Heteromesus inaffectus, Cunha & Wilson, 2006

Cunha, Marina R. & Wilson, George D. F., 2006, The North Atlantic genus Heteromesus (Crustacea: Isopoda: Asellota: Ischnomesidae), Zootaxa 1192 (1192), pp. 1-76 : 42-48

Heteromesus inaffectus sp. nov.

( Figs 19–23)

Etymology Inaffectus means natural or simple.

Material examined

Holotype: female (4.5mm), North Atlantic , offshore of Cork, Ireland, 50º12.3’N, 13º35.8’W, 20 Aug 1972, R / V CHAIN cruise nº 106, stn. WHOI 321 , epibenthic sled, 2890– 2868 m ( ZMUC CRU­9876 ).

Paratypes: male (3.1 mm), same data as holotype ( ZMUC CRU­9877 ) ; 68 specimens including males, females, mancae and 1 intersex, same data as holotype ( AM P 72122) ; adult male (4.4 mm) same data as holotype (dissected, mouthparts on slide, AM P 72121) ; preparatory female, same data as holotype ( SEM stub AW573, AM P 72245) ; preparatory female, same data as holotype ( SEM stub AW574, AM P 72246) .

Remarks on type material. Several specimens in this lot showed developmental oddities. One large specimen that had the female form and obvious pores to the spermathecal ducts, also had somewhat distorted male pleopods. Various asellote species are known to have facultative sex determination, but we suspect that this individual was only a single aberration, perhaps caused by a parasite. Three adult specimens had pereopods in the middle of the pereon that were in a rudimentary state, similar to the limbs of a manca III. Apparently, these limbs were being regenerated in subsequent molts. This species thus shows an ability to lose and regenerate limbs, perhaps as a predation escape mechanism. If the isopod needed to lose a limb, the break could occur at the thin neck between the shaft of the basis and the coxa.

Diagnosis

Pereonite 2 in female with no median dorsal spine or tubercle. Pereonite 5 in female length 3.1 width; in male length 6.9 width. Pleotelson posterolateral margin anterior to uropods without spines or tubercles. Antennula with 3 articles altogether. Pereopod bases proximal shoulder with bifid spines. Uropods elongate, length in female 0.50, in male 0.50–0.55 length of pleotelson.

Description

Body length in female 4.5 mm; granular. Head in female length 0.45 width; dorsal surface with pair of cephalic bullae; lobe on ventrolateral margin present in lateral view; dorsal cuticle tubercular granulation. Pereonite 1 in female width 0.20 total body length; female with 1 pair of anterolateral simple spines, stout, length distinctly less than length of pereonite 1. Pereonite 4 in female length 0.65 width. Pereonite 5 in female length 0.35 total body length. Pleotelson in female length 1.4 width.

Antennula in female article 2 length 0.48 head width, with 3 elongate stiff ventromedial setae, distal tip produced distally beyond insertion of next article, distal article inserting subapically; article 3 terminal, squat, wider than long. Antenna in female length 0.49 anterior body length; article 3 length 0.43 anterior body length, length 4.9 width, with 1 stout distomedial pedestal spine and 1 ventromedial stout pedestal spine placed proximally.

Labrum knobs present, low, rounded.

Pereopod bases proximal shoulder with bifid spines. Pereopod I carpus palm length near proximal region length, with 3 robust setae on palm distal to elongate seta; propodus ventral margin with 1 robust seta. Pereopods II–VII bases granulate.

Male specific characters. Body length 3.1 mm. Head length 0.45 width. Pereonite 1 width 0.20 total body length; with median tubercle, 1 pair of anterolateral simple spines, stout, length distinctly less than pereonite 1 length. Pereonite 2 with median tubercle, 1 pair of dorsal tubercles. Pereonite 3 with median tubercle. Pereonite 4 length 0.70 width. Pereonite 5 length 0.45 total body length. Pleotelson length 1.45 width. Antennula article 2 length 0.50 head width. Antenna length 2.8 anterior body length; article 3 length 0.51 anterior body length, length 4.4 width, article 3 with distomedial spine; article 5 length 0.42 anterior body length; article 6 length 0.52 anterior body length; flagellum length 0.42 total antennal length, flagellum with 17 articles. Pleopod I distal tip without lateral horns. Pleopod II protopod apex rounded, stylet thick and blunt, heavily calcified, extending beyond distal margin of protopod.

Distribution North Atlantic Ocean: Celtic Sea, South­West of Ireland, 2890– 2868 m.

Remarks

Heteromesus inaffectus sp. nov. differs from similar Heteromesus species ( H. granulatus , the female of H. frigidus and H. schmidtii ) in having an elongated pereonite 5 and long uropods both in the male and female. In some specimens (females and males) we can observe 3–5 pairs of dorsal tubercles arranged in a transverse row near to the anterior margins of pereonites 1–4. These are not easily detected in optical microscopy but are relatively conspicuous in the female specimen used for SEM micrographs ( Fig. 23).