Nannonisconus Schultz 1966

Nannonisconus Schultz, 1966

Nannonisconus Schultz, 1966: 24. Siebenaller & Hessler, 1977: 22; 1981: 249; Wilson, 1997: 102; Kussakin, 1999: 65. Type species. Nannonisconus latipleonus Schultz, 1966.

Composition. N. carinatus Mezhov, 1986; N. intermedius (Siebenaller & Hessler, 1981), comb. nov.; N. latipleonus Schultz, 1966.

Diagnosis. Body with concave lateral outline in dorsal view. Pereonal tergites projecting laterally from pereopodal coxae; pereonite 6–7 and pleon articulations absent medially. Pleon broad and elongate, wider than head or pereon, distinctly longer than pereonites 5–7, posterolateral spines angled medially, posterior margin blunt, not projecting. Antennula extending beyond margin of head, with 5 segments, distal article bulbous, article 4 distal margin with ventromedial projection. Opercular pleopods elongate and broad, covering pleon ventral surface.

Remarks. The variability of the posterior body somite articulation, a central theme of this paper, necessitates a reconsideration of the genus Nannonisconus Schultz, 1966. A survey of the published descriptions of Nannoniscus and specimens in the Australian Museum's collection (Table 1) showed several species with an absent ventral articulation between the pleon and pereonite 7, as mentioned above. N. oblongus, however, does have a suture between the pleon and pereonite 7 (Figs 2 A, 4A, 6A–B), suggesting that the absent pleonal suture on the dorsal surface in Nannonisconus, previously the only definitive character of this genus, is only an endpoint in a continuum. My inspection of N. intermedius Siebenaller & Hessler, 1981 (nontype specimens from the type locality WHOI 297; AM P.74558) showed that, although the suture is present dorsally between the pleon and pereonite 7, it is only a groove in the surface, which appeared to be absent in some specimens. Nannonisconus, however, does define a distinctive clade within the Nannoniscidae. Although the absence of pleonal articulation is a weak character, the genus can still be recognised by its unique pleotelson shape (Siebenaller & Hessler 1981: 242). Consequently, the species N. intermedius is moved to Nannonisconus and the generic diagnosis modified to reflect these changes. The genus may also be characterised by pleopods I–II covering entire ventral surface of pleotelson, although this feature is only incompletely known in some species. Additionally, the terminal article 5 of the antennula appears to have two segments but without an interannular margin (e.g., Wilson, 1997: fig. 1.42; Mezhov, 1986: fig.36), which may be significant once the full detail on this structure is known.