Heteromesus similis Richardson, 1911

Cunha, Marina R. & Wilson, George D. F., 2006, The North Atlantic genus Heteromesus (Crustacea: Isopoda: Asellota: Ischnomesidae), Zootaxa 1192 (1192), pp. 1-76 : 56-59

Heteromesus similis Richardson, 1911

( Fig. 27)

Heteromesus similis Richardson 1911: 532–533 ; Menzies 1962: 123; Wolff 1962: 85, 265. Material examined

Holotype: Manca III male (3.5 mm), North Atlantic, Azores, northwest of S. Miguel, 38º38’N, 27º26’W, 22 Aug 1883, R / V Talisman , stn 31, 2995 m (MNHN IS­1775 ).

Remarks on type material. A manca III male with developing pereopods VII: the body is decalcified, the right uropod and both antennulae are missing (except for the first article) and pereopods II– VI are broken.

Diagnosis (Male manca III only)

Pereonites 1–4 with transverse rows of pointed tubercles. Pereonite 5 length 4.6 width. Pleotelson with no posterolateral spines. Pereopod bases proximal shoulder with simple spines only. Pereopods II– VII bases denticulate, sometimes sharply toothed, with pedestal spines.

Description (Male manca III characters only)

Body length 3.5mm, granular. Head length 0.75 width, with 2 dorsal tubercles, on frons between antennulae; lobe on ventrolateral margin present in lateral view; dorsal cuticle not evident (specimen decalcified). Pereonite 1 width 0.20 total body length; with median tubercle, 2 pairs of dorsal tubercles, 1 pair of anterolateral simple spines, 1 pair of lateral simple spines. Pereonite 2 with median tubercle, 2–3 pairs of dorsal tubercles, 1 pair of anterolateral simple spines, 1 pair of lateral tubercles. Pereonite 3 with median tubercle, 2–3 pairs of dorsal tubercles, 1 pair of anterolateral simple spines, 1 pair of lateral tubercles. Pereonite 4 length 0.70 width; with median tubercle, 2–3 pairs of dorsal tubercles, 2 pairs of posterolateral stout rook spines. Pereonites 1–3 anterolateral spines decreasing in length, length distinctly less than pereonite 1 length, tubercles arranged in transverse row on anterior margin of pereonites. Pereonite 5 length 0.3 total body length. Pleotelson length 1.65 width.

Antennula with 5 articles altogether (estimated from original description). Antenna article 2 with ventrolateral spine; article 3 length 0.44 anterior body length, length 4.4 width, with distomedial stout pedestal spine and distolateral shorter pedestal spine and 2 medial pedestal spines placed ventrally and dorsally in proximal third of article.

Labrum knobs absent.

Pereopod bases proximal shoulder with simple spines only. Pereopod I carpus palm length near proximal region length, with 3 robust setae on palm distal to elongate seta (decreasing in size distally); propodus ventral margin with 2 robust setae. Pereopods II– VII bases denticulate, sometimes sharply toothed, with pedestal spines (robust setae on short pedestal with tuberculate shape), positioned more distally on dorsal margin than on ventral margin.

Uropods length 0.30 length of pleotelson.

Distribution North Atlantic: northwest of the Azores, 2995 m. Remarks

Richardson (1911) provided no illustrations of the specimens of Heteromesus similis , so this species is often overlooked in compendia on deep­sea isopods (e.g., Kussakin 1988). She described antennulae that are now missing from the holotype, with 4 articles in the “flagellum”, the fourth perhaps being the base of a broken terminal seta; no other Heteromesus species has this many articles. This species, although only represented by a manca III, is distinct from other Heteromesus species by its joint possession of 1 pair of lateral spines on pereonite 1, 2 pairs of posterolateral rook spines on pereonite 4 (also seen in the male of H. oryktus ), and rows of dorsal tubercles on pereonites 1–4.

H. similis is similar to H. spinescens (the transverse rows of more or less pointed tubercles on the anterior margin of pereonites 1–4 in H. similis are not much different from the low spines in H. spinescens ), but it has a longer pereonite 5 and lacks pleotelson posterolateral spines. Richardson (1911) mentioned posterolateral spines on the pleotelson, but these are absent in the holotype ( Fig. 27).