Unravelling the diagnostic characters of the subadult male in Phronima atlantica (Amphipoda, Hyperiidea), with an overview of the genus distribution in the Mediterranean Sea

During growth, amphipods undergo successive moults accompanied by changes in body size and morphology. Several morphological features can be used as reliable indicators of the growth stage. However, the limited availability of information on stage-specific morphological variation may hinder accurate taxonomic identification, reducing knowledge of species distribution. In this paper, the diagnostic characters of the subadult stage of the male Phronima atlantica from the Aegean Sea, eastern Mediterranean Sea, were examined and compared to the mature stage. As a result of the comparison, all taxonomic diagnostic features of P. atlantica that differentiate the species from its congeneric species are confirmed as robust, except for the outer ramus of uropod 2 being twice as long as the inner ramus. Such analysis poses some questions about the distribution pattern of the species within the Mediterranean Sea, which could be underestimated. Moreover, the documentation of P. atlantica in this study validates the occurrence of the species in the Aegean Sea, following an absence of records in this region since 1924, probably due to misidentifications.