Cymbasoma astrolabe Suárez-Morales & Mckinnon, 2016, sp.nov.

Cymbasoma astrolabe sp.nov.

(Figs 44, 45)

Material examined. Holotype: adult female from Warneet, Western Port Bay, Victoria, Australia (38°13.200’ S, 145°18.758’ E), partially dissected, ethanol-preserved; dissected parts mounted on 2 slides in glycerine, sealed with Entellan®. Date of collection: 12th June 1984. Slides deposited in the collection of MTQ, Australia (cat. MTQ W34397).

Description of adult female. Body elongate, slender (Fig. 44 A, B); body length of holotype female 1.87 mm. Cephalothorax approximately 1.2 mm long, slender, representing 64% of total body length. Midventral oral papilla moderately protuberant, located at 21% of cephalothorax length. Pair of medium-sized ocelli present, pigment cups moderately developed, medially separated, pigmented only at inner section; ventral cup as large as lateral cups (Fig. 44 A). Cephalic area narrower than post-oral region in dorsal view, with straight lateral margins and slightly produced "forehead”. Cephalic frontal area with medial depression flanked by pair of sensilla and few longitudinal wrinkles, otherwise smooth (Fig. 45 B). Anterior and middle areas of cephalothorax ornamented with wide fringe of transverse striae arranged in tight pattern including ventral, lateral and dorsal surfaces (Fig. 44 A, B); striae covering surface from post-oral region down to 60% of cephalothorax length. Ventral surface with additional cuticular elements: 1) pair of symmetrical, small nipple-like processes on anterior ventral surface located posterior to bases of antennules, with few adjacent transverse striae; 2) curved, “U”-shaped row of 12 spiniform elements adjacent to nipple-like processes (Fig. 44 C, arrowed in Fig. 45 B); 3) secondary pair of minute nipple-like processes in medial position (Fig. 45 B).

Urosome consisting of fifth pedigerous somite, genital double-somite and anal somite, together representing 12.5% of total body length. Relative lengths of urosomites (fifth pedigerous, genital double- and free anal somites) 28.7:50.1: 21.2 = 100, respectively (Fig. 44 D). Fifth pedigerous somite with slightly expanded lateral margins, dorsal surface with transverse wrinkles. Genital double-somite longest of urosome; ventral surface with field of transverse wrinkles near base of ovigerous spines; dorsal surface corrugated; lateral margins moderately swollen in dorsal view with postero-lateral rounded processes (arrowed in Fig. 44 F). Ovigerous spines paired, basally separated, slender, straight at their bases, distally acute; spines measuring 0.63 mm, about 34% of total body length (Fig. 44 B). Anal somite short, about half as long as genital double-somite, without medial constriction; dorsal and lateral surfaces weakly corrugated (Fig. 44 F). Caudal ramus subquadrate, about 1.2 times as long as wide, armed with three caudal setae.

Antennule length 0.31 mm, relatively short, representing about 17% of total body length and 25% of cephalothorax length; 4-segmented. Relative length of distal antennulary segment 45%. In terms of pattern described by Grygier & Ohtsuka (1995) for female monstrilloid antennulary armature, spiniform element 1 present on first segment; elements on second segment 2d1-2, 2v 1-3, and IId. Third segment with element 3 being short, spiniform, elements IIId and IIIv setiform, of normal aspect. Segment 4 bearing elements 4d1,2, 4v 1-2, element 4d1 not observed; element 4v 3 longest of group. Setae IVd, IVv, Vd, Vv, Vm, and 4aes present. Element 5 absent. Subterminal elements b1-3 and b 4 present, unbranched, apical elements 61-2 and 6aes present in specimens (Fig. 45 A).

Incorporated first pedigerous somite and succeeding three free pedigerous somites each bearing a pair of biramous legs. Pedigerous somites 2–4, together accounting for 22% of total body length. Intercoxal sclerites of legs 1–4 subrectangular; surface with pattern of finely spinulose subquadrate units; posterior margin smooth (Fig. 45 C–E). Bases of legs 1–4 with hair-like lateral seta; on leg 3, this seta 4.5 times as long as in other legs (Fig. 45 E). Endopods and exopods of legs 1–4 triarticulated. Ramal setae all biserially plumose except spiniform outer seta on exopodal segments 1 and 3, and inner seta of first exopodal segment, these latter being slender, sparsely setulated. Outermost distal spines on third exopodal segment of legs 1–4 short, 0.2 times as long as segment. Outermost apical exopodal setae of legs 1–4 with inner margin setulose, outer margin spinulose.

Armature formula of legs 1–4:

Fifth legs medially conjoined, bilobate, outer (exopodal) lobe cylindrical, distally expanded. Outer lobe armed with two distal and one subdistal setae, subequal in length, innermost seta slightly narrower (Fig. 44 E). Inner lobe wide, thumb-like, unarmed, shorter than outer lobe, reaching to about ¾ the length of outer lobe (Fig. 44 E, D).

Male: unknown.

Type locality. Warneet, Western Port Bay, Victoria, Australia (38°13.200’ S, 145°18.758’ E).

Etymology. Name in apposition, after the corvette Astrolabe commanded by Jules Dumont d’Urville, who visited Western Port in 1826.

Diagnosis. Cymbasoma with antennules representing about 17% of total body length and 25% of cephalothorax length; fourth antennulary segment representing 45% of antennule length. Cephalothorax with wide fringe of transverse striations in tight pattern covering about half the cephalothorax length from post-oral region to about 60% of way back along cephalothorax surface. Cephalic area with medial frontal depression flanked by sensilla; distinctive U-shaped curved row of spiniform cuticular processes adjacent to nipple-like processes. Legs 1–4 with spinulose intercoxal sclerites. Genital double-somite not ventrally protuberant in lateral view; weakly globose in dorsal view. Fifth leg with outer lobe armed with three subequally long setae; inner lobe arising from inner middle margin of exopodal lobe, unarmed. Ovigerous spines representing 34% of total body length.

Remarks. This species can readily be distinguished from its congeners by a unique combination of characters including the presence of a medial frontal depression, a wide fringe of striae around almost half of the cephalothorax, a ventral row of spiniform elements arranged in a “U”-shaped pattern between the antennulary bases and the oral papilla, a genital double-somite with produced posterior corners, and corrugated dorsal surface on the fifth pedigerous and genital double-somites.

Considering the first character, there are only a few other species in the genus that display a fringe of striae around the cephalothorax, i.e. C. striifrons, C. striatus, and the Australian C. markhasevae and C. annulocolle. The coverage, width and details of the striated area clearly differ in the new species with respect to these other species; in the new species the striation is absent from the anteriormost section of the cephalic area, starts post-orally and covers more than 43% of the cephalothorax (Fig. 44 A). Cymbasoma striifrons has a narrow fringe (Chang 2012: fig. 1A, B), covering about 20% of the cephalothorax while in C. markhasevae the fringe is even shorter (13%); in both cases the fringe ends abruptly at the post-oral area. In C. annulocolle the fringe coverage is up to 31% (Fig. 13 A, B, D) and starts at the insertion of the antennules (Fig. 13 B). In the latter, the shape of the cephalic region is also different being wide and laterally protuberant while displaying straight lateral margins in the new species. Another congener with a similar striation pattern stretching around the cephalothorax is C. striatus (Isaac, 1974) (Suárez-Morales 2000b) but its fifth leg has only one lobe, thus diverging from the new species which exhibits a bilobate fifth leg. Both C. annulocolle and C. markhasevae have an elongate, narrow inner lobe; the new species, however, has a relatively shorter and more robust inner lobe. Another striking character of this species, not previously described in any other Cymbasoma, is the peculiar wide “U”-shaped set of ventral spiniform processes adjacent to the usual nipple-like processes.