Boroecia antipoda ( Müller, 1906 )

Stępień, A., Błachowiak-Samołyk, K., Krawczuk, M. & Angel, M. V., 2018, A revision of the genus Boroecia Poulsen 1973 (Ostracoda, Halocypridae) with the descriptions of three new species, Zootaxa 4394 (3), pp. 301-346 : 315-321

2.2. Boroecia antipoda ( Müller, 1906)

( Figures 4–6 )

Synonymy:

1906 Conchoecia antipoda Müller : 110, Pl xxvi 5–16 (‘ mollis group’). 1912 Conchoecia antipoda —Müller: 97.

1920 Conchoecia borealis antipoda —Skogsberg: 718–719, Fig. 137 1921 Conchoecia antipoda —Barney: 182.

1967 Conchoecia antipoda —Hillman: 199–200. 1969 Conchoecia antipoda —Hillman: 189

1972 Boroecia antipoda —Rudjakov: 52–62

non 1973 Boroecia antipoda —Poulsen: 170–171, Fig. 87 A–G. 1974 Conchoecia antipoda —Deevey: 375, Fig. 87 A–G. 1976 Conchoecia antipoda —Deevey: 43–46.

1978 Conchoecia antipoda —Deevey: 65, Fig 18 . 1981 Conchoecia antipoda —Angel: 557, Fig. 194–21. 1985 Boroecia antipoda —Hopkins: 202.

1992 Boroecia antipoda —Benassi et al.: 247.

1992 Boroecia antipoda— Kock: 54–59, Fig. 15 . 1999 Boroecia antipoda —Angel: Fig. 9.30.

This extensive synonymy indicates that this species has been regularly recorded in the Southern Ocean, where it is one of the dominant species in the halocyprid assemblages at mesopelagic depths (e.g. Błachowiak-Samołyk & Osowiecki 2002). Müller (1906) reported it at 12 of the Tiefsee stations. However, the record from Tiefsee Station 54 (in the Gulf of Guinea) must be now considered as highly dubious, because subsequent extensive sampling in this region (e.g. Poulsen 1969, Angel et al. 2007, Angel 2010) has failed to confirm its presence (or that of any other Boroecia species). Błachowiak-Samołyk & Angel (2003) plotted its geographical distribution based on all published reports, supplemented by a considerable amount of unpublished data from Discovery Investigations and showed that this species is almost entirely restricted to latitudes south of 40°S, i.e. it is endemic to the Southern Ocean. When we examined the specimens identified as B. antipoda from the Dana Expedition ( Poulsen 1973) sampled at low latitudes in the Pacific Ocean, all but one (the female from Dana Station 3642 (46°43’S; 176°09’E)), were found to be a different and novel species, which is described below as B. danae n. sp.

Type locality: Müller (1906) failed to designate a type locality. The specimens reported here were obtained at Discovery Station 9969#8 at 59°00’S 20° 12’E at a depth of 600–700 m on 22 February 1979. The closest Tiefsee Station at which Müller (1906) recorded this species was 139 at 55° 01’S, 21°34’E. We, therefore, propose that this Tiefsee station position should now be considered the type locality for the species.

Male. Carapace ( Figures 4 A,B ). The carapace length (2.94 ± 0.065) is significantly greater than that of B. borealis ( Table 1), but both its relative height (39.2% CL) and relative width (37.7% CL) are smaller. An ornamentation of rhomboidal reticulations occurs over most of the carapace particularly in the anterior region, but tends to be less distinct than the ornamentation of B. borealis . In B. antipoda the shoulder vaults are smoothly curved and sharply angled. The rostra (10.7% CL) are slightly longer than the incisures (8.9% CL). The posterior dorsal corner is angled at about 80° and on both valves, it is armed with 3 to 4 short spines. The RAG is located on the rounded posterior ventral corner and the LAG opens just anterior to the posterior end of the hinge between the two valves. There is a group of edge glands on both valves, which open on the posterior margin just below the PDC.

Frontal organ ( Figure 4 C ). The stem (32.1% CL) extends almost to the end of the first antenna. There is a clear suture between the capitulum (11.5% CL) and the stem, and it is angled down about 45°. It is broadest at its base and tapers slightly distally, ending in a blunt point. It carries spines on its basal half.

First antenna ( Figure 4 C,D ). The limb is clearly segmented and has an overall length of 32.76% CL. The base of the a-seta (14.26% CL) is S-shaped and swollen. The b-seta (38.32% CL) carries a few fine spinules close to its end. The c-seta is quite short (13.3% CL). The d-seta (36.57% CL) is slightly shorter than the b-seta, and carries a dense row of fine spinules level with the distal half of the e-seta armature. The e-seta (47.06% CL) has an armature of 42–47 pairs of short spines ( Figure 4 D ), which point basally. At the distal end of this armature there are three more small spines that are directed distally. Just beyond the end of the armature the seta is hinged and is then flanged with very fine spinules along the edge of the flange.

Second antenna ( Figure 4 E ). The protopodite is 47.96% CL, and three times the length of the first exopodite segment (15.79% CL), and more than twice the combined lengths of the remaining exopodite segments (7.09% CL) that carry the swimming setae. As in the other species there is a small terminal seta at the distal end of the first exopodite segment that curves around the articulation with the second segment. The a-seta is about 75% the length of the b-seta, and both these setae are straight and bare. The c- and d-setae are bare and slim and a little shorter than the width of the second segment. There is a small curved e-seta inserted at the base of the f-seta. The g-seta (32.46% CL) is bare and slightly flanged towards its end. The f-seta is longer (44.84% CL) and more clearly flanged. The smaller left hook appendage ( Figure 4 G ) has a short base with a slight tubercle on the outer corner of the right angle. The distal arm is short and almost straight and its end is rounded and armed with a tiny curved thorn. The larger right hook appendage ( Figure 4 F ) has a basal section that is longer than the second endopodite segment and has two small inner tubercles. It is angled at about 135°, and its distal arm is long and curves back a further 45°. The end is pointed and extensively ridged sub-terminally.

Mandible ( Figure 4 H ). The setation of the limb is similar to that of B. borealis . The longest terminal seta is 16.7% CL. The toothed edge of the basale ( Figure 4 I ) has the sharp pointed and blunt spindle-shaped setae well off-set, and both are bare. The first of the six subserrate triangular teeth is also off-set from the other five. The broad inner tooth overlaps the three outermost small teeth as in B. borealis .

Labrum ( Figure 5 B ). It has a smooth concavity and is flanked on either side by 24–25 filaments.

Maxilla ( Figure 5 A ). There is a stout basal seta. The first exopodite segment has three setae on its anterior face, a lateral seta, and six setae on its posterior face. The terminal segment has a patch of four spines near its base on its inner face. There are three terminal hook setae, with the central one subtended by a pair of slim setae.

Fifth limb ( Figure 5 C ). The limb resembles that of B. borealis . The basale has six setae ventrally and two distal setae. Laterally it has a pair of lateral setae on the inner face, and the plumose seta that is normally dorsal appeared to be displaced laterally. The long dorsal seta extends to just beyond the bases of the terminal setae on the terminal segment. The first segment of the endopodite has a pair of medial setae ventrally and a medial dorsal seta. On the last segment there are three setae, the central terminal seta is the longest (7.0% CL), which is relatively shorter than in all the other species ( Table 3).

Sixth limb ( Figure 5 D ). The epipodial seta formula is 7 + 5 + 5. Ventrally, the basale has two plumose setae, and three short bare setae distally. The dorsal seta is exceptionally short, and no plumose seta was seen. The first endopodite segment has only a single short medial ventral seta, and the second segment has a single short medial seta both ventrally and dorsally. The three long terminal setae on the terminal segment are all plumose, sub equal and long (29.0% CL).

Seventh limb ( Figure 5 E ). The limb is obscurely two-segmented, and has two unequal terminal setae—the longer being twice as long as the shorter.

Copulatory appendage ( Figure 5 G ). The shape of the appendage is similar to that of the other species in the genus. Its length is 27.5% CL and the width/length ratio is 21.6. There were 8 oblique muscles in the specimen examined.

Caudal furca ( Figure 5 F ). The caudal furca has the typical structure for the genus with eight pairs of spine setae, and the marked disparity between the lengths of the fourth and fifth pairs that is typical of the genus (see Table 2). The tips of the longest first pair of spine setae are curved into slim points and have lengths of 22.4% CL. The first six pairs are coarsely spinose but the terminal two pairs are bare. There is an unpaired seta.

Female. Carapace ( Figures 6 A,B ). The carapace lengths range from 3.00– 3.36 mm. We have measured 339 specimens collected during Discovery Investigations between 1925 and 1936 and found the mean length to be 3.19 ± 0.065 mm. A further 108 specimens that were collected at various depths at Discovery station 9969 (22 February 1979, 59°S 20°W) had a mean length of 3.18 ± 0.061 mm, and we note from Kock’s data (1992, 1993) 29 specimens with a mean length of 3.23 ± 0.048 mm. So, the carapace lengths of this species are stable both spatially and through time (5 decades). This species is only slightly smaller than the largest species in the genus B. maxima . The carapace height is 45.5 ± 1.54% CL (n=24) and width is 34.5 ± 1.48% CL (n=24). The maximum carapace height is at three-quarters length and well anterior to the level of the PDC. The carapace has a marked taper. The shoulder vaults are sharply edged and smoothly curved and are clearly visible in ventral view ( Figure 6 B ). Due to the shoulder vaults the specimen cannot roll over, when turned on its dorsal side. The posterior dorsal corner is obliquely angled and on the right carapace valve are four small spines, whereas the left valve has none. The posterior ventral margin curves smoothly into the ventral margin with the RAG opening on the curve. The LAG opens close to the posterior dorsal corner just anterior to the posterior end of the hinge between the two valves. The entire carapace is covered with a diamond pattern of striae, which is clearest anteriorly.

Frontal organ ( Figures 6 C,D ). The long stem (20.2%CL) projects beyond the end of the first antenna. The capitulum ( Figure 6 C ) (10.2% CL) is clearly sutured from the stem and is angled down about 30°. It is S-shaped with a pointed tip, and is coarsely spinose dorsally along the basal third, and along most of its ventral surface.

First antenna ( Figure 6 D ). The segmentation of the limb (18.8% CL) is clear and both the two basal segments have ventral patches of fine spinules (note the base of stem is anterior to the posterior margin of the limb so the frontal organ extends much further anteriorly than the measurements seem to suggest). The dorsal seta on the second segment is long (13.4% CL) and nearly extends on a level with the tip of the capitulum. The e-seta (37.8% CL) is three times longer than the a–d setae (12.3% CL), and its distal quarter is slightly flanged, and lined with very fine spinules.

Second antenna ( Figures 6 E,F ). The protopodite (41.1% CL) is more than twice as long as the slightly curved first exopodite segment (16.1% CL), which is also over twice the length of the remaining exopodite segments (6.8% CL). The first segment of the endopodite has a processus mamillaris with a triangular end and the a-seta is only slightly shorter than the b-seta, both of which are bare. The second endopodite segment is quite small. The fseta is 27.6% CL, the g-seta 42.4% CL and the h–j setae are 18.7% CL.

Mandible and maxilla. Could not be differentiated from those of the male.

Fifth limb ( Figure 6 G ). This limb closely resembles that of the male. The basale has four proximal setae and two distal setae ventrally, and laterally a divergent pair of setae and a single distal seta. Both setae on its dorsal surface are long. The long bare dorsal seta reaches just beyond the end of the terminal segment and the adjacent plumose seta is not much shorter. The first endopodite segment has a pair of medial ventral setae that just reach the level with the end of the segment, and a slightly longer single dorsal seta. The last endopodite segment has three terminal setae, the central seta of is the longest (7.7% CL).

Sixth limb ( Figure 6 H ). The epipodial setal formula is 6+5+5. The basale has a line of fine spinules along its basal half, in front of the setae, which are arranged as a pair of plumose setae, a pair of bare setae, and terminally another single plumose seta. On its dorsal surface, the basale has a short bare terminal seta, and laterally below it, a short plumose seta. The first endopodite segment has a single short medial ventral seta, and the second has a single medial ventral seta and a slightly longer medial dorsal seta. The three setae on the terminal segment are quite slim, the central one being the longest (10.4% CL).

Caudal furca. Resembles that of the male.

Figures

FIGURE 4. Boroecia antipoda male 2.92 mm (Exemplar specimen Discovery Station 9969#8 59° 00’ 00”S; 20° 12’ 18”E 600–700m depth): A. Carapace, lateral view; B. Carapace, ventral view; C. First antenna and frontal organ; D. First antenna, armature of d- and e-setae; E. Right second antenna; F.Second antenna, endopodite, right hook appendage; G. Secondantenna, endopodite, left hook appendage;H.Mandible; I. Mandible, toothed edge of basale.Scales in mm.

FIGURE5. Boroecia antipoda male 2.92 mm (Exemplar specimen Discovery Station 9969#8 59° 00’ 00”S; 20° 12’18”E 600– 700m depth):A.Maxilla; B. Labrum; C. Fifth limb; D. Sixth limb; E.Seventh limb.F.Caudal furca; G.Copulatory appendage. Scales in mm.

FIGURE 6. Boroecia antipoda female 3.62 mm (Exemplar specimen Discovery Station 9969#8 59° 00’ 00”S; 20° 12’18”E 600–700m depth):A. Carapace, lateral view; B Carapace, ventral view; C. Frontal organ capitulum; D. Frontal organ and first antenna; E. Second antenna; F.Second antenna endopodite; G.Fifth limb; H, Sixth limb.Scales in mm.

FIGURE 15. Boroecia alaska female 3.04 mm, Paratype. A. Carapace, lateral view; B. Carapace, ventral view; C. Capitulum of frontal organ; D. First antenna and frontal organ; E. Second antenna; F. Endopodite of second antenna; G. Detail of end of first segment of second antenna endopodite showing the terminal seta; H. Labrum; I. Sixth limb; J. Caudal furca; K. Detail of 6–8th caudal furca spines.Scales in mm.

FIGURE 18. Boroecia hopcrofti female 2.76 mm. Paratype. A. Carapace lateral view; B. Carapace, ventral view; C. Frontal organ,capitulum; D. First antenna and frontal organ; E. Second antenna; F. Second antenna endopodite; G.Fifth limb; H. Sixth limb. Scales in mm.