Haploniscus unicornis Brökeland, 2010, n. sp.

Haploniscus unicornis complex

Composition: Haploniscus unicornis Menzies, 1956; H. aduncus Lincoln, 1985 b; H. bihastatus n. sp.; H. monoceros n. sp.; H. machairis n. sp.; H. angolensis n. sp.

TABLE 1. DIVA stations, where specimens were sampled, EBS, multicorer (MUC) and boxcorer (GBC) deployments. Diagnosis. Body oval, length at least 2.6 width. Head with conspicuous rostrum; rostrum triangular in dorsal view, anterior tip curved dorsally, acute, dorsal margin almost straight basally in males, strongly concave in females. Sutures between pereonites 5–7 and pleotelson visible in males, less distinct in females. Pleotelson processes large in males, distinctly smaller in females. Body surface (Fig. 2, details) covered by small depressions, merging into fine ridges on the surface of pleotelson. Antenna 1 article 1 slender, but wider than article 2. Flagellum in adult males and females with 4 articles. Antenna 2 dorsal tooth of article 3 somewhat shorter than article width, with 2 setae. Mandibular palp article 3 with 6 denticulate setae. Surface of carpus, propodus and dactylus covered with curved cuticular combs. Lateral apical combs of carpus setose, and in pereopods 2–7 of different size, outer comb slightly larger; additionally a slightly spinose dorsal comb. Dactylus accessory claw minute or absent. Pleopod 1 lateral margins of neck (narrowest part) with two setae each; distal margins undulating; distal setae of each side divided into a lateral group of 3–4 closely spaced shorter setae and a medial group of 3 widely spaced longer setae. Pleopod 2 endopod extending beyond distal margin of protopod, article 1 flexed slightly outwards, basal sperm chamber with at least one internal strut. Pleopod 3 outer plumose seta longest, exopod with 1 simple seta distally.

Remarks. Species of the Haploniscus unicornis complex can be recognized by their characteristic rostrum, which is triangular in dorsal view and has a typical sickle-shape in lateral view, especially in females. The Haploniscus unicornis complex belongs to a group of Haploniscus species (including also H. bicuspis (Sars, 1877) and H. spinifer Hansen, 1916), which is closely related to Mastigoniscus. This group of Haploniscus species shares some important characters with species of Mastigoniscus. These include the trend towards the elongation of the endopod of pleopod 2 in males and the setation of the pereopods, especially the lack of sensory setae (described as“sensory spines” in Brandt 1988), the ornamentation of the pereopods, especially the dorsal comb on the apical carpus, and the reduction of the accessory claw. Similarities also exist in the antennae and the somewhat flattened body shape, although these may be also found in other Haploniscus species. Yet, Mastigoniscus species lack a rostrum, contrary to the Haploniscus unicornis complex and also H. bicuspis, which has a small and inconspicuous rostrum. All these species share the pronounced sexual dimorphism. Therefore allocation of juveniles and females to a species, that is well defined by its adult males can be difficult within the whole group, especially since the distinguishing characters are largely restricted to the habitus and pleopods 1 and 2 of the males. The setation of the pereopods is rather conserved within Mastigoniscus and related species and distinguishing characters in the mouthparts are nearly absent within the whole family.