Nyctobadistes Merrin, 2011, gen. nov.

Nyctobadistes gen. nov.

Type species. Nyctobadistes hamatus sp. nov., here designated.

Diagnosis. Cephalic frons wide, rectangular; cephalic anterior flanges absent. Pereonites 1–4 anterior margins with spines, apical seta absent; pereonites 5–7 without lateral extensions, natasome reduced. Pleon longer than wide, with dorsal spines, anterolateral spines absent. Antennae positioned closely together; antenna 1 article 1 lateral flange present; antenna 2 article 1 with no anterolateral spine. Mandible incisor massive, rounded; lacinia mobilis present, reduced; spine row present; mandibular palp present, short, not extending past incisor; mandibular fossa angular, across mandible. Pereopod 2 ambulatory, not enlarged; pereopods 5–7 carpi and propodi narrow, elongate, not expanded and oar-like; superior margin of ischia, superior and inferior margins of carpi and propodi with row of plumose setae, plumose setae sparsely plumose. Operculum large, with medial keel. Pleopod 4 exopod with more than 1 plumose seta.

Species included. Nyctobadistes hamatus sp. nov.

Remarks. Nyctobadistes gen. nov. is the eighth genus to be described from the subfamily Ilyarachninae which also includes: Aspidarachna Sars, 1897; Bathybadistes Hessler and Thistle, 1975; Echinozone Sars, 1897; Epikopais Merrin, 2009; Ilyarachna Sars, 1870; Notopais Hodgson, 1910; and Pseudarachna Sars, 1897. Along with Epikopais and Notopais, it is the third genus to be known exclusively from the southern hemisphere.

Nyctobadistes can be identified by the combination of: the reduced natasome; pereonites 5–7 without lateral extensions; dorsal body spines without apical setae; antenna 1 article 1 with lateral flange; the presence of a mandibular palp; pereopods 5–7 carpi and propodi elongate, not expanded and their marginal plumose setae are sparsely plumose.

Within the Ilyarachninae, the presence or absence of a mandibular palp is an important and diagnostic character when identifying a particular genus. Thus, the presence of a mandibular palp can easily distinguish Nyctobadistes from those genera that do not have one, namely Epikopais, Notopais, Echinozone and Pseudarachna.

Nyctobadistes is in some ways similar to Bathybadistes as both genera have a mandibular palp, numerous dorsal body spines and a natasome which is reduced from the streamlined one seen in Ilyarachna. Nyctobadistes, however, can be distinguished from Bathybadistes as it lacks lateral extensions on the natasomal pereonites, does not have apical setae on the dorsal body spines and the anterolateral margin of the pleon lacks the spine and apical seta which is present in Bathybadistes.

The other genera in the Ilyarachninae typically have expanded paddle-like carpi and propodi in pereopod 5, these segments expanded to a lesser extent in pereopod 6 and can also be expanded in pereopod 7, for example, in Epikopais. The margins of these segments are generally outlined with heavily plumose setae. In Nyctobadistes, although pereopod 5 is not known for this genus, the propodus and carpus of pereopod 6 are not expanded and their marginal plumose setae are sparsely plumose. This is also the case for the corresponding setae on pereopod 7. This reduction in plumosity is unique within the subfamily.

At present, Nyctobadistes is only known from the type species, Nyctobadistes hamatus sp. nov. which is located off the Freycinet Peninsula, Tasmania, Australia, between 720–800 metres.

Etymology. Nyctobadistes means night walker and is derived from the Greek words nyktos (night) and badistes (walker), in reference to the dark, deep ocean in which they inhabit.