Current status of Stenothoe gallensis (Amphipoda, Stenothoidae): Towards the re-appraisal of the species complex with new records from Greece and challenges arising from intraspecific morphological variation.

Abstract

Background: Stenothoe gallensis Walker, 1904 is one of the most widely reported species, with records from various localities across the globe, including the Indian, the Pacific and the Atlantic Oceans. Nevertheless its cosmopolitan distribution has increasingly been questioned and, in recent years, the taxon has been established as a species complex whose members are recognised for their invasion potential.

New information: This study revealed the presence of specimens belonging to the S. gallensis complex, which closely matched the original description of Stenothoe irakiensis Salman, 1985. This species is currently considered a junior synonym of S. gallensis. A thorough examination of key linear and meristic morphological characters across sexes and maturity stages revealed significant intraspecific variation in several characters commonly used for species diagnosis. This size-, sex- and maturity-dependent variation within the population, when combined with incomplete or conflicting species descriptions from different localities, introduces uncertainty regarding the reliability of the diagnostic features that were used to identify members of the Stenothoe gallensis complex. This, in turn, complicates the interpretation of previous records and the delimitation of taxa within the complex. These findings provide a compelling rationale for a comprehensive re-assessment of records attributed to the S. gallensis complex, underscoring the necessity for an integrative taxonomic revision incorportating both morphological and molecular data from material collected at the designated type localities.