Morphological and genetic evidence supports that invasive Carcinus Leach, 1814 in South Australia are C. maenas (Linnaeus, 1758)

Accurate identification of invasive marine species is essential for understanding species range expansions and developing taxon-specific surveillance tools. The globally invasive European green crab Carcinus maenas, and the Mediterranean green crab C. aestuarii, which has a more limited introduced range and is a lesser concern as a pest, can be difficult to distinguish morphologically and can also hybridise. Although the species are visually similar, it has been proposed that they can be distinguished using carapace morphometrics. In temperate South Australia, invasive Carcinus have been reported as C. maenas. This study aimed to confirm the identity of invasive Carcinus in South Australia using carapace morphometrics and mtDNA COI gene sequencing. Carapace width:length ratios of 1,311 Carcinus from South Australia were variable within sexes and corresponded to those of both C. maenas and C. aestuarii. However, COI sequencing of 59 of these crabs revealed ~ 100% sequence matches to C. maenas and diverged from C. aestuarii sequences on GenBank by an average of 11.5%. Results supported that the South Australian population is C. maenas and that intraspecific morphological variation prevents carapace ratios alone from distinguishing species of Carcinus. DNA sequencing should be used to confirm the species identity of crabs identified as Carcinus during new incursions.