Gaps in Bioinvasion Phenomenon Studies: The First Occurrence of Serejohyale spinidactyloides (Schellenberg, 1939) (Crustacea: Amphipoda) in the Mediterranean Sea Raises Some Issues on the Taxonomic Identifications Within the Genus

Reliable identification of non-indigenous species (NIS) is essential for preventing and managing biological invasions. In marine environments, this process is often hampered by limited research on small-sized taxa and a decline in taxonomic expertise. Misidentifications have been reported for decades and continue to occur in certain invertebrate groups. This study highlights this issue within an amphipod genus currently invading the Mediterranean Sea. The Atlantic benthic amphipod Serejohyale spinidactyloides (Schellenberg, 1939) was sampled between 2016 and 2017 at two Sicilian intertidal sites: Stagnone di Marsala Lagoon and Altavilla Milicia (southern Italy, central Mediterranean). Both sites were characterized by mytilid beds of the Lessepsian mussel Brachidontes pharaonis (P. Fischer, 1870). The finding represents the first record of both the genus and the species in Italian waters, and the second record of the genus in the Mediterranean since 2019 (Bizerte Lagoon, Tunisia). We provided a redescription of S. spinidactyloides based on the Sicilian specimens and the holotype from the Museum für Naturkunde, Berlin. Diagnostic characters of congeneric Serejohyale species were reassessed to improve future monitoring of NIS. Based on growth-related morphological changes, we suggest a synonymy between Serejohyale ramalhoi (Reid, 1939) and Serejohyale spinidactylus (Chevreux, 1926), and propose an updated Atlanto-Mediterranean distribution for the genus.