Haplomesus angustus Hansen, 1916

Kavanagh, Fiona A., Wilson, George D. F. & Power, Anne Marie, 2006, Heterochrony in Haplomesus (Crustacea: Isopoda: Ischnomesidae): revision of two species and description of two new species, Zootaxa 1120, pp. 1-33 : 21-24

Haplomesus angustus Hansen, 1916 ( Figs 13–14 )

Haplomesus angustus Hansen, 1916: 61 –62, pl. V, fig. 2a–e; Gurjanova, 1932: 43, tabl. XIV, 50; 1933: 410; Gorbunov, 1946: 76 –77; Menzies, 1962: 119, fig. 20A–D; Wolff, 1962: 86, 217, 262, 289; Gurjanova, 1964: 259; Kussakin, 1988: 454 –456, fig. 375.

Remarks on Type Material. The holotype male is in relatively good condition. The spines on pereonite 1, however, have been broken off since Hansen's original illustration. Hansen also designated a male paratype that he illustrated but did not describe. The paratype specimen is missing the head and anterior pereonites 1 and 2, but otherwise is in good condition. Pereonite 5 of the paratype is 4 times as long as wide, whereas the holotype has a ratio of five times as long as wide; this may be due to the smaller size of the paratype. A small difference in the posterolateral spines of the pleotelson may be attributed to slight damage of both specimens.

Material examined. Holotype male 4.8 mm, ZMUC CRU­ 5338, Ingolf Station 18, south of Denmark Strait, 61º44'N, 30º29'W, 1135 Danish fathoms (2137 m). Paratype male 4 mm (estimated), ZMUC CRU­ 5338, Ingolf Station 125, 68º08'N, 16º02'W, 729 Danish fathoms (1373 m).

Diagnosis. Male with anterolateral spines on pereonite 1 only. Pereonite 7 length reduced, less than pereonite 6. Pleotelson posterolateral margin anterior to uropods with simple spines, length subequal to uropods. Antennula with 3 distal flagellar articles. Pereopod I carpus ventral margin with 1 proximal robust seta and 1 robust seta on palm. Pleopod I of male without simple setae on lateral or distal margins.

Description of male (female unknown). Body length 4.8 mm. Head length 0.9 width. Head cuticular structure granulated; pereonite 7 length reduced, less than pereonite 6. Head lobe on ventrolateral margin absent in lateral view. Pereonite 1 width 0.1 total body length, anterolateral simple spines, short, length near pereonite 1 length, acutely pointed. Pereonite 5 length 5 width, 0.5 total body length. Pleotelson dorsal surface axial ridge weakly vaulted, separated from lateral fields only by shallow elongate concavities; Pleotelson length 1.4 width; posterolateral margin adjacent to uropods convex.

Antennula and antenna. Antennula with 5 articles; article 2 length 0.6 head width, with 2 elongate ventromedial setae; distal articles in total small, shorter than article 2; article 3 cuticle smooth, length similar to article 4, elongate and tubular, much longer than wide; terminal article longer than penultimate article; aesthetascs absent, length 3.9 width.

Mouthparts. Maxilliped palp article 3 wider than article 2.

Pereopods. Pereopod I propodus ventral margin with 1 robust seta (see Hansen, 1916: pl. V, fig. 2C). Pereopod VII absent in adults.

Pleopods and uropods. Pleopod I distal tip with lateral horns. Uropods extending near posterior margin of pleotelson, length 0.2 length of pleotelson.

Distribution. Northeast Atlantic Ocean, south of Denmark Strait; Arctic Ocean east of Greenland, 1333–2076 m.

Remarks. The holotype of Haplomesus angustus Hansen, 1916 was considered by Hansen to be a juvenile male, due to the lack of pereopod VII. Hansen (1916: 64) estimated that a fully grown male of H. angustus would measure about 9–12 mm. This would be an unusually large specimen for a Haplomesus species, although H. gigas Birstein, 1960 is in this size range. Species described from the North East Atlantic generally range from 4–5 mm, similar to the holotype of H. angustus (4.3 mm, fig. 13A).

The type specimen is sexually mature. The first pleopods ( Fig. 13 E) are fully developed, with lateral horns on the distal tips, in contrast to the rounded tips that would be expected in a juvenile specimen ( Fig. 6 E). The seventh pereonite is also much reduced, as noted by Hansen, a feature seen in most species that lack a pereopod VII as adults. We can conclude that the holotype is not a juvenile but rather is an adult without expression of the seventh pereopod.

H. angustus males are distinct from those of other '6 pereopod' group species by having only short (approximately as long as width basally) anterolateral spines on pereonite 1 only. H. corniculatus males have long (much longer than wide) anteriorlycurving robust anterolateral spines on pereonite 1. Other species also have anterolateral spines on pereonite 4, which are absent in H. angustus .

Figures

FIGURE 6. Haplomesus hanseni (male paratypes juvenile AM P. 71669, manca 3 AM P. 71670). A, juvenile lateral; B, juvenile ventral; C, manca 3 ventral; D, manca 3 lateral; E, juvenile pleotelson ventral; F, manca 3 pleotelson ventral (scale bar 0.5 mm).

FIGURE 13. Haplomesus angustus (male holotype ZMUC CRU­ 5338), A, dorsal view; B, ventral view; C, pereopod VI; D, lateral view E, pleotelson ventral; F, maxilliped; G, antennula (scale bar 1 mm).

FIGURE 14. Haplomesus angustus (male paratype ZMUC CRU­ 5338). A, dorsal view; B, pleotelson ventral; C, lateral view (scale bar 1 mm).