The Ericthonius group, a new perpective on an old problem (Crustacea: Amphipoda: Corophioidea)
The Ericthonius group is established based on corophioid taxa with dorsal spines on the telson. Within this group there are three non-aligned genera (Ericthonius, Pseudericthonius and Pseudischyrocerus) and two robust clades known as the Cerapus clade and the Siphonoecetes clade. The Cerapus clade (five genera) is defined by four synapomorphic characters: a posteriorly directed dactylus on peraeopod 7; accessory spines on the dactyli of peraeopods 5-7; uniramous uropod 2; and rows of recurved spines on the telson. The Siphonoecetes clade (10 genera, Siphonoecetinae of Just, 1983) is defined by: five synapomorphic characters: a short, slender first antenna; a reduced mandibular palp; a simple or weakly subchelate male gnathopod 2; an anteriorly inserted carpus on peraeopods 5-6; and no recurved distal spines on the rami of uropod 3. The relationships between these taxa are discussed and the Cerapus clade and the Siphonoecetes clade are considered to be sister taxa. Although the Ericthonius group is an obvious member of the Corophioidea there is currently no evidence to place it in an existing family. All genera in the Cerapus clade (Bathypoma n.gen., Cerapus Say, 1817, Notopoma n.gen., Runanga J.L. Barnard, 1961, and Paracerapus Budnikova, 1989) are diagnosed and a number of species are placed in new combinations. The two new genera, Bathypoma and Notopoma, are both characterised by a medially expanded peduncle on antenna 1 which functions as an operculum and distinguishes them from all other members of the group. Bathypoma enigma n.sp. is the only member of the Cerapus clade with a subchelate gnathopod 2 in the male. Notopoma contains N. africana n.sp., N. crassicornis (Bate, 1857), N. fallohidea (Lowry, 1981), N. harfoota (Lowry, 1981), N. lutkini (Tzvetkova, 1990), N. moorea n.sp., N. opposita (K.H. Barnard, 1931), N. sismithi (Stebbing, 1888), N. stoddartae n.sp. and N. stoora (Lowry, 1981). A generic key to the Ericthonius group is included.