Seborgia vietnamica, Jaume & Sket & Boxshall, 2009

Jaume, Damià, Sket, Boris & Boxshall, Geoff A., 2009, New subterranean Sebidae (Crustacea, Amphipoda, Gammaridea) from Vietnam and SW Pacific, Zoosystema 31 (2), pp. 249-277 : 260-268

publication ID	https://doi.org/ 10.5252/z2009n2a3
persistent identifier	https://treatment.plazi.org/id/038887C1-FFD4-E84C-BE09-008AFDB6A040
treatment provided by	Marcus
scientific name	Seborgia vietnamica
status	sp. nov.

Treatment

Seborgia vietnamica View in CoL n. sp.

( Figs 7-11 View FIG View FIG View FIG View FIG View FIG )

TYPE MATERIAL. — Vietnam. Vinh Ha Long (Ha Long Bay), Gulf of Tonkin, phreatic lake in cave Đông Duc Tiên ( GPS coordinates: 20°50.34’N, 107°16.77’E), island Đáo Van Giò, P. Trontelj & B. Sket coll., 17. VI.2003, holotype brooding ♀ (oostegites developed, setose) 2.76 mm, completely dissected and mounted on single slide ( BMNH 2009.38).

Paratypes same data as holotype: 18 specimens ( BMNH2009.39 - 48 ) including 7 brooding ♀♀, of which 5 measured (2.55, 2.55, 2.53, 2.60 and 2.88 mm) ; 32 specimens, sex unknown ( OB BF UL) .

OTHER MATERIAL EXAMINED. — Polje Viêt Hái headwater, between rootlets in spring (20°48’08’’N, 107°02’29’’E; coordinates approximate, derived from Google Earth), Đáo Cát Bà island , B. Sket & P.Trontelj coll., 15.VI.2003, 2 specimens, one clearly juvenile ( OB BF UL) GoogleMaps .

DIAGNOSIS. — Posterior margin of coxa of female pereiopod IV strongly oblique. Mid-lateral armature of protopod of uropod I comprising several flagellate robust setae.

ETYMOLOGY. — Species name derived from Vietnam, where it was found.

DESCRIPTION OF BROODING FEMALE

(MALE UNKNOWN)

Body ( Fig. 7A View FIG ) completely unpigmented, eyeless, similar to S. sanctensis n. sp. except for narrow triangular head lobe ( Fig. 8A View FIG ), variably produced rostrum (cf. Figs 7A View FIG ; 8A View FIG ), and posterodorsal margin of pleonites lacking serrations ( Fig. 11C, F View FIG ). Outline of epimeral plates variable, from all with posterodistal angle distinct and produced into pointed process to only angles of plate II produced into pointed process (Fig. 19G).

Antennule ( Figs 7A View FIG ; 8A View FIG ) peduncle segments length ratio 44: 33: 23. Peduncle-to-main flagellum length ratio 64: 36. Main flagellum 4-articulate, proximal article longest, all articles with aesthetasc.Accessory flagellum with three slender simple setae, one shorter simple robust seta and one penicillate seta.

Antenna ( Figs 7A View FIG ; 8A View FIG ) differing from S. sanctensis n. sp. in gland cone, directed laterally (vs. posteriorly); in length ratio of peduncle segments 4-5 (57.5: 42.5; vs. 47: 53 in S. sanctensis n. sp.); in length of peduncle segment 5 relative to first peduncle segment of antennule (segment shorter; vs. longer in S. sanctensis n. sp.); and in articles of flagellum, all approximately similar in length, with short aesthetasc present on articles 2 and 4 only.

Labrum ( Fig. 9A View FIG ) as in S. sanctensis n. sp. Paragnaths ( Fig. 8C View FIG ) with inner lobes longer than outer lobes (reverse of S. sanctensis n. sp.).

Right mandible differing from S. sanctensis n. sp. in composition of spine row, with series of buds probably corresponding to additional spines located proximal to three well-defined, ordinary distal spines ( Fig. 8B View FIG ); left mandible spine row with similar buds, although here there are four rather than three well-defined distal spines ( Fig. 9B View FIG ). Molar process of both mandibles with smooth anterior margin (vs. margin ornamented with setules or microspinules in S. sanctensis n. sp.) and distal seta smooth with trifid tip (vs. pappose and unicuspid in S. sanctensis n. sp.). Mandibular palp ( Fig. 9C View FIG ) differing in relative proportions of segments (22: 52: 26; vs. 28: 43: 29 in S. sanctensis n. sp.), demonstrating elongation of segment 2 in S. vietnamica n. sp., and in segment 3 narrow and subrectangular instead of rhomboidal; in addition, armature of distal segment reduced to four E-setae only (cf. D-setae also present on medial margin of segment in S. sanctensis n. sp.).

Maxillule ( Fig. 9D View FIG ) differing from S. sanctensis n. sp. in ornamentation of coxal endite, covered with long simple setules (vs. endite smooth in S. sanctensis n. sp.), in having only two setae instead of three on distal segment of endopod, and in much shorter endopod segment 2 (relative proportions of segments 50: 50; vs. 20: 80 in S. sanctensis n. sp.).

Maxilla ( Fig. 8D View FIG ) reduced to single lobe equivalent to outer lobe of S. sanctensis n. sp. based on common display of four armature elements; ornamentation of elements different; two medial elements tricuspidate, two lateral elements simple. Protuberance covered with weak setules positioned close to base of lobe on medial side presumably equivalent to inner lobe of maxilla.

Maxilliped ( Fig. 8E View FIG ) differing from S. sanctensis n. sp. in relative proportions of carpus-to-unguis of palp (41: 34: 19: 6; vs. 40: 31: 20: 9 in S. sanctensis n. sp.), especially on proportionately longer propodus (= palp segment 3) and noticeably shorter and more slender unguis; and in absence of transverse comb of long spinules on dactylus, among other features.

Coxa IV ( Fig.9F View FIG ) with portion of posterior margin below proximal square excavation strongly oblique; anterior margin of plate straight.

Gnathopod I ( Fig. 10A View FIG ) subchelate, propodus about 1.3 times longer than broad (vs. 1.5 times longer in S. sanctensis n. sp.), with pointed palm angle located at 54% of maximum length of segment (vs. at 47% in S. sanctensis n. sp.). Palm margin convex, densely ornamented with slender triangular denticles and row of about five submarginal flagellate robust setae; palm angle ( Fig. 10B View FIG ) with two robust setae, one on each side of angle, that on lateral side shorter (vs. longer in S. sanctensis n. sp.) and bifid, other simple. Dactylus ( Fig. 10C View FIG ) with two tiny robust setae terminally on medial margin (vs. long spine in S. sanctensis n. sp.); unguis short and stout, about 12% length of dactylus (vs. 33% in S. sanctensis n. sp.).

Gnathopod II ( Fig. 10D View FIG ) propodus subquadrate, with convex palm margin covered with stout triangular denticles and submarginal row of about nine robust flagellate setae; palm angle with short and stout, hardly visible simple robust seta ( Fig. 10F View FIG ); dactylus with two short simple robust setae terminally on medial margin, and with hyaline sheath almost completely covering unguis ( Fig. 10E View FIG ); unguis tiny, about 3% length of dactylus.

Pereiopods III-IV ( Fig. 9E, F View FIG ) with unguis fully incorporated into dactylus; latter unarmed.

Pereiopods V-VII ( Fig. 7 View FIG B-D) bases each about 1.5 times longer than broad, that of pereiopod V longest (vs. basis of pereiopod V shortest in S. sanctensis n. sp.); anterior margin of bases provided with 3 or 4 flagellate robust setae; posterior margin evenly convex with 3-5 serrations each provided with slender simple seta.

Pleopods with protopod variably setulose ( Fig.11A, B View FIG ); that of pleopod I with proximal swelling covered with long setules ( Fig. 11A View FIG ).

Uropods I and II ( Fig. 11 View FIG C-E) differing from S. sanctensis n. sp. in display of additional robust setae on posterolateral margin of protopod and on both rami. Uropod III ( Fig. 11F View FIG ) uniramous, with ramus considerably longer than protopod.

REMARKS

Seborgia vietnamica n. sp. is unique in the possession of a series of flagellate robust setae mid-laterally on the protopod of uropod I. The other species of the genus have a single robust seta at most in the homologous position.The strongly oblique posterior margin of coxa IV is also unique in Seborgia , although in S. relicta – where coxa IV is sexually-dimorphic – the margin in the female is slightly oblique (see Holsinger &Longley 1980: fig. 25d). The peculiar serrated tip of the long setules present on some limb segments is probably also characteristic of the species, although it is only visible under the highest magnification and could have been overlooked in the other species.Additional diagnostic features of the new taxon compared with other Seborgia species are given in Table 1.

ECOLOGY

A moderately dense population of these amphipods was found in the cave lake in Đông Duc Tiên on the small island Đáo Van Giò (eastern part of Ha Long Bay, 15 km off the coast). At the time of the visit it was a water body up to 1.5 m deep and approximately 100 × 20 m in surface area, fragmented by large boulders into a system of interconnected basins with loamy bottom. Water was slightly brackish (4-5 ppt salinity). From the nature of its fauna, we infer that this body of water is, at least during highest levels, connected to and included in the general body of phreatic water of the island. The island is slightly over 1 km 2 in surface area, with a complex branching outline, its width in few locations reaching as much as 400 m. Copepods and oligochaetes were extremely scarce. The lake is inhabited by a small population of a troglobiotic freshwater fish (Pisces, Balitoridae , Nemacheilinae ). It is surprising that a freshwater fish could have survived on such a small island, possibly since the Pleistocene, when it was a continental hill.

Only a few specimens were found on the larger island Đáo Cát Bà, in an inland karst spring, most probably in the permanently freshwater zone.