Majidae Samouelle, 1819

Marin, Ivan N., Spiridonov, Vassily A. & Ng, Peter K. L., 2019, Ophiacantha scissionis Lee & Stöhr & Bae & Shin 2019, sp. nov., Zoological Studies 58 (26), pp. 1-11 : 2-3

Family Majidae Samouelle, 1819 Subfamily Planoterginae Števčić, 1991

Remarks: Serène (1965) discussed at length that Planotergum has morphological affinities with the eastern American mithracine genus Hemus A. Milne- Edwards, 1875, and the two genera may be closely related ( Števčić 1991; Garth 1958; Hendrickx 1999). Planotergum , like Hemus , has a relatively broad basal antennal article (like other mithracines) and shares with Hemus not only a similar carapace and short, hooklike ambulatory pereiopods, but also relatively delicate chelipeds that are barely visible in dorsal view, and the prominently foliaceous third antennal article visible in dorsal view. On the basis of these features, Števćić (1991) established Planoterginae as a monotypic subfamily within Majidae sensu lato. Ng et al. (2008) redefined Majidae and recognised four subfamilies: Majinae s. str., Mithracinae MacLeay, 1838 , Planoterginae Števćić, 1991 , and Eurynolambrinae Števćić, 1994 . Ng et al. (2008) transferred Hemus to the Planoterginae with Planotergum , commenting that the genus Thoe Bell, 1835 , apparently connects the two subfamilies. Windsor and Felder (2011 2014) showed that the morphological similarities are superficial, with Hemus and Thoe actually closely related, but both belonging to the Mithracidae , which was recognized as a separate family ( Windsor and Felder 2017). Tissues of Planotergum they examined, however, did not yield any DNA and so its phylogenetic position, and that of the Planoterginae could not be studied (Amanda Windsor, personal communication). The Planoterginae is here retained in the Majidae sensu Ng et al. (2008) , with the removal of Mithracidae as a separate family.

Species of Planotergum are apparently heterochelous. Prominent heterochely is not common in the Majidae or other families in the Majoidea (Schäfer 1956; Griffin and Tranter 1986; Windsor and Felder 2014) and the degree of symmetry/asymmetry of chelipeds can be diagnostic in some brachyuran families ( Schubart et al. 2002; Tavares and Cleva 2010; Spiridonov et al. 2014). The heterochely of Planotergum observed here needs to be further examined, possibly with genetic data to see if it helps resolve its systematic position.