Bathynomus wilsoni, Ahyong, 2025

Bathynomus wilsoni View in CoL , new species

( Figs. 1–7, 8A)

Bathynomus kensleyi View in CoL . — Lowry & Dempsey, 2006: 184 [ Sulu Sea specimens only; not B. kensleyi Lowry & Dempsey, 2006 View in CoL ].

Material examined. HOLOTYPE: AM P42711 , female ( 215 mm), Sulu Trough , Sulu Sea, Philippines, 8°58.4′N, 121°21.1′E to 8°58.4′N, 121°21.2′E, 2,500 m, thermally insulated tube trap, PAPATUA Expedition Leg 08, PPTU08 WT, RV Thomas Washington, coll. A.A. Yayanos, 10–11 April 1986 GoogleMaps . PARATYPE: ZRC 2025.0098 View Materials , male ( 212 mm), collected together with holotype GoogleMaps .

Diagnosis. Cephalon anteromedian surface without shallow, irregularly ridged sulcus; cephalic ridge above eyes discontinuous; maxillipedal somite incisions deep, extending inward from posterolateral margin for length equivalent to about ⅓ distance between left and right incisions. Pleonites 1–5 combined length 17–18% body length; pleonites 3–5 pleural apices extending posteriorly to about same level or with pleuron 4 apices extending slightly beyond those of pleonites 3 and 5. Clypeus subpentagonal, lateral margins weakly convergent, distal margins concave, apex bluntly rounded. Pleotelson length about 0.8 × width, dorsal outline (excluding spines) evenly rounded, almost semicircular; posterior margin with 11–13 spines (lateralmost spine on either side short, small; remaining spines slender, upwardly curved), central spine apically bifid. Uropodal exopod lateral margin strongly convex, length 2.1–2.2 × width, setal fringe continuous (length 80%); endopod triangular, distolateral corner approximately right-angled, apex produced to short point.

Description. Body longitudinally ovate, widest at pereonite 4, body length 2.3 times as long as pereonite 4 width (excluding coxae), 2.1 including coxae ( Figs. 1A, 3A, 4A, 7A, C); surface densely punctate, without sculpture or granulation. Cephalon width half width of pereonite 1 (including coxae); ridge above eyes discontinuous ( Figs. 2A, 3C, 6A, 8A); anterior surface pitted and with fine, short, irregular striae, and narrow smooth, arcuate band, neither sulcate nor bordered by irregular ridges ( Figs. 2A, 6A, 8A); maxillipedal somite marked by deep, wide posterolateral incision extending inward from lateral margin, incision length equivalent to about ⅓ distance between left and right incisions ( Figs. 2A, 3C, 6A, 8A). Frontal lamina triangular, wider than long, margins carinate ( Figs. 2A, 3C, 6A, 8A). Clypeus subpentagonal, lateral margins weakly convergent, distal margins concave, apex blunt, narrowly to broadly rounded ( Figs. 2B, 3D, 6B).

Antennular peduncle ( Fig. 3C) 3-articulate; flagellum 1.3× longer than peduncle ( Fig. 3C). Antennal peduncle ( Fig. 3E) 5-articulate; article 5 longest, about 1.3× longer than article 4; flagellum extending to within pereonite 2, of 61 annulations in holotype.

Mandible ( Fig. 4D) incisor tridentate; molar process occlusal margin evenly dentate; palp not reaching incisor, article 1 half length of article 2; article 2 lateral margin setose on distal half; article 3 arcuate, lateral margin setose, length 0.4 × length of article 2. Maxillule ( Fig. 4E) lateral lobe occlusal margin with 11 corneous, simple, robust setae in U-shaped row; mesial lobe with 4 robust, circumplumose setae. Maxilla ( Fig. 4F) lateral lobe occlusal margin with 9 long simple setae, followed by [1]–9 short simple setae; middle lobe occlusal margin with [9] or 10 long simple setae followed by 6–[8] short simple setae; mesial lobe with fringe of plumose setae of varying length. Maxilliped ( Fig. 4G) endite with 5 coupling hooks.

Pereopods 1–5 coxal plates ( Figs. 1A, C, 3A, 5A, C) without fine central ridge, apically rounded on 1–3, bluntly angular on 4, acute on 5, coxa anterior margin gently concave to straight in dorsal view on pereopod 1, gently convex on coxae 2–5; holotype female with ovate oostegites ( Fig. 2B), length about 1/3 corresponding basis length.

Pereopod 1 ( Fig. 3A) basis length about 3.7 × width. Ischium half as long as basis; with row of 4 or 5 robust setae along posterior margin and 3[4–5] robust setae on posterodistal margin. Merus with 3 robust setae on anterodistal angle; inferior margin with proximal row of 4 robust setae and distal row of 4 robust setae. Carpus inferior distal lobe with 2 or [3] robust setae. Propodus length 2.3 × width; posterior (occlusal) margin with 6 [6 or 7] robust setae. Dactylus falcate, two-thirds propodus length.

Pereopod 2 ( Fig. 3B) basis length about 3.9 × width. Ischium half as long as basis; with row of 3 [4 or 5] robust setae along posterior margin and 3[3 or 4] robust setae on posterodistal margin. Merus anterodistal angle produced to midlength of propodus, lined with 7 [or 8] robust setae; inferior margin with 3 or 4 robust setae in proximal cluster and distal row of 4[3–4] robust setae. Carpus inferior distal lobe with 5 or [6] robust setae. Propodus length 2.7 × width; posterior (occlusal) margin with 4 or 5 robust setae. Dactylus falcate, two-thirds propodus length.

Pereopod 6 coxal plate ( Figs. 1C, 3B, 5C, 6D) with fine central ridge, apex narrowly acute, pointed.

Pereopod 7 ( Fig. 4C) coxal plate distally narrowed, gently curved posteriorly, dorsal margin gently sinuous, ventral margin convex, apex narrowly acute, pointed; outer surface with fine central ridge ( Figs. 1C, 2J, 3B, 5C, 6D). Basis length 3.7–3.8 × greatest width; inferior margin convex; superior margin (flexor) lined with long plumose setae. Ischium length 0.4 times × basis length, inferior margin with 1 or [2] robust setae near midlength; superior distal angle with 6–8 robust setae; inferior distal angle with 8–11 robust setae. Merus length 2.3 × width, 0.9 × ischium length; inferior margin with 1 or 2[2] robust seta near midlength, superior distal angle with 12–14 robust setae; inferior distal angle with 9[9–11] robust setae. Carpus as long as ischium, about 2 × as long as wide; inferior margin with 2 [2 or 3] robust setae near midlength (as 1+1 or 1+2); superior distal angle with 29–31 [21–23] robust setae encircling distal margin. Propodus as long as ischium, length 5.2 × width; inferior margin with 2 or 3 pairs of robust setae and 0–3 single setae, superior distal angle with 11 or 12 robust setae, inferior distal angle with 1 or 2 robust setae. Dactylus 0.5 × propodus length.

Pleonites 1–5 combined length along midline 17–18% body length ( Figs. 1A, 3A, 5A, 7A, C). Pleonite 3–5 pleural apices ( Figs. 1, 2G, I, J, 3A, B, 56D, E, 7A, B) extending posteriorly to slightly beyond mid-posterior margin of pleonite 5, pleonite 4 pleural apex extending slightly beyond those of pleonites 3 and 5; pleura 1–3 with fine central ridge, 4 or 5 without ridge. Penial processes ( Figs. 4K, 5B, 6C) as flat lobes, ovate, distally subtruncate, length 1.9 × width, separated by 0.17 × width of sternite 7.

Pleopod 1 ( Fig. 4H) exopod about twice as long as wide, widest slightly distal to midlength, lateral margin almost straight, mesial and distal margins convex; endopod subquadrate, with obtuse triangular lobe on lateral margin, length about 1.8 × width (excluding lobe).

Pleopod 2 ( Fig. 4I) exopod subtriangular, distal margin broadly rounded, slightly shorter than pleopod 1 exopod; endopod subquadrate, with obtuse triangular lobe on lateral margin, length about 1.6 × width (excluding lobe), as long as pleopod 1 endopod, male holotype without appendix masculina.

Pleopods 3–4 with form and proportions similar to pleopod 2.

Pleopod 5 ( Fig. 4J) exopod subovate, twice as long as wide, widest slightly distal to midlength, lateral margin strongly convex, mesial margin weakly convex, anterior distomedial surface with row of 2 or 3 [3 or 4] blunt nodules; endopod spatulate, mesial margin sinuous, distal margin rounded, lateral margin strongly convex, with obtuse triangular lobe on lateral margin, length about twice width (excluding lobe). Pleotelson length 0.7 × greatest width, dorsal outline (excluding spines) evenly rounded, almost semicircular, widest at anterior 0.3 ( Figs. 1A, 2C, 3A, 5A, 6E, F, 7); dorsal surface smooth (minutely pitted), evenly convex in cross section, with inconspicuous (scarcely discernible, even when surface is dried) longitudinal median carina, straight in lateral view; posterior margin with 11–[13] spines of which lateralmost spine on either side short, small ( holotype with minute, inconspicuous incipient tubercle in position of lateralmost spine in paratype; Figs. 2E, 3A, 7A); remaining spines slender, upwardly curved, flattened proximally, distally becoming circular in cross-section; Figs. 1C, 2G, H, 3B, 4C), with row of setae between spines, central spine bifid ( Figs. 2D, 3H, 4L).

Uropods not reaching end of pleotelson ( Figs. 1, 2E, 3A, 4A, 5, 6E, 7). Peduncle with 3 or 4 robust setae on anteroventrolateral margin ( Fig. 3F, G); exopod and endopod with smooth lateral and distal margins. Exopod ( Figs. 2E, F, 3F, G) length 2.1–2.2 × width; lateral margin strongly convex, with 9–11[13] robust setae along margin, setal fringe continuous (length 80%); mesial margin straight; distomedial corner convex with 4[6 or 7] robust setae, distolateral corner obtuse, weakly produced to short blunt angle. Endopod ( Fig. 3F, G) triangular, length 1.8 × width; lateral margin broadly convex, with 4–6[5] robust setae; mesial margin straight, distomesial corner rounded; distal margin straight with 9 or 10 [11–13] robust setae; distolateral corner approximately right angled, apex produced to short point.

Etymology. The type specimens were collected during the Scripps Institution of Oceanography expedition, PAPATUA, and brought to the Australian Museum by George D. F. (Buz) Wilson, my former Professor. It is most fitting to name the new species after Buz.

Remarks. Bathynomus wilsoni , new species, belongs to the group of species within the supergiants that have upcurved posterior spines on the pleotelson ( Figs. 1C, 2H, 3B, 5C) as adults. Upcurved pleotelson spines are present in B. jamesi Kou, Chen & Li in Kou, Chen, Li, He & Wang, 2017 (South China Sea), B. kensleyi Lowry & Dempsey, 2006 (Coral Sea), B. lowryi Bruce & Bussarawit, 2004 (Andaman Sea), and B. vaderi Ng, Sidabalok & Nguyen, 2025 (South China Sea). Bathynomus wilsoni is readily distinguished by the combination of: 1) maxillipedal somite incisions measuring about ⅓ the distance between the left and right incisions; 2) the narrowly subpentagonal clypeus with weakly convergent lateral margins; 3) semicircular pleotelson with a bifid, rather than simple central spine on the posterior margin; and 4) proportionally broad uropodal exopod (length 2.1–2.2 × width). The maxillipedal somite incisions of B. wilsoni extend deeply medially, their length being equal to about ⅓ of the distance between the left and right incisions ( Figs. 1A, 2A, 3A, 5A, 6A, 7A, C, 8A), similar to that of B. kensleyi and B. vaderi ( Fig. 8C, D). Conversely, the incisions in B. jamesi ( Fig. 8B; Soong, 1992: fig. 2a; Huang et al., 2022: fig. 8a) and B. lowryi (see Bruce & Bussarawit, 2004: fig. 1A) are shorter, their length being about ¼ the distance or less between left and right incisions. The bifid central pleotelson spine in B. wilsoni ( Figs. 2D, 3H, 4L) is noteworthy because other species of the genus previously reported to have a bifid central pleotelson spine ( B. bruscai Lowry & Dempsey, 2006 , B. decemspinosus Shih, 1972 , B. kapala Griffin, 1975 and B. pelor Bruce, 1986 ) are smaller species (“giants” sensu Lowry & Dempsey, 2006; maturing between 80 and 140 mm body length) and have fewer pleotelson spines (7 or 9 compared with 11–13 in B. wilsoni ). Although Bathynomus wilsoni is the first supergiant to be documented with a bifid central pleotelson spine, a bifid spine was also present in several specimens of what are currently referred to as B. giganteus (AM P20202, male 214 mm; AM P68685, female 231 mm) and B. kensleyi (QM W29630, male 315 mm), respectively, indicating that the feature is present in more species than reported to date. The bifid central pleotelson spine in supergiants warrants further study as a taxonomic character, but its practical diagnostic utility may nevertheless remain limited given that the tip is often broken or regenerating from damage in captured specimens.

Bathynomus wilsoni is morphologically most similar to B. kensleyi , with which it was misidentified by Lowry & Dempsey (2006), but differs in the stouter uropodal exopod with a more prominently convex lateral margin as reflected in the length-width proportions (length 2.1–2.2 width versus 2.4–2.5 in B. kensleyi ); the unadorned mid-anterior surface of the cephalon ( Figs. 2A, 3C, 6A) (versus the presence of a smooth, transverse sulcus across the front of the cephalon that is bordered along its lower margin by a low, irregular ridge in B. kensleyi ( Fig. 8C; Lowry & Dempsey, 2006: fig. 18D); a proportionally slightly shorter pleon (free pleonites 17–18% of body length vs 21–22% in B. kensleyi ); and in the more evenly rounded pleotelson outline. Despite having similar length-width proportions, the pleotelson of B. wilsoni (like that of B. vaderi ) is evenly rounded, almost semicircular in its posterior half ( Figs. 1A, 2C, 3A, 5A, 6E, F, 7A, C), whereas in B. kensleyi , the lateral margins transition more abruptly to the spinose posterior margin, which itself is much more broadly convex (straighter) than in B. wilsoni . As a result, the pleotelson outline of B. kensleyi is distinctly more subtrapezoid ( Lowry & Dempsey, 2006: fig. 18D) than in B. wilsoni .

Features that distinguish B. wilsoni from B. kensleyi also distinguish the new species from B. jamesi and B. vaderi (except in the near semicircular telson). Bathynomus wilsoni , however, also differs from both B. jamesi and B. vaderi in the shape of the uropodal endopod in which the posterolateral corner is almost right-angled ( Figs. 2E, F, 3F, G) (versus strongly obtuse, resulting in a more projecting posteromesial margin in B. jamesi and B. vaderi ; see Ng et al., 2025: fig. 8D, E) and in the proportionally longer pleonite 3 pleuron, whose tip reaches or slightly exceeds that of pleonite 5, versus not reaching pleonites 4 or 5 (e.g., Figs. 1, 2G, I, J, 3A, B, 5, 6D, E, 7 versus Soong, 1992: fig. 2, Ng et al., 2025: fig. 2, 5A, D, 6A, D, E). Bathynomus vaderi further differs from B. wilsoni in the distinctly more rectangular clypeus (cf. Ng et al., 2025: fig. 5C, 6C versus Figs. 2B, 3D, 6B), more strongly sinuous dorsal margin of the pereopod 7 coxa tapering to a more curved, slender, distal spine (see Ng et al., 2025: fig. 6E, 9B versus Figs. 1C, 2I, J, 3B, 5C, 6D), rugose body surface with dense dorsal granulation, especially on the free pleonites and pleotelson (versus smooth, albeit pitted, body surface), and somewhat swollen pleotelson dorsal surface with gently convex, instead of straight, lateral profile (compare Ng et al., 2025: fig. 9D with Fig. 2G, H, 3B, 5C). Another character of possible diagnostic utility is the number of coupling hooks on the mesial margin of the maxillipedal endite: five in B. wilsoni ( Fig. 4G), four in B. vaderi ( Ng et al., 2025: fig. 7A, B). Further study is required, however, to assess the species-specific taxonomic stability in the number of coupling hooks. From B. lowryi , B. wilsoni is readily distinguished by the more evenly rounded and more spinose pleotelson (versus more subtrapezoid in B. lowryi and with 9 instead of 11–13 spines) and more extensive uropodal exopod setation (80% versus 67%).

It should be noted that the western Atlantic B. giganteus (type locality: between Tortugas and Yucatan Bank, Gulf of Mexico) has been variously reported as having straight or upcurved posterior pleotelson spines (e.g., Milne-Edwards & Bouvier, 1902; Bruce & Bussarawit, 2004; Lowry & Dempsey, 2006; Sidabalok et al., 2020; Huang et al., 2022). Intraspecific variation in pleotelson spine shape has been documented in other species, such as B. jamesi (see Ng et al., 2025), but the spine orientation (upcurved or straight) in adults is considered to be species specific (noting that the curvature of the pleotelson spines does not become pronounced until maturity). The 226-mm holotype of B. giganteus has weakly upcurved pleotelson spines, reflecting its presumed immaturity. Revision of B. giganteus is beyond the scope of this study but it seems likely that what is currently known as B. giganteus in the western Atlantic includes more than one species, especially given the recent discovery of B. yucatanensis in the Gulf of Mexico ( Huang et al., 2022). Nevertheless, what is currently known as B. giganteus is readily separated from B. wilsoni by the distinct median carina on the pleotelson ( Soto & Mincarone, 2001: fig. 5A) (nearly absent in B. wilsoni ; Figs. 1A, 2C, 5A, 6E, F), the presence of a shallow transverse sulcus across the front of the cephalon (usually bordered along its lower margin by a low, irregular ridge) ( Sidabalok et al., 2020: fig. 4D; absent in B. wilsoni , Figs. 2A, 6A, 8A), and the granulate, rugose surface of the free pleonites and pleotelson in specimens exceeding 200 mm body length (surface pitted but smooth in B. wilsoni ).

Both specimens of B. wilsoni have fully developed pereopods 7, but the female oostegites are rudimentary ( Fig. 1B) and the male lacks an appendix masculina on pleopod 2 ( Fig. 4I). This could indicate that the specimens are subadults. As observed by Barradas-Ortiz et al. (2003) for B. giganteus , however, based on detailed gonadal examination, the condition of the female oostegites also varies between moults during the reproductive cycle, so mature females can variously have rudimentary or functional oostegites depending on reproductive staging ( Barradas-Ortiz et al., 2003). Similarly, the absence of the appendix masculina does not necessarily indicate immaturity because the structure appears not to be permanent, but can be lost and regained during the reproductive cycle. Therefore, such cyclic dimorphism can make inferring maturity based on the condition of the appendices masculinae or oostegites unreliable. Either way, both specimens of B. wilsoni exhibit well developed diagnostic characters that readily distinguish the new species from all known congeners.

The two specimens of B. wilsoni differ from each other in minor overlap in robust setal counts on the pereopods and uropods. Otherwise, the most obvious differences are in the shape of the apex of the clypeus (narrowly to more broadly rounded; Figs. 2B, 3D, 6E) and the presence of 11 posterior pleotelson spines in the female holotype ( Figs. 1A, 2C, 3A, 7A) compared with 13 in the male paratype ( Figs. 5A, 6E, F, 7C). In both specimens, the lateralmost spine on each side is small and conical, the remainder being elongate and spiniform—similar variation has also been observed in other supergiants such as B. giganteus , B. jamesi , B. kensleyi and B. vaderi ( Magalhães & Young, 2003; Lowry & Dempsey, 2006; Huang et al., 2022; Ng et al., 2025). The additional two spines in the male paratype ( Figs. 6E, F, 7C) are represented in the female holotype by a minute, inconspicuous incipient tubercle ( Figs. 2E, 3A, 7A). The central spine in both specimens of B. wilsoni is intact and distally bifid ( Figs. 2D, 3H, 4K).

Having been taken at 2,500 m depth, B. wilsoni is the deepest occurring species of the genus, the nearest record being that of B. giganteus at 2,140 m off the Bahamas ( Holthuis & Mikulka, 1972). Given its bathyal occurrence, B. wilsoni possibly differs ecologically from other supergiants in the region, all of which occur between outer-shelf and mid-slope depths: B. jamesi ( 300–925 m), B. kensleyi (590– 1,000 m), B. lowryi ( 690 m), and B. vaderi (outer shelf, depth not known). The specimens of B. wilsoni were collected by A. Yayanos as part of an expedition sampling deep-sea benthic habitats to examine the influence of temperature and pressure of distributions and diversification, which included trapping benthic bathyal crustaceans as potential sources of barophilic bacterial strains ( White, 1986; DeLong et al., 1997). The two specimens of B. wilsoni were brought to the surface and maintained alive after capture for more than five weeks at atmospheric pressure, but decompression in landing the specimens is believed to have adversely affected their nervous system ( White, 1986); during that time, Edith and Bill, as they came to be affectionately known, did not feed, possibly as a result of the presumed neurological damage (G. D. F. Wilson, pers. com.).

Distribution. Currently known only from the Sulu Trough, Sulu Sea, Philippines, at 2,500 m depth.