Seba marianaensis, Tokumori & Watanabe & Tanaka & Ohara & Tomikawa, 2025

Seba marianaensis sp. nov.

( Figs 2–6 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype: female 4.8 mm, AMNH _IZC 00361527; the surface of the chimney named ‘Chim 4’ in the CH 3 site in the Shinkai Seep Field ( Okumura et al. 2016) , Mariana Trench (11.656°N, 143.048°E), 5689– 5683 m, collected by H.K. Watanabe, 17 July 2014. GoogleMaps Paratypes: 7 females, 5.1 mm, 5.0 mm, 5.3 mm, 4.7 mm, 5.2 mm, 4.4 mm ( SEM), 4.3 mm ( SEM) ( AMNH _IZC 00361528); data same as for the holotype.

Diagnosis. Epimeral plate 3 posterodistal corner weakly produced. Mandible palp articles 2 and 3 with 1 and 2–3 setae, respectively. Maxilla 1 inner plate with apical seta. Maxilla 2 inner and outer plates with 3 and 3–4 apical setae, respectively. Gnathopod 1 coxa with tooth at posteroventral corner. Gnathopod 2 coxa with prominent tooth at posterodistal corner. Uropod 3 proximal article of ramus with dorsal setae.

Description of holotype female. Body ( Fig. 2 View FIGURE 2 ) relatively thick, dorsally smooth; eyes absent; urosomites 2 and 3 coalesced. Epimeral plate 1 ( Fig. 3A View FIGURE 3 ) rounded without setae; epimeral plate 2 ( Fig. 3B View FIGURE 3 ) ventral margin with two setae, posterodistal corner weakly produced; epimeral plate 3 ( Fig. 3C View FIGURE 3 ) ventral margin with seta, posterodistal corner weakly produced.

Antenna 1 ( Fig. 3D View FIGURE 3 ) 0.08 times as long as body length; length ratio of peduncular articles 1–3 in 1.0: 1.2: 0.6, peduncular article 1 with two penicillate setae on posterodistal corner; peduncular articles 2 and 3 with setae; accessory flagellum 2-articulate; primary flagellum 5-articulate, articles 1 and 2 each with two aesthetascs. Antenna 2 ( Fig. 3E View FIGURE 3 ) length 0.95 times as long as antenna 1; anterodistal corner of peduncular article 2 strongly produced; length ratio of peduncular articles 3–5 in 1.0: 2.1: 1.3; peduncular articles 4 and 5 with low of setae on anterior margins; flagellum 3-articulate.

Upper lip ( Fig 3F View FIGURE 3 ) anterior margin weakly concave without setae. Mandibles ( Fig. 3G, H View FIGURE 3 ) with left and right incisors 5-dentate; left lacinia mobilis comprising five teeth, right lacinia mobilis absent; molar process reduced; palp consisting of three articles, length ratio of articles 1–3 in 1.0: 3.6: 3.2 (left) and 1.0: 3.4: 3.0 (right), article 1 bare, article 2 with single seta, article 3 with three apical setae. Lower lip ( Fig. 3I View FIGURE 3 ) with broad, rounded outer lobes bearing fine setae; inner lobes indistinct. Maxilla 1 ( Fig. 3J View FIGURE 3 ) inner plate ovate with seta; outer plate with seven teeth, some of which serrate; palp 1-articulate with two apical setae. Maxilla 2 ( Fig. 3K View FIGURE 3 ) inner and outer plates with three and four apical setae, respectively. Maxilliped ( Fig. 4A View FIGURE 4 ) inner plate with two apical robust setae; outer plate not exceeding distal end of palp article 1, medial margin with three robust setae; palp 4-articulate, articles 2 and 3 with medial setae, article 4 curved with minute setae on medial margin and distal seta.

Gnathopod 1 ( Fig. 4B View FIGURE 4 ) chelate; coxa rounded, with posteroventral tooth, bearing marginal setae; basis slender, broadened distally, with long seta on posterior margin; ischium, merus and carpus with one, four and five setae, respectively; carpus length 1.4 times the width, 0.5 times as long as length of propodus; propodus length 2.2 times the width, posterior margin lined with setae, projected posterodistally, tip of projection with a pair of robust setae, palmar margin lined with setae; dactylus narrow, weakly curved, with distal setae. Gnathopod 2 ( Fig. 4C View FIGURE 4 ) slender, strongly chelate; coxa squarish, with prominent tooth at posteroventral corner, bearing marginal setae; basis weakly curved, with short setae; ischium, merus and carpus weakly setose; carpus length 2.2 times the width, 0.5 times as long as propodus; propodus length 4.4 times the width, projected posterodistally, tip of projection with small robust seta, palmar margin lined with setae; dactylus not exceeding tip of propodus, with distal setae.

Pereopod 3 ( Fig. 5A View FIGURE 5 ) coxa roundish square, without posteroventral tooth, with submarginal setae; basis almost straight, with setae; carpus and propodus with robust setae on posterior margins. Pereopod 4 ( Fig. 5B View FIGURE 5 ) coxa expanded bearing posterior concavity, posterodistal corner of ventral margin angled; basis almost straight, with setae; propodus with robust setae on posterior margin. Pereopod 5 ( Fig. 5C View FIGURE 5 ), coxa bilobed without setae; basis ovoid, narrowing distally, posterodistal corner lobate, anterior and posterior margins with setae; merus not expanded, produced posterodistally, with robust setae on posterior margin; carpus length 0.8 times as long as propodus; carpus and propodus with robust setae on anterior margins. Pereopod 6 ( Fig. 5D View FIGURE 5 ) coxa weakly bilobed, posterior lobe with marginal setae; basis ovoid, with posterodistal lobe, anterior and posterior margins with setae; merus not expanded, produced posterodistally, with robust setae on posterior margin; carpus 0.8 times as long as propodus; carpus and propodus with robust setae on anterior margins. Pereopod 7 ( Fig. 5E View FIGURE 5 ) coxa rounded, with ventral seta; basis widely expanded posteriorly, posterodistal lobe broad, anterior and posterior margins with setae; merus not expanded, produced posterodistally, with robust setae on posterior margin; carpus 0.8 times as long as propodus; propodus with robust setae on anterior margin.

Coxal gills on gnathopod 2 to pereopod 6. Oostegites ( Fig. 6A View FIGURE 6 ) on gnathopod 2 to pereopod 5.

Pleopods ( Fig. 6B View FIGURE 6 ) peduncles each with two coupling hooks and associate seta, surface with setae, outer distal corner strongly produced; inner and outer rami with five and four articles, respectively, plumose seta of rami long.

Uropod 1 ( Fig. 6C View FIGURE 6 ) peduncle with one dorsomiddle and two dorsodistal robust setae; inner ramus almost as long as peduncle, with small seta, distal part with tiny slit; outer ramus 0.9 times as long as inner ramus, without setae, distal part with tiny slit. Uropod 2 ( Fig. 6D View FIGURE 6 ) peduncle with two dorsodistal robust setae; inner ramus 1.1 times as long as peduncle, with robust setae; outer ramus slightly shorter than inner ramus, with robust seta. Uropod 3 ( Fig. 6E View FIGURE 6 ) uniramous; peduncle 0.6 times as long as ramus, with two dorsodistal robust setae; ramus 2-atriculate, proximal article with two dorsal and two distal setae, terminal article short, 0.1 times as long as proximal article.

Telson ( Fig. 6F View FIGURE 6 ) roundish, tapering distally, length 1.3 times the width, with a pair of distal setae.

Variation. Female 4.7 mm has three apical setae on the outer plate of maxilla 2. Two females 5.0 mm and 5.2 mm have two apical setae on the palp article 3 of mandible.

Etymology. The specific name is an adjective derived from the type locality, the Mariana Trench.

Remarks. S. marianaensis sp. nov. is most similar to S. bathybia Larsen, 2007 in having a prominent tooth at the posterodistal corner of the coxa of gnathopod 2. However, this new species differs from S. bathybia as follows (features of S. bathybia in parentheses): mandibular palp article 2 with seta (without seta), gnathopod 1 coxa with tooth at the posteroventral corner (two teeth, one at anterodistal and another at posteroventral corners), and uropod 3 proximal article of ramus with dorsal setae (without setae).

Interestingly, three of the four species of Seba reported from bathypelagic zones ( S. bathybia , S. longimera , and S. profunda ), were reported from hydrothermal vents ( Shaw 1989; Larsen 2007; Wang et al. 2019). In deep waters, where sunlight does not reach, detritus from surface water is the main food source for most animals. However, the transport of organic matter to the deep sea decreases significantly with depth, resulting in energy-deficient deepsea ecosystems ( McClain et al. 2012). In exceptional cases, hydrothermal vent and cold-water seep form special ecosystems in which chemosynthesis by chemosynthetic bacteria is the source of energy ( Bell et al. 2017). This study is the first to record this genus in cold-water seep with chemosynthesis-based communities. This suggests that the chemosynthetic ecosystems in hydrothermal vent and cold-water seep are important habitats for Seba in the deep sea. Although the species diversity of Seba in deep waters is not fully understood, intensive studies in such environments are essential to determine the true species diversity of this genus.

Disatibution. Known only from the type locality.