Heteromesus dentatus Hansen, 1916

Cunha, Marina R. & Wilson, George D. F., 2006, The North Atlantic genus Heteromesus (Crustacea: Isopoda: Asellota: Ischnomesidae), Zootaxa 1192 (1192), pp. 1-76 : 29-32

Heteromesus dentatus Hansen, 1916

( Figs 13–14)

Heteromesus dentatus Hansen 1916: 66–68 , pl. V, figs. 6a–d, pl. VI, figs. 1a–c; Gurjanova 1932: 45, tabl. XV­54, 1933: 411; Menzies 1962: 122; Wolff 1962: 85, 217, 259; Kussakin 1988: 474–475, fig. 387.

Material examined

Syntypes: 2 males (posterior fragments, fragment illustrated 3.0 mm), 1 female (anterior fragment illustrated 1.7 mm), 2 heads and 1 pleotelson of undetermined sex, North Atlantic : southwest of Iceland, 60º37’N, 27º52’W, 1895–1896, R / V Ingolf stn 78, 1505 m (799 Danish fathoms) ( ZMUC CRU­6258 ) .

Remarks on type material. Specimens in ZMUC CRU­ 6258, are considered to be syntypes. We do not designate a lectotype here because all specimens are fragmentary. The mutilated and fragile specimens illustrated by Hansen were identified and re­illustrated (preparatory female: anterior fragment with head and pereonites 1–4, both pereopods I are missing; male: posterior fragment from pereonite 2 to pleotelson, right uropod missing). Male pleopods I and II were illustrated from a third specimen (male: posterior fragment P5­pleotelson).

Diagnosis

Pereonite 1 in female with 1–3 pairs of dorsal rook spines; pereonite 5 in male with posterodorsal rook spines; pereonite 6 in male with dorsal rook spines; pereonite 7 in male with lateral rook spines. Pereonite 5 in male length 5.5 width. Pleotelson (only male known) with posterolateral rook spines. Antennula with 5 articles altogether.

Description

Body granular. Head in female length 0.75 width; dorsal surface with pair of cephalic bullae; lobe on ventrolateral margin present in lateral view; dorsal cuticle fine spinous granulation. Pereonite 1 in female with median rook spine, 1–3 pairs of dorsal rook spines, 1 pair of anterolateral simple spines, 1 pair of lateral simple spines. Pereonite 2 in female with median rook spine and median tubercle, 2–3 pairs of dorsal rook spines, 1–2 pairs of dorsal tubercles, 1 pair of anterolateral rook spines, 1 pair of lateral rook spines. Pereonite 3 in female with median rook spine and median tubercle, 2–3 pairs of dorsal rook spines, 1 pair of anterolateral rook spines, 1 pair of lateral rook spines. Pereonite 4 in female length 0.70 width; with median rook spine, 2 pairs of dorsal rook spines. Pereonites 1–4 dorsal and median rook spines short to stout placed along anterior margin, anterolateral rook spines short to stout, length distinctly less than length of pereonite 1, male and female similar (pereonites 5–7 with male only).

Antennula in female article 2 length 0.73 head width, with 5 elongate stiff ventromedial setae; article 3 length similar to article 4, elongate and tubular, much longer than wide, terminal article shorter than penultimate article. Antenna in female length 2.2 anterior body length; article 2 with 1 ventromedial short pedestal spine; article 3 length 0.44 anterior body length, length 7.5–8.0 width, with stout distomedial pedestal spine and short distolateral pedestal spine, 1 long ventromedial pedestal spine placed proximally, length near distal article width; article 5 length 0.28 anterior body length; article 6 length 0.38 anterior body length; flagellum with 20 articles, flagellum length 0.37 total antenna length.

Labrum knobs absent.

Pereopod bases proximal shoulder with rook spines. Pereopod I ( Hansen, 1916: fig. 6d) carpus palm length distinctly less than proximal region length, with 2 elongate robust setae, with 1 robust seta on palm distal to elongate seta; propodus ventral margin with 2 robust setae. Pereopods II– VII bases denticulate, sometimes sharply toothed.

Male specific characters. (Head and pereonite 1 only with female) Pereonite 2 with median rook spine and median tubercle, 2–3 pairs of dorsal rook spines, 1 pair of dorsal tubercles, 1 pair of anterolateral rook spines, 1 pair of lateral rook spines. Pereonite 3 with median rook spine, 2–3 pairs of dorsal rook spines, 1 pair of anterolateral rook spines, 1 pair of lateral rook spines. Pereonite 4 length 0.80 width; with median rook spine, 2–3 pairs of dorsal rook spines. Pereonite 5 with lateral row of unconspicuous tubercles. Pereonites 5–7 rook spines short to stout, 1 pair in pereonite 5 and 2 pairs in each of pereonites 6 and 7. Pleotelson length 1.45 width. Pleonite 1 region of pleotelson with 1 median and 2 pairs of dorsal rook spines short to stout. Pleotelson with 5 pairs of dorsal rook spines short to stout arranged in 2 median longitudinal rows; posterolateral margin anterior to uropods with paired elongate curved rook spines; terminal margin with 1 pair of distal short simple spines. Pleopod I distal tip with lateral horns. Pleopod II protopod apex narrow, tapering, stylet tapering and pointed, not heavily calcified, extending to distal margin of protopod. Uropods length 0.30 length of pleotelson.

Distribution North Atlantic, southwest of Iceland, 1505 m.

Remarks

The “spiniform tubercles” on the pereonites and pleotelson of Heteromesus dentatus described by Hansen (1916: 67) are complex, each consisting of a short pedestal ending in around four (3–5) denticles usually with one seta in the centre herein designated as "rook spines". H. dentatus is distinctive among the other species in the genus because of its abundant rook spines on most dorsal surfaces, and particularly on the lateral margin of pereonite 7. Male pleopod I appears to have a different shape in the two illustrated specimens ( Fig. 14) because it is embedded laterally in pleopod II on the more intact specimen and, in the enlargement from a fragment, it is partially exposed, showing the lateral margins. Pereonite 7 in the males also has a ventral rounded tubercle.

Heteromesus dentatus is similar to H. greeni and H. oryktus in having spinous granulation of the body, 5 antennular articles altogether and inconspicuous ornamentation of the pereopods (in which these species differ from H. spinosus and H. ctenobasius that also have 5 antennular articles) that are characterised, however, by the presence of rook spines on the proximal "shoulders" of the bases. The most distinctive features of H. dentatus are the rook spines arranged in dorsal rows in the pereonites and pleotelson, the anterolateral rook spines on pereonites 2 and 3 and posterolateral rook spines on the pleotelson.