A new species of Halacarsantia Wolff, 1989 (Crustacea, Isopoda, Asellota, Santiidae) from Wistari Reef, southern Great Barrier Reef, Australia

Abstract Halacarsantia acuta sp. n. is described from Wistari Reef, Capricorn Group, southern Great Barrier Reef, the first record of the genus from Australia. The new species differs from its congeners in having antenna flagellum composed of 8 articles; epipod apically acute, without setae, broad maxilliped endite and pereopod 1 basis with a short projection. A key to species of the genus is provided.


Introduction
The Santiidae is a small family of the Asellota with 28 species in five genera (Schotte et al. 2008 onwards). Species of Halacarsantia Wolff, 1989 are tiny (0.62-0.85 mm) and include six species, all from marine benthic habitats (Menzies and Miller 1955, Wolff 1989, Wolff and Brandt 2000, Müller 1992, Shimomura and Ariyama 2004. Two species, Halacarsantia uniramea (Menzies & Miller, 1955) and H. kussakini Müller, 1992 has been so far described from the southern Pacific, from New Zealand and coral reefs at Mooréa (Society Islands) respectively.
The marine asellote fauna of Australia is diverse in shallow and deep waters (Kensley 1982, Wilson 1989, Merrin and Poore 2003, Just and Wilson 2004, but remains particularly poorly documented in tropical Australia, with only three named species reported from the Great Barrier Reef (Bruce 2009, Just and Wilson 2004, Kensley 1982. The genus Halacarsantia has not previously been recorded from Australian waters, though the genus is known from tropical and coral-reef habitats (e.g. Müller 1989, Wolff 1989, Wolff and Brandt 2000. In the present paper, we report a new species of Halacarsantia from the shallow coral-reef habitats of the Capricorn Group, southern Great Barrier Reef.

Material and methods
Collections of isopods were obtained by the CReefs program organized by the Australian Institute of Marine Science (AIMS) in Heron Island in 2009. Pieces of coral rubble collected by hand during SCUBA were washed in a bucket, and isopods were extracted by decanting the suspension through a sieve with a pore size in 0.3 mm. All the specimens obtained were fixed and preserved in 95% ethanol. Each individual was dissected and prepared for observation by a light microscope (Nikon E600). The total length as indicated in "Material examined" was measured from the tip of the head to the end of the pleotelson.
The type specimens are deposited in the Museum of Tropical Queensland, Townsville (MTQ).
Diagnosis (modified from Wolff 1989). Body depressed, lanceolate or ovate, widest at pereonite 3, with robust setae on or near lateral margins of head, pereonites and pleotelson. Head with large, broadly rounded, frontal lobe. Eye lobes obsolete or small.
No pleonite visible anterior to pleotelson. Second article of antennula with dorsolateral projection; article 5 as long as two preceding equally sized articles combined, with aesthetasc. Third article of peduncle of antenna with outer projection, with robust seta. Pereopods short, robust or slender, with robust setae, particularly on carpus. Pereopod 1 with two claws. Female operculum broader than long. Uropods uniramous.  Paratype. 1♀ (0.83 mm), same data as holotype (MTQ W31550). Description of the holotype female. Body (Fig. 1A) 1.9 times as long as maximum width, with dark reddish brown pigment in patches. Head 1.4 times as broad as long, narrower than pereonite 1, with 1 robust seta on anterior part of eyes; frontal lobe broad and long, 0.32 times as wide as maximum width of head, with 6 long robust setae on anterior margin; lateral margins of head, each with 1 slender and 3 robust setae; posterior margin of head convex. Eyes each with 7 ommatidia. Pereonites laterally rounded; pereonites 1, 4 and 7 with 2 long robust setae near lateral margin on each side; pereonite 2 with 2 short slender setae on each side of lateral margins and 2 long robust setae near lateral margin on each side; pereonites 3, 5 and 6 with 1 short slender setae on each side of lateral margins and 3 robust setae near lateral margin on each side. Pereonites 1 to 3 increasing in length; pereonites 3 and 4 subequal in length; pereonites 5-7 subequal in length. Pereonites 1 to 3 increasing in width; pereonite 4 slightly narrower than pereonite 3; pereonites 4 to 7 decreasing in width. Coxal plates dorsally visible on all pereonites, laterally rounded; coxal plate of pereonite 1 with 1 short seta; coxal plates of pereonites 2 and 3 each with 1 robust seta; coxal plates of pereonites 4-6 each with 2 robust setae; coxal plate of pereonite 7 with 1 short seta. Pleotelson (Figs 1A, 3E, 3F) pyriform, about 1.3 times as long as wide, with 7 robust setae near dorsolateral margin on each side and 4 robust setae near ventrolateral margin on each side.
Remarks. Halacarsantia acuta sp. n. may be distinguished from its congeners in having long robust setae on the pereonites. The body shape of Halacarsantia acuta is similar H. kussakini Müller, 1992 from the Society Islands, French Polynesia (type locality), but H. acuta can be separated from H. kussakini by the (those of H. kussakini in parentheses): 3 robust setae on each side of the head (2 setae); antenna flagellum composed of 8 articles (11 articles); maxilliped epipod apically acute, without setae (apically blunt, with 2 slender setae); maxilliped endite broad (moderately narrow); and pereopod 1 basis with a conspicuous short projection (without conspicuous projections).
Etymology. The species is named after the apically acute maxilliped epipod.