited a longer thorax while females displayed a larger cephalus. Regional differences in the cephalothorax shape were observed, with females from the southern and northern Adriatic as the primary contributors. Morphological variation in shape of the median tooth was also detected, particularly between individuals from channel areas and those from the open Adriatic Sea. Despite consistent male-biased sexual size dimorphism across populations, an inverse allometric pattern was observed, with dimorphism decreasing at larger body sizes, contradicting Rensch's rule. These morphological outcomes suggest a complex interplay of natural and anthropogenic factors shaping sexual dimorphism and phenotypic divergence in Adriatic Nephrops populations.