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

Abstract

The Japanese spider crab, Macrocheira kaempferi (Temminck, 1836), the largest living arthropod in the world, was originally placed in the family Majidae Samouelle, 1819 as a monotypic subfamily, Macrocheirinae Dana, 1851. 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 MacLeay, 1838 (superfamily Majoidea Samouelle, 1819). 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 Haan, 1839 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 Garth, 1958 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.

A new subfamily classification of the highly diversified Dorippidae H. Milne Edwards, 1837 (Crustacea, Decapoda, Brachyura, Dorippoidea), using morphological, molecular and palaeotonlogical data, with special emphasis on its unique female reproductive system

“…Based on the brachyuran fossil material studied at hand, the current view is that the earliest Eubrachyura (heterotremes) did not appear until the Early Cretaceous, but there are arguments for dating the first 'true crabs' to the Jurassic. This is what is suggested in our recent article (Guinot 2019), admittedly provocative and containing assumptions that will certainly prove false, but hopefully not in vain. For example, for the Late Jurassic Lecythocaridae Schweitzer & Feldmann, 2009, which do not appear to be podotreme crabs as currently believed, there is conclusive evidence to advocate for their eubrachyuran, probably majoid, affiliation, implying that the evolutionary history of Brachyura started much earlier than assumed (Guinot et al 2019: fig.…”

“…The evolutionary history of the Dorippoidea has been documented by fossils since the Early Cretaceous, and this early occurrence had confirmed that they were, with the Ma- Guinot D. joidea, the earliest splitting brachyuran branches (Luque 2015; Guinot et al 2013Guinot et al , 2019Charbonnier et al 2017;Guinot 2019;Vega et al 2019;Van Bakel et al 2020). Our view that Hymenosomatoidea is even more basal in Eubrachyura, with its relationship to Dorippoidea (Guinot 2011) apparently supported by consistent data sets, will be discussed in light of the recent discovery of even older hymenosomatoid fossil representatives, namely in the Barremian (Mendes et al 2022).…”

“…However, leading scientists, not least, have recognised the priority of H. Milne Edwards: De Haan (1841: 119, 120) who writes "Dorippidea, Edwards"; Alcock (1896: 273) who writes "Dorippiens, Milne Edwards"; Miers (1886: 326) who first cites "Dorippiens H. Milne Edwards, 1837"; Chen & Sun (2002: 208) who first cite "Dorypiens" and " Dorippiens" of H. Milne Edwards;and Holthuis & Manning (1990: 5) first mentioning the name of H. Milne Edwards for the family Dorippidae but explicitly regarding "Doryppiens" and "Dorippiens" as French vernacular names. For most carcinologists including Guinot et al (2013Guinot et al ( , 2019 and Ng et al (2008: 59), although the priority of H. Milne Edwards is implicitly or explicitly acknowledged, the authorship is commonly attributed to MacLeay, 1838(Davie 2002Poore 2004: 324), or sometimes to De Haan, 1833(Guinot 1977, 1978, or to Dana, 1852(Ortmann 1892Rathbun 1937: 75).…”

“…Such a rim all along the wide, concave posterior margin of the carapace is present in podotreme crabs, namely in the extant Cyclodorippoidea Ortmann, 1892, but this without lateral extension: for example, in the cyclodorippine Tymolus japonicus Stimpson, 1858 and in the xeinostomine Xeinostoma eucheir Stebbing, 1920, Krangalangia rostrata (Ihle, 1916 The dorippid strip is unrelated to the flange present in various fossils, notably in the Cretaceous Archaeochiapasidae Guinot, Carbot-Chanona & Vega, 2019(Guinot et al 2019, in which the lateroposterior and posterior regions of the carapace are expanded, forming a flange all around, i.e., a narrow, continuous, undivided, channel-like depressed area, bordered by a deeply concave, strongly rimmed and raised posterior margin. It seems premature to recognise as homologous all the rims and flanges found in various Jurassic and Cretaceous families.…”

“…Such a position contrasted with a larger multigene study by Tsang et al (2014) that returned Dorippidae as sister to Leucosiidae Samouelle, 1819, at some distance from Majidae Samouelle, 1819. The basal position of Dorippidae, long based on morphology (Guinot 1978;Guinot et al 2008Guinot et al , 2013Guinot et al , 2019, is recognised by the latest molecular phylogenetic analyses (Chu et al 2015; see also Chu et al 2009a), which place the family among the most basal heterotremes (see also Bracken et al 2009). However, this basal position is not supported by the genetic results obtained by Wolfe et al (2022, pers.…”