Unravelling the identity of Phronima atlantica (Amphipoda, Hyperiidea), diagnostic characters of the subadult male and its overlooked 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 on 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 one, the outer ramus of uropod 2 being twice as long as inner ramus. Such analysis poses some questions on the distribution pattern of the species within the Mediterranean Sea, which could be underestimated. Furthermore, the records of P. atlantica in this paper confirm the presence of the species in the Aegean Sea after no records in this area since 1924, possibly because the species was being misidentified.