Pseudarachna Sars, 1897

Merrin, Kelly L., 2006, The first record of the crustacean isopod genus Pseudarachna Sars, 1897 (Isopoda: Asellota: Munnopsidae) from the Southern Hemisphere, with description of a new species from New Zealand, Zootaxa 1370, pp. 59-68 : 60-61

Pseudarachna Sars, 1897

Mesostenus Sars, 1864 : partim: 9 (preoccupied).

Pseudarachna . — Sars, 1897: 142.— Nierstrasz & Schuurmans-Stekhoven, 1930: 129.— Kussakin, 2003: 261.

Not Pseudarachna . — Schultz, 1976: 13.

Type species: Mesostenus hirsutus Sars, 1864 ; by monotypy.

Diagnosis

Cephalic frons enlarged, broad, semi-circular, without defined ridge around antennae; cephalon anterior flanges absent. Pereonites 1–4 not short or compact, dorsal spines present, apical setae present. Pereonite 2 largest; pereonites 5–7, lateral margins not narrow, natasome not reduced. Pleon anterolateral margins rounded, without spine. Antennae positioned widely apart; antenna 1 small, article 1 with lateral flange; antenna 2 article 1 with no anterolateral spine. Mandible lacinia mobilis, spine row and palp all absent; mandibular fossa angular, sub-horizontal across mandible. Pereopod 2 elongate, robust; ischium compact, wider than other articles; pereopods 5 and 6 ischium superior margin with plumose setae; carpus paddle shaped; carpus and propodus with marginal plumose setae; pereopod 7 more ambulatory, with setation similar to that of pereopods 5 and 6; dactyli of natatory pereopods flattened, slightly broadened. Pleopod 4 exopod with more than 1 plumose seta. Pleon triangular, domed in centre. Operculum partially vaulted, with small medial keel. Pleopod 5 simple lobe. Uropods flat, uniramous.

Remarks

Pseudarachna has many unique apomorphies, the most notable being the enlarged, semi-circular shaped cephalic frons, the widely spaced antennae, the small antenna 1, enlarged pereonite 2 and a robust and enlarged pereopod 2 with a wide, compact ischium. Pseudarachna is easily distinguished by these characters from other closely related genera such as Ilyarachna Sars, 1870 and Echinozone Sars, 1897 .

Males are not known for this genus, and thus there is no data specific on sexual dimorphism.

Distribution

In the North Atlantic, from western Ireland to Denmark ( Wolff 1962) at depths between 50–478 metres ( Svavarsson et al. 1993); off eastern Australia at approximately 36° south and, Bass Strait at 38° south off Point Hicks (both records of undescribed mancas) at depths between 960–1277 metres; and the Challenger Plateau, west of the South Island, New Zealand at depths between 1005–1463 metres.