Seba bathybia Larsen 2007, n. sp.

Seba bathybia n. sp.

(Figs. 8–11).

Material examined. Holotype male, 2.8 mm (FMNH # 13758), Station code VOIJALV4046F, Dive 4046, near Wuzza Bare Mount, 3 September-2004 47°47.085’N 127°41.478’E. Depth 2656 m. Paratypes: 1 male, 2.1 mm, (KMNH IvR 700250). 1 male (dissected), 3.2 mm, 1 female, 3.3 mm (dissected), 1 female 3.5 mm (FMNH # 12771). 29 males 3 females, 5 sex? 1.8–3.6 mm, (FMNH # 12883). 9 males, 3 females, 2 sex? 1.2- 2.9 mm, (FMNH # 12906). 1 male 3.4 mm (dissected) 33 males, 9 females 12 sex? 1.3-3.6 mm (FMNH # 12987). 9 males, 3 females 2.0– 3.5 mm, (FMNH # 12995). 41 males, 5 females, 8 sex?, 2.2–3.8 mm, (FMNH # 13009). 10 males 4 females, 2.8–3.4 mm, (FMNH # 12027). 1 male 2.2 mm, (FMNH # 13127). 1 female, 3.2 mm, (FMNH # 13194).

Diagnosis. Coxa 1 and 3 with small posteroventral notch. Coxa 2 with prominent tooth at posteroventral corner. Coxa 4 clearly largest, posterior margin deeply excavated. Mandibular palp article 3 almost as long as article 2, with blunt apex.

Etymology. Named after the deep collection site, the deepest of any Seba (bathybia from Greek: bathys, bios = 'deep living’).

Description. Holotype, male 2.8 mm (only external body). Paratype, female, body length 2.4 mm (appendages).

Body (Fig. 8A). Smooth. Rostrum blunt. Eyes absent. Lateral cephalic lobe prominent. Epimeron without posteroventral spiniform processes. Epimeron 1-2 subrectangular, corners with small tooth, anterior margin straight, posterior margin weakly concave. Epimeron 3 larger, posteroventral corner smoothly rounded.

Coxae (Fig. 11A–G). Coxa 1 (Fig. 11A) shorter but wider than coxa 2, widest midlength, with 2 small distal teeth, w/d ratio 1.4. Coxa 2–4 size increasing, coxa 2 (Fig. 11B) elongated, widest proximally, with large posteroventral tooth at corner, w/d ratio 0.4. Coxa 3 (Fig. 11C) widest distally, with small posteroventral notch, w/d ratio 0.5. Coxa 4 (Fig. 11D) largest, posterior margin deeply excavated, corner acute, w/d ratio 0.7. Coxa 5 (Fig. 11E) bilobed, w/d ratio 1.4. Coxa 6 (Fig. 11F) weakly bilobed, posterior lobe much larger, w/d ratio 0.9. Coxa 7 (Fig. 11G) oval, with tiny distal notch, w/d ratio 0.8.

Antenna 1 (Fig. 8B, 8B 1). Longer than head and pereonites 1–3, length subequal to antenna 2. Peduncular article 1 0.9 times as long as article 2. Peduncular article 2 twice as long as article 3. Flagellum shorter than peduncle, with 4–5 progressively shorter articles, distally with setae. Accessory flagellum (Fig. 8B 1) with 2 articles, the proximal about 7 times longer than the distal. Peduncular article 3 & 4 with aesthetascs.

Antenna 2 (Fig. 8C). As long as antenna 1. Peduncular article 1 with head. Peduncular article 2 short, gland cone feeble. Peduncular article 3 square, one third as long as article 4. Peduncular article 4 1.33 times as long as article 5. Peduncular article 5 twice as long as article 3. Flagellum with 2–3 articles.

Mouthparts. Epistome-labral complex (Fig. 8D, 8D 1): epistome separate, wider than upper lip, naked; upper lip, weakly setose distally, slightly bilobed. Mandibles (Fig. 8E, F) incisor well developed, with blunt denticles; spine row (Fig. 8F 1) with 3 spiniform setae. Mandibular body with several setules. Molar reduced. Palp with 3 articles, article 2 and 3 subequal, naked. Article 3 without A3-seta, with 2 terminal E3 setae. Left mandible lacinia mobilis broad, with blunt denticles. Right mandible lacinia mobilis absent. Lower lip (Fig. 8G) inner lobes not visible, outer lobes small and sparsely setose, mandibular lobes blunt and rounded. Maxilla 1 (Fig 8H) inner plate with 1 distal seta, outer plate with 2 denticulate, 3 bifurcate and 2 simple spiniform apical setae arranged in a pseudocrown (Fig. 8H 1) and with several setules. Palp uniarticulate, with 2 terminal setae. Maxilla 2 (Fig. 8I) plates feeble. Inner plate with 3 simple setae. Outer plate with 3 pinnate setae and outer setules. Maxilliped (Fig. 8J) inner plates, small, narrow, just reaching the base of palp article 1 rectangular, apically with 2 spiniform and 1 simple setae. Outer plates subovate, short, reaching just beyond palp article 1, medially and distally with few simple and spiniform setae but without denticles, lateral margin naked. Palp articles wide but not elongated, articles 2-3 medially with scattered setae, lateral margin naked, article 3 medially with blunt bifurcated setae, article 4 claw-like.

Gnathopod 1 (Figs. 10A, 10A 1, 11A). Subchelate, but tending to chelate. Basis slender, 4 times as long as ischium and naked except for 1 large subdistal and 1 small distal setae. Ischium naked, as long as merus. Merus naked, marginally shorter than carpus. Carpus ventrally lobate, 0.3 times as long as propodus, with several ventral pinnate setae. Propodus rectangular, slightly widening distally (Fig. 10A 1) inclusive of palm 0.85 times as long as basis, ventral margin with many pinnate setae. Palm bent ventrally, relatively smooth, with evenly spaced inner setae. Dactylus proximally with dorsal seta and fitting palm.

Gnathopod 2 (Figs. 10B, 10B 1, 11B). Chelate. Basis naked, marginally longer than propodus (inclusive palm). Ischium marginally longer than merus, with 1 ventrodistal seta. Merus 0.2 times as long as basis, with 1 ventrodistal seta. Carpus longer than ischium, widening distally with 1 ventral seta. Propodus slender, narrowing distally (Fig. 8B 1) more than 3 times as long as carpus, with 1 dorsodistal seta. Lower finger of chela, straight, slender, and with only 1 ventral seta and 4 setae on palmar edge. Dactylus slender, straight, fitting palm, with 1 proximal dorsal seta.

Pereopods (Fig. 9A–E). Pereopod 3 (Figs. 9A, 11C) basis 5.4 times as long as ischium, with 1 small distal seta. Ischium one third as long as merus, with 1 small distal seta. Merus 1.2 times as long as carpus, with anterodorsal extension, with few scattered setae. Carpus marginally shorter than propodus, posterior margin with few scattered setae. Propodus more than half as long as basis, with 3 short spines. Dactylus less than half as long as propodus, naked. Pereopod 4 (Figs 9B; 11D) larger than pereopod 3, but of similar appearance. Pereopods 5–7 (Figs 9C–E; 11E–G) similar, basis subovate, broadened, posterior margins very weakly serrated, posteroventral lobes not extending beyond ischium. Merus with strong posteroventral lobes, reaching about half length of carpus. Carpus, about 0.66 x length of propodus. Dactylus about half as long as propodus, with 1 dorsal seta.

Brood plates (Fig. 9F). Slender, with 7 long setae.

Gills (Fig. 9F). Elongated, not pleated.

Pleopods (Fig. 10C, 10C 1). Fairly short and with only 4–5 articles on each ramus. Peduncle retinaculae slightly bent, blunt and articulated.

Uropods (Fig 10D–F). Uropod 1 (Fig.10D) peduncle with 2 distal and 1 medial bifurcfate spiniform setae, rami clearly longer than peduncle, outer ramus marginally longer than inner, with 1 proximal inner seta. Uropod 2 (Fig. 10E) peduncle with 2 distal bifurcate spiniform setae. Rami marginally longer than peduncle, both rami with 1 medial bifurcate spiniform seta. Uropod 3 (Fig. 10F) uniramous, peduncle with 1 simple distal seta; ramus longer than peduncle, with 2 spines and 2 setae at base of minute terminal article.

Telson (Fig. 10G). Entire, laminar, tapering distally, smoothly rounded, reaching to end of uropod 3 peduncle, with 1 distolateral seta on each margin.

Remarks. The genus Seba contains 14 species excluding S. tropica McKinney, 1980 (following the exclusion of this species from Seba by Shaw 1989) and the new species. Unfortunately many of these species are poorly described and often only from one sex. For these reasons a key is not created. This species is most similar to S. profundus Shaw, 1989 also reported from hydrothermal vents at the nearby ‘Explorer Ridge’ (Shaw 1989). However, it does diverge from Shaw’s (1989) description in several respects. Seba profundus is described with a heavy denticulation on the gnathopod 1 palm inner margin and most importantly with a small notch on coxa 2 posteroventral margin and small, non-excavate coxa 4. The new species lacks the gnathopod 1 palm denticulation, has a large notch on the posteroventral margin of coxa 2 and the coxa 4 is much bigger (clearly larger than the other coxae) and strongly excavated. For these reasons the new species is erected. Shaw (1989) did not illustrate the pereopods or the entire (total) gnathopods, so it is possible that these appendages would have revealed additional characters.

According to Holman & Watling (1983) this genus is notorious for its sexually dimorphic characters and allometric variations. However, little dimorphic difference were found between the male and female of this species. The gnathopods are identical and the merus of the last 4 pereopods (Fig. 12) reveals only minor allometric variation visible in a slight elongation in the female article. The possibility remains that none of the males present are fully mature terminal males, but given the large material, this seems unlikely.

Ecology. This species is the most abundant species found in the Juan de Fuca vent field. Unfortunately, it is not known if this species is associated with other invertebrates, as typical for Seba (Thurston 1974; Shaw 1989). Such an association is possible since the recovered wood blocks contained a number of other invertebrates, especially wood boring clams.