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

Cymbasoma constrictum sp. nov.

(Figs 36, 37)

Material examined. Holotype: Adult female from Davies Reef, Queensland, Australia (18° 48.78' S, 147° 39.30' E), partially dissected, ethanol-preserved; dissected parts mounted 2 slides in glycerine, sealed with Entellan®. Date of collection: 3rd February 1989. Slides deposited in the collection of MTQ, Australia (cat. MTQ W34393).

Description of adult female. Body robust, relatively wide in dorsal view, cephalothorax globose in lateral view; body length of holotype female = 1.31 mm. Cephalothorax approximately 0.76 mm long, representing 58% of total body length. Midventral oral papilla not particularly protuberant, located at 27% of cephalothorax length. Pair of relatively large ocelli present, pigment cups well developed, medially conjoined, intensely pigmented at inner half; ventral and lateral cups with similar diameter (Fig. 36 A). Cephalic area wide, with slightly produced "forehead” (Fig. 36 C), frontal area ornamented with pattern of transverse and concentric striations (Fig. 36 C), frontal sensilla absent. Dorsal surface of cephalothorax smooth except for field of transverse striae and field of reticulation overlying ocellar area (Fig. 36 C), ventral surface with transverse cuticular wrinkles at preoral area. Ventral surface also with: 1) pair of symmetrical, crescent-shaped processes on anterior ventral surface located near bases of antennules, with adjacent striae (Fig. 37 A); 2) pair of nipple-like processes with concentric pattern of striae; 3) pair of minute papilla-like processes between nipple-like processes and oral area (Fig. 37 A).

Urosome consisting of fifth pedigerous somite, genital double-somite and anal somite, together representing 18% of total body length. Relative lengths of urosomites (fifth pedigerous, genital double- and free anal somite) 24: 37: 39 = 100, respectively (Figs. 37 E). Lateral margins of fifth pedigerous somite moderately produced, rounded, with field of transverse wrinkles on dorsal surface covering posterior half of somite (Fig. 37 E). Genital doublesomite slightly shorter than anal somite, with ventral and dorsal surfaces ornamented with wrinkles (Fig. 37 E, F), anterior half swollen, with strong constriction at posterior 2/3 of somite; posterior half with protruding margins.

Ovigerous spines paired, basally separated, slender, straight at their bases, relatively long, 62% of total body length (0.81 mm) (Fig. 36 B, G). Anal somite longest of urosome, with strong medial constriction and faint longitudinal wrinkles at constriction area (Fig. 37 E, F). Caudal ramus subrectangular, about 1.6 times as long as wide, armed with three caudal setae.

Antennule length 0.29 mm, representing about 23% of total body length and 39% of cephalothorax length, 4- segmented. Relative length of distal antennulary segment 43.5%. In terms of pattern described by Grygier & Ohtsuka (1995) for female monstrilloid antennulary armature, short, spiniform element 1 present on first segment; elements on second segment: 2d1-2, 2v 1-3, and IId. Third segment with element 3 being strong, spiniform, elements IIId and IIIv setiform, of normal aspect. Segment 4 bearing elements 4d1,2, 4v 1, elements 4v 2-3 not observed; setae IVd, IVv, Vv, Vm, and aesthetasc 4aes present. Element 5 absent. Subterminal elements b1-3 and b6 present, unbranched, elements 61 not observed, probably broken off; element 62 present, small (Fig. 37 B).

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, with patches of minute spinules (Fig. 37 C, D), posterior margin smooth. Bases of legs 1–4 articulating with large, rectangular coxa along oblique line; with hair-like lateral seta; on leg 3, this seta about twice as long as and noticeably thicker than those on other legs (Fig. 37 D). 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 short, slender. Outer spines on first exopodal segments of legs 1–4 remarkably reduced in size. 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 separate, bilobate, outer (exopodal) lobe subrectangular, distally truncate, and armed with three terminal, subequally long setae (Fig. 37 F). Inner lobe thumb-shaped, noticeably shorter than outer lobe, unarmed, arising from exopodal lobe but not reaching its distal margin (Fig. 36 F).

Male: unknown.

Type locality. Davies Reef, Queensland, Australia (19°7.340’ S, 146°53.024’ E).

Etymology. The species name, an adjective derived from the Latin constrictus (= drawn together), refers to the strongly constricted anal and genital double-somites of this species.

Diagnosis. Cymbasoma with relatively long cephalothorax, representing almost 60% of total body length, third antennulary segment representing approximately 47% of antennule length, with pair of crescent-like processes on ventral surface of cephalothoracic region. Eyes well-developed, intensely pigmented at inner half. Cephalothorax with dorsal field of reticulations covering part of cephalothorax. Fifth pedigerous somite with deep transverse striae in posterior half. Genital double-somite with constricted margins in distal half, proximal half rounded. Anal somite remarkably long, slightly longer than genital double-somite, with deep medial constriction. Fifth leg with elongate, outer lobe with three distal setae, inner lobe distinctly arising medially, narrow, not reaching distal end of outer lobe, unarmed. Ovigerous spines long, 62% of total body length.

Remarks. This species most closely resembles the Australian C. apicale. Both share similar body shapes and proportions, a reticulated field on the dorsal surface of the cephalic area, a bilobate fifth leg with a short inner lobe and the outer lobe bearing three subequally long setae, ovigerous spines proximally separated, and intensely pigmented eyes. They differ in several characters including the preoral margin is more strongly produced ventrally (lateral view) in the new species (Fig. 36 B) than in C. apicale (Fig. 34 A). In addition, the cephalic region is clearly wider in C. apicale (Fig. 34 A) than in C. constrictum sp. nov. (Fig. 36 A) and the cephalothorax is shorter in the former. In the new species, the fifth pedigerous somite has a distinctive set of deep, symmetrical, transverse striae that is absent in C. apicale, which has only a few faint lateral wrinkles on this somite (Fig. 35 B). The fifth leg inner lobe is different in both species; it arises subdistally from the inner margin of the exopodal lobe in C. apicale, reaching its distal end (Fig. 34 D), whereas in the new species the inner lobe arises on the middle of the inner margin of the outer lobe and does not reach its distal end (Fig. 36 F). Both the genital double- and the anal somite are strongly constricted in the new species (Fig. 37 E), thus differing from the different shape of these somites in C. apicale. The most distinctive character of C. constrictum sp. nov. is its long anal somite in the female, being slightly longer than the genital double-somite; this character has not been observed in any other female Cymbasoma. Only two other species have an anal somite that is almost as long as the genital double-somite, C. striifrons (cf. Chang 2012: fig. 2A) and the Australian C. paraconstrictum sp. nov., but it is not longer, as in the new species. A strongly constricted genital double-somite is also exhibited by C. germanicum (cf. Suárez-Morales 2006: figs.4b, d), but it differs from C. constrictum in having an anal somite that is shorter than the genital doublesomite. In C. germanicum the fifth leg inner lobe is long, reaching the distal end of the outer lobe (Suárez-Morales 2006: fig. 4b), whereas in the new species it barely reaches beyond halfway down of the inner margin of the outer lobe.