Carapace quite large (for the genus); lateral outline subtrapezoidal, rectilinear; anterior and posterior margin narrowly rounded; dorsal angle conspicuous; ventral margin fairly straight. Zone of concrescence mostly medium sized, but fairly wide anteroventrally; radial pore canals mostly straight, but also few ramified; calcified inner lamella wide, vestibules fairly closed. In all specimens LV larger than, and overlapping RV anterodorsally, posterodorsally and ventrally.
Antennulae and antennae with slender podomeres and setae. Podomeres II and III of antennulae fused, no suture visible. Length of podomere IV of Antenna approximately two times length of podomere III. Base of mandibula with one conical plus four or five tricuspidate teeth-like setae; exopodite with one reduced plus six or seven medium-sized setae. Vibratory plate of maxillula with two strahlen plus around 16 feathered setae. Exopodite of fifth limb with three proximal and seven distal setae; podomere V of fifth limb (= palp podomere IV) with short and thick, terminal claws, dorsal one shortest, medial one longest. Male fifth limb very asymmetrical, right appendage with one very long and thick modified seta (= peg), plus one short, modified seta (= peg) and one short, simple seta on podomere II (palp podomere I); podomere III smoothly curved; left appendage with two short, modified setae (= pegs), and one short, simple seta on podomere II (palp podomere I); podomere III pointed at 90°. Podomere II of sixth limb with three long setae; podomere VI with one long and one medium-sized claw, and one short seta. Furca with symmetrical, barbed rods, and short, thick, distal setae fused to rod. Male copulatory limb subtrapezoidal in outline; copulatory process trisegmented, proximal element thick, robust, rod-shaped; weakly sclerotized, irregularly shaped medium element; and ‘cactus’-shaped terminal element. Zenker’s organ with thin central, sclerotized tube, terminal bulb medium-sized; vas deferens arranged in few loops, shorter than central tube. Genital lobes suboval.
Carapace quite large (for the genus); lateral outline subtrapezoidal, rectilinear; anterior and posterior margins narrowly rounded. In all specimens studied herein, LV larger than, and overlapping RV anterodorsally, posterodorsally and ventrally. Podomere V of female appendage V (= palp podomere IV) with short and thick, terminal claws, dorsal one shortest, medial one longest. Furca with symmetrical rods. Hemipenis subtrapezoidal in outline; copulatory process trisegmented, with ‘cactus’-shaped terminal element. Zenker’s organ with thin central, sclerotized tube, terminal bulb medium-sized; vas deferens arranged in few loops, shorter than central tube.
Macroscapha cactus is most similar to the species included in the informal group Macroscapha tensa-opaca (see below).
Valve measurements: Holotype, RV L 2.16 mm, H 0.76 mm; LV L 2.18 mm, H 0.78 mm. A F L 2.16–2.17 mm, H 0.81–0.82 mm; A M L 2.05–2.31 mm, H 0.76–0.82 mm; (A–1) L 1.74–1.79 mm, H 0.64–0.65 mm; (A–?3) L 1.04 mm, H 0.40 mm.
The informal species group Macroscapha tensa-opaca includes the following species: Mh. opaca Maddocks, 1990, Mh. cactus Brandão, 2010, Mh. falcis Brandão, 2010, and three new but unnamed species (Mh. tensa was considered a nomen dubium).
This species group is characterized by:
(1) an elongate, often rectilinear outline, with acutely pointed posterior angle;
(2) podomeres IV and V (= palp podomeres III and IV) of the female fifth limb present medium-sized, thick claws;
(3) symmetrical furcal rods;
(4) ‘reverse’ valve overlap; the LV is larger than and overlaps the RV ventrally, antero-, and posterodorsally. Noteworthly, in only the adult and juvenile females of Mh. opaca from the Ross Sea (stations ‘H in 4’ and ‘H in 5’), the right valves are always larger than and overlap the left valves;
(5) the male copulatory limbs are subtrapezoidal with a copulatory process composed of three segments, the most proximal one is elongate, fairly straight, and quite invariable amongst species; the second and third segments are shorter and vary greatly in shape. These two latter segments are the most important characters differentiating amongst the species of the informal group Macroscapha tensa-opaca.
Additionally, the valve lateral outline of Mh. cactus differs from other previously described Macroscapha species in: (1) it is more subtriangular and rectilinear than Mh. atlantica, Mh. gyreae, Mh. heroica, Mh. jiangi, Mh. inaequalis, Mh. inaequata, Mh. marchilensis, Mh. opaca, Mh. sinuata, Mh. rehmi, Mh. scotia, Mh. turbida, and Mh. walterae; (2) it presents a more narrowly rounded anterior margin and more acutely pointed posterior margin than Mh. tensa nomen dubium (based on drawings of the lost lectotype (Maddocks, 1990: figs 14.2, 15.2)).
For the soft part anatomy, I follow Horne et al., 2002 (Horne DJ, Cohen A, Martens K. 2002. Taxonomy, morphology and biology of Quaternary and living Ostracoda. In: Holmes JA, Chivas AR, editors. The Ostracoda. Applications in Quaternary Research. Washington, DC: American Geophysical Union. p 5-35.).
We use the following terms for the limbs/furca from anterior to posterior with alternative names in parenthesis:
antennula (antenna I, Aa, AI, first antenna)antenna (antenna II, AII, Au, second antenna)mandibula (mandible, Md)Maxillula (first maxilla, maxilla I, maxillule, MxI, Mxu)Fifth limb (ApV, Appendage V, first thoracic leg, maxilla, maxilla I, MxI, Mxu, maxilliped, clasper (for males), P1, second maxilla, walking leg)Sixth limb (ApVI, Appendage VI, first thoracic leg, second thoracic leg, P2, walking leg)Seventh limb (ApVII, Appendage VII, cleaning limb, P3, second thoracic leg, third thoracic leg, walking leg)Male copulatory limb (eighth limb, hemipenis, uropod). Whether the Male copulatory limb is homologous to a crustacean limb remains controvertial.Furca (caudal rami, Fu, furcal rami, uropods). The homology of the podocopan furca (anterior to the anus) to a myodocopan furca (posterior to the anus) remains controversial. Similarly, the homology of the ostracod furca to the furca of other crustaceans remains controvertial.
Antennula 1, 2(0/.2), +3(.1/.1.), 4(.1/ .1.), 5(.1/.1), 6(.2/.3), 7(0/0:4). Antenna 1(0/:1), 2(0/ 0:1), Exopodite (0/0:2,1r), 3(0/.6.4), 4[female (.1r./ .1r.1c,3)] [male (.1r./.1r.1c,2mod,1)], 5(0/.1c,1:4c,1), 6(0/0:2c,3). Mandibula 1(.1./4t,+5.), 2(0/.2:1), Exopodite (0/0:1r,7), 3(0/.4:4), 4(.3.2/.4), 5(0/0:3c,2). Maxillula vibratory plate (3re,+17), palp 1(.1/0), 2(.4/0), 3(0/0:5- 6). Fifth limb 1(0/.1), Exopodite (0/0:3.7), [female 2, 3, 4(0/.1) 5(.1./0:1c,1)]; [male 2(.1r/2mod,1), 3(0/ 0:1mod)]. Sixth limb 1(.2/0), 2(.2.1/.1), 3(.1/.0), 4(.1/0), 5(1,1r/0) 6(0/0:1,2c). Seventh limb 1(0/0:1), 2(.1.1.1/1) 3(.1/0), 4(.1/0), 5(.2/0), 6(0/0:2,1re). Furca 1(0/0:3r.1).
Holotype: 1 A M (SNB 0041), EASIZ II, # 171, ZMH K-41483.
Paratypes: 2 A F, 4 A M (SNB 0042, 0764-0766), 2 (A-1), 1 (A-3?), EASIZ II, # 171, ZMH K-40824.
2010 Macroscapha cactus Brandão: Figs. 43, 44A, 45A, 47A–C, 47P, 48A–C, 49A–B, 50, 51A, 52E–F, 52.2
For the resemblance of the terminal element of the copulatory process of the hemipenis to a cactus (name used in apposition).
Only known from the type locality in the Eastern Weddell Sea, 231 m.