Formal re-establishment of Macrocheiridae Dana, 1851 (Decapoda: Brachyura: Majoidea) for the giant spider crab Macrocheira kaempferi (Temminck, 1836) based on a reappraisal of morphological and genetic characters

The Japanese spider crab, Macrocheira kaempferi (), the largest living arthropod in the world, was originally placed in the family Majidae as a monotypic subfamily, Macrocheirinae . While various authors have treated the taxon as belonging to its own superfamily, family, subfamily, or even tribe, the consensus of modern taxonomic system has for the most part considered it to be simply a genus within Inachidae (superfamily Majoidea ). Strangely, however, considering its size and fame, its adult morphology has remained poorly studied. Recent larval and genetic studies have raised questions regarding its suprageneric status, suggesting an independent evolutionary position within Majoidea and the recognition of a separate family. We examined and compared a large suite of adult characters in Macrocheira De against other potentially related majoid groups, and found unique characters that support the larval evidence for monophyly of the genus. It is thus necessary to re-establish a monotypic Macrocheiridae for Macrocheira. A molecular phylogeny reconstructed from mitochondrial 16S rRNA gene is presented in further support of this conclusion, showing Macrocheira as sister to Oregoniidae and distant from Inachidae. A new diagnosis for the family is provided. Macrocheiridae differs from the morphologically most similar families, Inachidae and Oregoniidae, by characters associated with the carapace, thoracic sternum, male pleon, gonopod, and vulvae.