Nannoniscidae Hansen 1916

Nannoniscidae Hansen, 1916

Desmosomidae Sars, 1897a: 118 (part)

Nannoniscini.— Hansen, 1916: 83; Nordenstam, 1933: 251–252.

Nannoniscidae.— Gurjanova, 1932: 50, 1933: 413; Menzies, 1962a: 29, 1962b: 133; Wolff, 1962: 31; Birstein, 1963: 78; Menzies & George, 1972: 95; Siebenaller & Hessler, 1977: 18, 1981: 229; Svavarsson, 1982: 179; Kussakin, 1999: 28; George, 2001: 1843.

Type genus. Nannoniscus Sars, 1870.

Composition. Austroniscus Vanhöffen, 1914 (= Nannoniscella Hansen, 1916); Exiliniscus Siebenaller & Hessler, 1981; Hebefustis Siebenaller & Hessler, 1977; Micromesus Birstein, 1963; Nannoniscoides Hansen, 1916 (= Nannoniscella George, 2001, junior homonym of Nannoniscella Hansen, 1916); Nannonisconus Schultz, 1966; Nannoniscus Sars, 1870 (type genus); Panetela Siebenaller & Hessler, 1981; Rapaniscus Siebenaller & Hessler, 1981; Regabellator Siebenaller & Hessler, 1981; Thaumastosoma Hessler, 1970 (= Leutziniscus George, 2001).

Excluded. Sugoniscus Menzies & George, 1972 (family incertae sedis).

Diagnosis. Head longer than deep, frons broad, antennulae and antennae well separated, insertions recessed into broad posterolateral margins of head; eyes absent. Pereonites approximately parallel sided; pereonites 2–4 anterolateral margin with robust seta. Pleonite 1 posterior articulation absent (except in Thaumastosoma), pleon articular margin reduced to single suture, if present. Pleotelson with ventral anus, separated from pleopodal chamber. Antennulae shorter than head, with 5–9 articles; distal article with a single aesthetasc, often enlarged and bulbous in taxa with 5 articles. Antenna article 7 (first flagellar article) distinctly longer than more distal articles, conjoint. Mandible left incisor with dorsal subdistal cusp, distinctly separated from distal cusps; molar process triangular flattened spinose lobe with dentate or spinulate spines on distal margin. Pereopods length less than body length; dactyli with ventral claw shorter than major dorsal claw; coxae II–IV without robust seta; coxae insert ventrally on body, coxae V–VII insertions converging to midline posteriorly. Pleopod III exopod uniarticulate, subtriangular or ovoid, shorter than endopod. Uropods inserting posteroventrally, adjacent to anus.

Sexual dimorphism, males compared to females. Head rostral crest enlarged, more projecting or thickened. Pereonites and pleotelson lateral margins more projecting and flattened, occasionally indurate and narrower. Antenna flagellum basally enlarged often with many curled setae. Pereopods V–VII propodus dorsal margins with more elongate distally setulate setae.

Remarks. The diagnosis is substantially revised from Siebenaller and Hessler (1977: 18), including information on sexual dimorphism and some new characters. Variable features, such as that of the ventral keels and spination were omitted. The apomorphic bulbous antennular article 5, which characterises many, but not all species, is retained in the diagnosis because it is easily recognisable and, if present, unequivocally diagnoses a specimen as a nannoniscid. Variable articulation of the posterior somites is removed, but this is replaced with the absence (in most species) of pleonite 1. The mandibular palp presence has been removed from the diagnosis, because it is absent in Exiliniscus and Micromesus. The shape of the maxillipedal palp articles 4–5 is seen in many deep­sea Asellota so this character state is not particularly diagnostic. The inclusion of Micromesus into the Nannoniscidae by Svavarsson (1984) requires the removal of the free head and biramous uropod characters, both of which are putative plesiomorphies anyway, from the diagnosis. As discussed above, the ventral insertion of the coxae with laterally extended tergites in most species, a conjoint article 7 on the antenna and the dorsal subproximal cusp on the mandibular incisor process are added as new defining features of the Nannoniscidae. Several characters are apomorphies of larger clades of isopods, but are included for completeness, in the absence of a useful division of the Janiroidea. The ventral anus separate from the pleopodal chamber is common in many but not all deep­sea asellotans. The flattened, triangular mandibular molar and the pleopod III exopod shape are synapomorphies of the Nannoniscidae, Desmosomatidae and Macrostylidae.