Proceroecia rivoltella, Benassi & Ferrari & Menozzi & McKenzie, 1994

Benassi, G., Ferrari, I., Menozzi, P. & McKenzie, K. G., 1994, Planktic ostracodes from the antarctic and subantarctic collected by the 1989 – 1990 Italian Antarctic Expedition, Records of the Australian Museum 46 (1), pp. 25-37 : 28-32

Proceroecia rivoltella n.sp. McKenzie & Benassi

Figs 2-3

Dimensions. HOLOTYPE, Museum of Natural History , University of Parma reg. no. 994/1, male, length [L] 1.49 mm, height [H] 0.725 mm, breadth [B] 0.675 mm, length to base of shoulder [S] 0.85 mm (Sample 6/3).

PARATYPES, Australian Museum P42286 - damaged female, L 1.42 mm, H 0.705 mm, S 0.825 mm (Sample 6/3); mature male, L 1.53 mm, H 0.765 mm, B 0.69 mm, S 0.86 mm (Sample 6/3). Other paratypes held in the working collections, Oceanographic Unit , Institute of Ecology, University of Parma - female, length 1.55 mm, height 0.705 mm, breadth 0.650 mm, length to base of shoulder 0.90 mm (Sample 7/ 3 bis); ovigerous female, L 1.59 mm, H 0.76 mm, B 0.710 mm, S 1.00 mm (Sample 6/2); damaged female, L 1.585 mm, H 0.745 mm, B 0.745 mm, S 0.98 mm (Sample 7/3 bis); damaged male, L 1.40 mm, H 0.63 mm, B 0.59 mm, S 0.84 mm (Sample 7/3 bis); juvenile female, L 1.235 mm, H 0.59 mm, B 0.59 mm, S 0.775 mm (Sample 7/3 bis); juvenile female, L 1.235 mm, H 0.59 mm, B 0.55 mm, S 0.78 mm (Sample 7/3 bis); juvenile male, L 1.27 mm, H 0.55 mm, B 0.49 mm, S 0.805 mm (Sample 5/2); juvenile male, L 1.18 mm, H 0.51 mm, B 0.51 mm, S 0.745 mm (Sample 10/2) .

Description. Shell ( Fig. 2 B-C) of the male regularly shaped; greatest height posterior of the middle and about 45-50% of the length; surface ornamented distinctively by concentric striations; rostrum arched with a rounded tip, incisure moderately deep and rounded, opening out towards the broadly rounded anteroventral margin; no selvage bulge; ventral margin evenly convex, with a slight protrusion posteroventrally (at the site of the asymmetric compound gland) in the RV, this protrusion not present in the LV; posterior margin broadly and evenly rounded; dorsal margin weakly concave from the posterodorsal area to the base of the shoulder, but straight to weakly convex from this point to the rostrum (the shoulder, therefore, is only moderately developed); greatest shell length is measured from the tip of the rostrum to level with the mid-posterior; greatest breadth posteromedial, ranging from a little less than to equal the height; asymmetric compound glands in the usual pOSItIOns (posteroventral in RV, posterodorsal in LV); lateral corner glands absent; dorsomedial glands in the males only, opening posterodorsally on the posterior margin; the ventral and posterior margins, from below the incisure posteriorly, are lined with numerous small subcircular medial glands.

Antennule ( Fig. 2 G-H). Both right (R) and left CL) antennules united proximally with the shaft of the frontal organ, which is further linked to them by a simple circlet attached mediodorsally to the second segment of each antennule (the arrangement is best observed in ventral view); scattered erythrophores can be seen in the ventral part of the first segment of each antennule and at the region of juncture with the stalk of the frontal organ. First and second antennule segments without any bristles; comparative lengths 330).lm and 250).lm respectively; terminal segment (or segment complex) short and bearing 5 bristles but no dorsal bristle. The 'a' bristle is a 'pipe' bristle, slightly broadened at its base and S-shaped proximally with a length of about 365).lm, the 'b' bristle is comparatively short and annulate with a length of 315).lm, the 'c' bristle is a very short (100).lm or less) 'pipe' bristle which is broadened basally; the 'd' bristle is longer and bent abruptly at about 75% of its length from its base, its total length being 585).lm; the 'e' bristle is very long (845).lm) and ornamented with a double row of 12-13 blade-like spines, pointing upwards and backwards along the bristle, plus 7-8 uniserial similar spines more proximally, this bristle also is annulate and flexes outwards where its ornament of spines terminates. The 'e' bristle armature is reminiscent of P. brachyaskos ( Mueller, 1906) .

Frontal organ ( Fig. 2 E-F). Shaft about 450).lm in total length, with a distinct suture about medially (proximal part 225).lm, more distal part 215).lm); distally of this suture about 40-50 pm it is attached by a simple circlet to the inner dorsal margin of the second segments of the Rand L antennules; cap has a length of 215 pm, is clearly separated from the shaft, and displays a well-rounded tip; in ventral view shape is digital and completely regular, but viewed laterally is seen to be slightly bent towards the front; bears scattered hairs ventrally.

Antenna ( Fig. 2 J-L). Muscular protopod has the characteristic halocyprid wedge-like shape and is 725 pm in length; exopod modified as a natatory structure, with the first segment about 255 pm long while segments 2- 8 (all very short) total about 140).lm in length; the natatory bristles are up to 785 pm in length and are characteristically annulate and feathered; the endopod first segment is about as high as it is long (length 150).lm) with a conical processus mamillaris proximally, and a lunate bulge distally on which latter are the 'a' and 'b' bristles; the 'b' bristle is about a third again as long as the 'a' bristle, both are annulate and appear to be adorned with minute hairs; second segment of the endopod is relatively short (about 65).lm), carrying 7 bristles; the 'e' bristle is minute, the 'c' and 'd' bristles both are short, the 'h', 'i' and 'j' bristles are sensory bristles and subequally long (150 jJm), the 'f' bristle measures about 685 jJm and the 'g' bristle is distinctly longer (785 jJm), both the 'f' and 'g' bristles are annulate and smooth; the clasping organ is dimorphic on the R and L antennae; the R antennal endopod is larger and curved back regularly and strongly; the L antennal endopod has a generally similar shape but is reflexed more acutely and is smaller; on both organs (R and L) tip is weakly retrousse and rounded terminally; on the R organ it bears several ventral corrugations which form an oval pad; whereas on the L organ the oval pad occurs but is minutely spinulose and does not show any corrugations.

Mandible ( Fig. 3 N-P). Coxale masticatory pad consisting of a densely-hirsute squared-off plate; above it what appear to be 2 minutely spinulose overlapping flaps; more proximally and laterally is a rounded structure off which radiate 4 rather broad-based subacuminate molar claws, behind these is a dense cluster of many (over 20) annulate spikes (the overall appearance is rather like a pincushion); proximal tooth list, below the masticatory pad, has a broad and flat anterior tooth followed after a brief gap by 4 conical teeth, then becoming denticulate and tapering away and up to the rear; median tooth list has a prominent pointed anterior tooth, the list then becomes denticulate and ends posteriorly with 2 stout teeth rather close together; distal tooth list begins anteriorly with a blade-like tooth then becomes denticulate and ends with a relatively powerful pointed tooth; the triangular anterior condyle is well chitinised; overall length of the coxale, from its base to the upper condyle is about 240 jJm. Basale wedge-shaped, oriented at right angles to the coxale and highest proximally at its meshing with the coxale, its length is 275 jJm; the basis is dentate and has a well-developed ventral tooth; on the anterior side of the basis are 2 moderately long annulate bristles set well apart, about level with the higher but set medially are 2 shorter annulate bristles, and medially, towards the juncture with the first endopod segment is a small annulate bristle. Epipod a minute bristle extruding from an oval depression on the epipodial hump, near the coxale. Exopod distinctive; shaped like a cocked pistol proximally but terminating as a rather elongate pilose bristle. Endopod 3-segmented; first endopod segment 140 jJm in length with a single annulate dorsodistal bristle, adorned medially by radiating spiky hairs, plus a prominent lunate bulge ventrodistally which bears 2 minute setules; second segment 100 jJm long, with 2 unequal smooth annulate ventrodistal bristles, plus 3 unequal annulate dorsodistal bristles, 2 of them slender and smooth, the third powerful with spiky hairs; third segment 90 jJm long and carrying 7 annulate terminal bristles - I rather short and smooth and set mediodorsally, 3 longer, slender and smooth, set ventrodistally, another like these but hirsute and set medioventrally, and 2 powerful but unequal bristles (the larger about one third longer than the smaller), both adorned with stiff spiky hairs.

Paragnath. Widest proximally and linguiform, clothed distally on its inner surface with 9-10 long hairs.

Front of the head. Conical in lateral view, trending anteroventrally; rounded in front, gently convex ventrally and gently concave dorsally; in lateral view 275 jJm long and 315 jJm high. Viewed from below it is broad and flat in front; posterior part of the upper lip has a small central notch in the median element, while the rake-like processes on either side each have 2 relatively coarse inner teeth and a row of 6-8 similarly sized outer teeth.

Maxillule ( Fig. 3Q ). First endite with 7, mainly spinelike, terminal bristles, the anteriormost adorned with spiky hairs; second endite with 8-9 mainly spine-like terminal bristles, the anteriormost subfa1cate, plus 2 more proximal bristles on the inner side; additionally, there are 1-2 longish bristles set just above the margin of the palp but projecting downwards along it; palp (endopod) first segment quite wide with respect to its length, carrying 5 unequal dorsal annulate bristles and 3 annulate ventral bristles, plus at least 1 annulate median bristle; second segment hand-shaped and directed backwards, with 5 curved unequal terminal bristles, 3 of these coarser and claw-like, the intervening 2 slender.

P1 ( Fig. 3S ). Subpediform; epipod tripartite with 3 sets of Strahlen numbering 5, 5 and 5 respectively from upper to lowermost; protopod rectangular; first endite with 1 short and 1 long annulate bristle, well separated, the latter hirsute; second endite with 3 bristles, 1 short and claw-like, the other 2 unequal but both are annulate and carry long hairs; endopod uni-segmented, plump, with 5 terminal bristles (2 more powerful than the rest and claw-like), plus 3 annulate bristles on the inner side of the ventral endopod; exopod 3-segmented; first segment with 3 proximoventral, 2 medioventral, 2 ventrodistal and 1 dorsodistal bristles (the latter being largest); second segment with 2 medioventral bristles; third segment with 3 unequal terminal bristles; lengths of the segments 140 jJm, 130 jJm and 20 jJm respectively.

P2 ( Fig. 3 U-V). Subpediform, reflexed backwards and upwards; epipod tripartite with 3 sets of Strahlen numbering 7 (one small), 5 and 5 respectively; endopod with a single distal bristle; exopod 4-segmented; first segment with at least 4 small medioventral and ventral bristles plus 1 dorsodistal bristle; second segment with 1 small medioventral bristle; third segment with a small bristle medially on each side; fourth segment with 3 long flexuous annulate terminal bristles which are clothed with long hairs distally and have a length of 650 jJm; lengths of the exopod segments are 235 jJm, 160 jJm, 175jJm and 20 jJm respectively. The sex dimorphism of this limb is characteristic for all halocyprids.

P3. Reduced and 2-segmented; second segment smaller, but slightly more expanded than the first segment, and carrying 2 very unequal terminal bristles, the muchlonger of them whip-like and strongly recurved.

Penis ( Fig. 3X ). Finger-shaped, relatively wide, and 245 jJm in length, with a few (4-5) medial transverse muscle bands.

Furca ( Fig. 3Z ). Lamellar and bearing 8 claws, the first of these is the largest and separated from the others by a distinct gap, the second and third claws are subequal, the fourth to eighth claws reduce regularly in size.

Female. Shell ( Fig. 2A ) resembles that of the male, but does not have posteroventral dorsomedial glands. Female antennules do not have a circlet connecting them with the frontal organ ( Fig. 2D ); each antennule carries 5 terminal bristles of which 4 ('a'-'d') are 'pipe' bristles and subequally long, while the 'e' bristle is longer and slender with fine lateral hairs. The cap of the frontal organ in the female is not clearly differentiated from the stalk, terminates in a pointed tip and is finely pilose ventrally. On the endopod of the female antenna [Fig. 21], setae 'c' and 'd' are missing; 'e' is minute and 'f' is only slightly longer than 'g', 'h', 'i' and 'j', all ('f'­ 'j') are sensory; and there is no clasping organ. The mandible ( Fig. 3M ), maxillule and PI ( Fig. 3R ) are all similar to those of the male, except that the epipod of the PI carries fewer Strahlen, 4, 3 and 4, although their arrangement remains tripartite. The P 2 in the female ( Fig. 3T ) is again very similar to that in the male but smaller, also the bristles of exopod segments 1, 2 and 3 are all stronger than in the male, and the 3 terminal bristles of the fourth segment are unequal, the strongest being claw-like; on the first exopod segment of the P2 the chaetotaxy is 2 medioventral, 2 ventrodistal and 1 dorsodistal. The P3 ( Fig. 3W ) and furca resemble those of the male. Adult females are easily recognised through the translucent valves because they are usually ovigerous; when first formed the eggs are small and densely packed, when released they about double their volume (large eggs) ( Fig. 3Y ).

Etymology. Rivoltella (Italian) = pistol, for the shape of the proximal mandible exopod.

Remarks. As noted earlier, the new species belongs among a small group of conchoeciines, the 'procera' group of Mueller (1906) [see also Angel, 1971] which Poulsen (1973) brought into Paraconchoecia Claus, 1891 . Reprising, this species group is characterised by asymmetric compound glands in the usual conchoeciine position, the absence of lateral corner glands, the occurrence of posterodorsal dorsomedial glands only in males, abundant medial glands along virtually the whole free margin below the incisure, the absence of a dorsal bristle on the furca, the occurrence of at most two annulate bristles with or without associated minute setules on a prominent ventral bulge of the first endopod segment of the mandible, and a straightedged, densely hirsute masticatory pad on the mandible coxale.

Our new species shares these features but is readily distinguished from others in the group by the single long annulate bristle and two minute setules on the lunate bulge of the first endopod segment of the mandible and the relatively short 'b' bristle of the male antennule (these features are readily observed at ordinary binocular magnification by removing the antenna from one side of the animal). Of the other 'procera' group species, P. procera , microprocera and macroprocera all have one long annulate bristle ventrally on the first mandible endopodite segment, but no minute setules ( Angel, 1971), P. brachyaskos has one and P. decipiens has two long annulate ventral bristles ( Poulsen, 1973), but no minute setules; P. vitiazi Rudjakov, 1962 has one such bristle plus three minute setules ( Angel, 1971); P. hoensis Poulsen, 1973 has neither long annulate bristles nor setules ( Poulsen, 1973). In all these other species, the 'b' bristle of the male antennule is nearly as long as the 'd' and 'e' bristles.

It is interesting to note that our most southerly record of P. rivoltella (about 71°S) was taken at 200 to 160 m in near surface waters; whereas the specimens found at 68°35'S were collected at 800 to 700 m depth; and the specimens from about 66°S were taken at depths between 1000 and 700 m. Since the Antarctic Convergence in this part of the Southern Ocean oscillates between 60° and 65°S, P. rivoltella probably follows antarctic water down the gradient at the boundary region. Now that it has been described and differentiated from other species in the 'procera' group, therefore, we expect that P. rivoltella will be determined in the deepwater zooplankton of the southern oceans at more northerly latitudes and, indeed, could become an index for tracing the movement of antarctic water into these latitudes.

Figures

Fig. 2. A - shell of adult ovigerous female, x40 (paratype, sample 7/3 bis); B - posterior of adult male shell, LV asymmetric compound gland (a), dorsomedial glands (d), medial glands (m), xl25 (holotype, sample 6/3]; C TODO posteroventral margin of adult male RV, asymmetric compound gland (a), medial glands (m), xl25 (holotype). All subsequent figures are of the same female paratype and male holotype. D - female Al (antennule) and frontal organ, xI25; E - male frontal organ (ornament of cap not shown), indicating position of erythrophores, x125; F - male frontal organ cap, detail, x500; G - male AI, detail of location of terminal bristles, 'a' and 'c' are 'pipe' bristles, 'b' is unusually short (good specific character), 'd' and 'e' are long, x125; H - male Al (antennule), detail of 'e' bristle armature, x500. I - female A2 endopod, showing 'a', 'b', and 'f'-'j' bristles, x125; J - male A2 endopod, processus mamillaris (pm) and 'a', 'b' bristles, x500; K - male A2 left (L) clasping organ, x500; L male A2 right (R) clasping organ, x500.

Fig. 3. M - female mandible, basale and endopod, showing location of exopod (e) and sites of minute setules (*) of the ventral first endopod segment, x125; N - male mandible general view, coxale, basale and endopod, showing minute epipodial bristle on medial hump of basale (near the coxale), exopod, and sites of minute setules (*) of the ventral first endopod segment, x125; 0 - male mandible, exopod showing the pistol-like proximal part and setal pilosity, x500; P - male mandible, detail of lunate bulge of ventral first endopod segment and its 2 minute setules x500; Q - male maxillule, second endopod segment (the 'hand'), detail of terminal claws and bristles, x500; R - female PI exopod, x125; S - male PI exopod third segment, detail of terminal bristles, x500; T - female P2 exopod, x125; U - male P2 epipod (detail of some Strahlen) and exopod (terminal bristles not shown), x125; V - male P2, detail of ends of reflexed terminal bristles, x125; W - female P3, x125; X - penis, x125; Y - female posterior of body, showing large eggs, x125; Z - male furca, x125.