Richardina A. Milne-Edwards, 1881

Richardina A. Milne-Edwards, 1881 View in CoL

Richardina A. Milne-Edwards 1881: 933 View in CoL ; Holthuis 1946: 40; Holthuis 1993: 313; Chen et al. 2016: 484, 487; Poore & Ahyong 2023: 56.

Diagnosis. [aberrant features of Richardina taina sp. nov. marked with *]. Body somewhat compressed. Carapace with dorsal cincture of spines along posterior margin of cervical groove (4–16 on each side); area posterior to cervical groove without spines; dorsal rostral carina usually with teeth, sometimes unarmed (*); ventral rostral carina usually with two or more subdistal teeth, sometimes with one tooth or unarmed; post-orbital area at least with some spines; anterolateral margin and pterygostomial angle serrate or with minute, widely spaced teeth. Pleon largely smooth, except for some spinules on sixth pleonite and/or ventral and posterior margins of some pleura. Telson more or less elongate, lance-shaped, typically with one pair of strong marginal spines near mid-length; posterior margin with or without two posterolateral spines and one median spine; dorsal surface usually with smallto medium-sized spines, rarely with very prominent subdistal spines (*). Eyestalks typically with spines or spinules, rarely unarmed (*). Third maxilliped with well-developed exopod. Third pereiopod chela with or without teeth on finger cutting edges. Dactylus of fourth and fifth pereiopod simple, usually long and slender, sometimes stout (*). Uropod with endopod bearing one or two smooth dorsal ridges (see below); exopod with two smooth ridges, lateral margin toothed. Pleurobranchs typically present above first to fifth pereiopods, rarely absent (*).

Type species. Richardina spinicincta A. Milne-Edwards, 1881 View in CoL .

Other species included. Richardina fredericii Lo Bianco, 1903 View in CoL ; R. parvioculata Saito & Komatsu, 2009 View in CoL ; R. ohtsukai Saito & Komatsu, 2009 View in CoL ; R. rupicola Komai, 2011 View in CoL ; R. crosnieri Goy, 2015 View in CoL ; R. taina sp. nov. (but see discussion below).

Remarks. As mentioned above, one of the most important diagnostic features of Richardina is the presence of a simple dactylus on the fourth and fifth pereiopods (walking legs), which largely influenced the present assignment of the below described new species to that genus. However, the molecular analyses in Chen et al. (2016: figs. 2, 3) revealed the non-monophyly of Richardina and a close relationship between its type species, R. spinicincta , and a taxon presently known as Odontozona spongicola , whereas Globospongicola spinulatus Komai & Satio, 2006 was recovered between R. aff. parvioculata and the R. spinicincta + O. spongicola clade. Both O. spongicola and G. spinulatus have biunguiculate dactyli on the fourth and fifth pereiopods, suggesting that the general configuration of the dactylus, i.e., its elongation and development of an accessory unguis on the dactylar flexor margin, may represent highly plastic characters. Furthermore, Saito et al. (2009) observed dactyli with one, two or three ungui on the fourth and fifth pereiopods in the same specimen of Stenopus goyi Saito, Okuno & Chan, 2009 . The small clade containing two species of Richardina , G. spinulatus and O. spongicola recovered by Chen et al. (2016) is a very significant result, but does not seem to represent the full morphological and ecological diversity of this stenopodidean group. Several other deep-water taxa, especially O. edwardsi , remaining species of Globospongicola and Richardina (including R. taina sp. nov.), as well as the two species of Jogoya ( Anker 2023) , will need to be included in a more comprehensive molecular analysis, which would enable further systematic rearrangements. For the time being, Richardina is treated as a presumably non-monophyletic, morphologically heterogeneous, deep-water stenopodid genus, with a worldwide distribution and containing both presumably free-living and sponge-associated species.