Unispinosus, Chim & Tong, 2020

Chim, C. K. & Tong, Samantha J. W., 2020, Two new species of paratanaoid tanaidaceans of the family incertae sedis (Crustacea: Peracarida) from polymetallic nodule fields in the eastern Clarion-Clipperton Fracture Zone, Zootaxa 4758 (3), pp. 461-485 : 464-465

Genus Unispinosus n. gen.

Diagnosis. Body elongate and robust. Pereonites 1 and 6 as long as wide; pereonites 2–5 subequal in length, all longer than wide. Pleotelson elongate, almost as long as four pleonites. Antennule with four articles; terminal article with complex aesthetasc. Antenna with six articles; article-2 with a dorsodistal process bearing a large spiniform seta. Mandible molar wide, with a few spines on the lower margin. Labium with two pairs of lobes; outer lobe membranous bearing one spiniform seta distally. Maxilliped endite without tubercles; palp without outer setae on all articles. Cheliped basis with a long posterior lobe, separated from pereonite-1 by a gap ventrally; chela robust, with large and strongly chitinised rounded process ventrally and distally near dactylus insertion, ventral margin smooth bearing one setulose spiniform seta. Pereopods 1–6 with coxa, each bearing one simple seta. Pereopods 4–6 dactylus and unguis unfused, combined length more than 1.5 times as long as propodus; pereopod-6 propodus with three dorsodistal setae. Pleopods rudimentary in females; well-developed in males, endo- and exopod brush-like, each with one slender article bearing nine stiff simple setae. Uropod endopod with two articles, article-2 less than half as long as article-1; exopod with one article.

Remarks. Unispinosus n. gen. shares many characters with the genera Monstrotanais Kudinova-Pasternak, 1981 , Robustochelia Kudinova-Pasternak, 1983 , Leptognathioides Bird & Holdich, 1984 , Robustochelia [?] solida Larsen, 2005 and Singula Błażewicz-Paszkowycz, 2005 belonging to the family Paratanaoidea incertae sedis, most notably in the presence of strong chelae ( Table 2). However, the chela of Unispinosus can be differentiated from those of the abovementioned genera by the presence of a large and strongly chitinised rounded process along the ventral and distal margins. Leptognathioides potens Bird & Holdich, 1984 and Leptognathioides rectus Kudinova- Pasternak, 1993 also possess these two features but they are much shallower. The two processes in Robustochelia [?] solida are similar to that in L. potens and L. rectus but do not appear to be heavily chitinised. Leptognathioides polita (Hansen, 1913 in Bird & Holdich, 1984) and Singula cuncta Błażewicz-Paszkowycz, 2005 only have a small pro- cess along the ventral margin of their chela. The two processes are absent in Leptognathioides vicina (Hansen, 1913 in Bird & Holdich, 1984) and Leptognathioides biarticulata Bird, 2014 . The chela ventral margins of Robustochelia and Monstrotanais are very different—there is a heavily calcified carina in Robustochelia robusta ( Kudinova-Pasternak, 1970) , Robustochelia longa Kudinova-Pasternak, 1983 , Robustochelia angusticephala Kudinova-Pasternak, 1986 , Robustochelia viridis Jóźwiak & Błażewicz-Paszkowycz, 2007 and Robustochelia pacifica Larsen, 2011 while it is a serrated triangular flange in Monstrotanais mirabilicheles Kudinova-Pasternak, 1981 and Monstrotanais ingens Jóźwiak & Błażewicz-Paszkowycz, 2007 .

Another distinguishing feature of the chela in Unispinosus is the presence of a setulose spiniform seta along the ventral margin. Singula cuncta and R. pacifica also have a single seta along their chela ventral margin but they are simple. The chelae of Robustochelia [?] solida , Leptognathioides , Monstrotanais and the other four Robustochelia species bear two simple setae ventrally. Other characters present in Unispinosus but not in the abovementioned taxa include the (1) large spiniform seta on antenna article-2; (2) presence of spiniform setae on the labium; (3) pereopods 4–6 dactylus and unguis together more than 1.5 times longer than propodus; (4) rudimentary pleopods in females; (5) uropod endopod article-2 shorter than article-1.

The long cylindrical pleotelson, the lack of tubercles on maxilliped endites and the reduction of female pleopods in Unispinosus suggest that this genus is affiliated to the family Colletteidae Larsen & Wilson, 2002 . Leptognathioides and Singula , the two genera most morphologically similar to Unispinosus , have also been linked to this family (see Błażewicz-Paszkowycz, 2005). However, the two-lobed labium in Unispinosus is not characteristic of that family (see Larsen et al., 2015). Furthermore, Colletteidae is widely accepted to be polyphyletic and its definition needs a systematic revision (Larsen & Araújo-Silva, 2014; Wi et al., 2015a). Due to these reasons, we restrain from placing Unispinosus in Colletteidae until its status has been stabilised, which is not within the scope of this study.

Etymology. The genus name refers to the large singular spiniform seta on the ventral margin of the chela as well as on the second article of the antenna. Gender masculine.

Type species. Unispinosus eopacificus n. sp., by original designation.