Bathynomus jamesi Kou, Chen & Li, 2017

Bathynomus jamesi Kou, Chen & Li, 2017 View in CoL

Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 9 E – H View Figure 9 , 10 B – D View Figure 10

Bathynomus kensleyi View in CoL — Lowry and Dempsey 2006: 184; Truong 2015: 80. (Not Bathynomus kensleyi Lowry & Dempsey, 2006 View in CoL ).

Bathynomus jamesi Kou, Chen & Li, 2017: 285, figs 1–5 — Huang et al. 2022: 890, figs 3–8, 9 a. View in CoL

Bathynomus sp. — Huang et al. 2022: 902, fig. 9 b.

Material examined.

Vietnam • 1 ♂; 300 mm; 1 ♀; 280 mm; collected by trawlers operating off Qu ảng Ngãi , Bình Đ ịnh, Khánh Hòa and / or Phú Yên Provinces, central Vietnam; purchased by Nguyen Thanh Son from seafood markets in Hanoi; April 2024; ZRC 2024.0088 View Materials • 1 ♀; 285 mm; same collection data as for preceding; MZB. Cru. Iso 118 • 4 ♂; 415 mm, 407 mm, 380 mm, 313 mm; 1 ♀; 303 mm; same collection data as for preceding; ZRC 2024.0118 View Materials • 1 ♀; 293 mm; same collection data as for preceding; ZRC 2024.0119 View Materials • 1 ♂; 325 mm; 1 ♀; 305 mm; same collection data as for preceding; RUMF -ZC-8375 • 1 ♂; 410 mm; same collection data as for preceding; ZRC 2024.0179 View Materials . Taiwan • 1 ♀; 303 mm; TMCD 3326 ; north part of South China Sea between North Vereker Bank (21.061 ° N, 116.109 ° E) and Pratas Island (20.717 ° N, 116.700 ° E); coll. bottom trawl, Keelung-based fishing vessel Jin Ruiyi 37; 17 June 2019 GoogleMaps • 1 ♂; 369 mm; same collection data as for preceding; TMCD 3327 GoogleMaps • 1 ♂; 314 mm; same collection data as for preceding; TMCD 3328 GoogleMaps • 1 ♀; 288 mm; same collection data as for preceding; TMCD 3329 GoogleMaps • 1 ♂; 342 mm; same collection data as for preceding; TMCD 3330 GoogleMaps • 1 ♀; 260 mm; about 300 km south-west of Pratas Island (19.084 ° N, 115.250 ° E); coll. bottom trawl, Keelung-based fishing vessel Jing Yang; 12 May 2020; TMCD 3331 GoogleMaps • 1 ♀; 267 mm; same collection data as for preceding; TMCD 3332 GoogleMaps • 1 ♂; 296 mm; same collection data as for preceding; TMCD 3333 GoogleMaps • 1 ♂; 330 mm; same collection data as for preceding; TMCD 3334 GoogleMaps . Philippines • 1 ♂; 320 mm; MUSORSTOM 2 station CP 75 , 13°51'N, 120°30'E, off Manila, Luzon Island 300–330 m; 25 March 1976; MNHN IS.2290 GoogleMaps .

Remarks.

The species was originally described from a subadult female and three juveniles by Kou et al. (2017) from off Hainan Island in the northern part of the South China Sea. Huang et al. (2022) subsequently obtained a series of specimens from Pratas (= Tungsha) Islands in the South China Sea and redescribed the species at length. Huang et al. (2022: 903) observed that there appeared to be two forms of B. jamesi , a slender type (with the body having the lateral edge of the pereon relatively straight; Huang et al. 2022: fig. 9 b) and a stout type (with the pereonal lateral edge convex; Huang et al. 2022: fig. 9 a). Huang et al. (2022) also observed that compared to the stout type, the pleotelson of the slender type was relatively longer (0.70 times as long as wide) (vs 0.42–0.56) and its pleotelson spines are flat and proximally broad (vs round and proximally narrow).

Huang et al. (2022: table 1) reported four males ( TMCD 3327, 3328, 3330, 3333) and three females ( TMCD 3329, 3331, 3332) of the stout type which they regarded as B. jamesi s. str., and one male ( TMCD 3334) and one female ( TMCD 3326) of the slender type which they considered as either a morphological variation of B. jamesi or possibly a separate species. In their photograph of the two forms, however, these authors depicted specimen TMCD 3329 as the slender type ( Huang et al. 2022: fig. 9 b) and TMCD 3326 as the stout type ( Huang et al. 2022: fig. 9 b) (sexes not stated). Their photograph of the slender type showed a proportionately longer pleotelson with the spines relatively broader and flatter while that of the stout type has a proportionately wider pleotelson with acute spines; these observations contradict what was discussed in Huang et al. (2022: 903).

We examined the specimens of Huang et al. (2022) in TMCD and found that the character states of the pereon and pleotelson discussed by them, including the associated body types in their table with codes TMCD 3326 and 3329, are indeed reversed. The slender type (based on their figured female specimen TMCD 3329) has the lateral edge of the pereon gently or distinctly convex and a proportionately wider pleotelson with the spines relatively wider and somewhat flatter (Figs 2 D, E View Figure 2 , 3 A View Figure 3 ); while the stout type (based on their figured female specimen TMCD 3326) has the lateral edge of the pereon gently convex to almost straight posteriorly and a narrower pleotelson with the spines acute and more cylindrical (Figs 2 F View Figure 2 , 3 B View Figure 3 ). The relative shape or convexity of the pereon and its lateral edge is not a reliable character as they are somewhat flexible; depending on how they are positioned or flexed, it can appear more slender or stout. For example, the female specimen (288 mm, TMCD 3329) figured as the slender type by Huang et al. (2022: fig. 1 b) has the lateral edge of the pereon appears gently or strongly convex depending on how they are stretched and photographed (Fig. 2 D, E View Figure 2 ). Other specimens from Taiwan regarded by Huang et al. (2022) as the stout type (e. g., TMCD 3327, 3333) or slender type (e. g., TMCD 3334) show varying forms of the pereon (Fig. 2 B, C View Figure 2 ).

There is some variation in the kind of pleotelson spines present. Huang et al. (2022) noted that the spines may be more acute or are flattened and broader. We did not detect any pattern with the kind of spines present. The pleotelson spines do tend to be more slender, with a rounder cross-section and are usually longer in smaller specimens (ca 300 mm TL or less) (e. g., Fig. 3 B, D View Figure 3 ), but we also have smaller specimens with more flattened spines as well (Fig. 3 A, F View Figure 3 ). The largest specimens (exceeding 350 mm TL), however, invariably have shorter spines which are more flattened (Fig. 3 C, E View Figure 3 ). We also note that the form of the median pleotelson spine also varies; in some specimens, the lateral margins have an additional tubercle, and its base may have an additional small spine or sharp tubercle (Fig. 3 A, E View Figure 3 ).

For the adult specimens from Taiwan and Vietnam, the shape and proportions of the pleotelson appears to vary rather considerably, from proportionately wider and subrectangular in shape to narrower and more rounded, with length-to-width ratios ranging from 0.57–0.72 (Fig. 3 View Figure 3 ). This is unexpectedly substantial for one species. As the shape of the pleotelson is a critically important and usually highly consistent species character in cirolanid taxonomy (as is the number of robust setae on the appendages), this was rather surprising. For the Taiwanese specimens examined by Huang et al. (2022), the pleotelson ratios are 0.57 and 0.71 for the two specimens of the “ slender type ” they reported (Fig. 3 A View Figure 3 ), with the rest of the specimens (the “ stout type ”) ranging from 0.58–0.72 (Fig. 3 B – D View Figure 3 ). As discussed above, we cannot differentiate their two types for the specimens from Taiwan and Vietnam we examined. There is, however, some correlation with size as the specimens below 300 mm TL tend to have relatively longer pleotelsons, with the ratios 0.70–0.73 (Fig. 3 A, D, F View Figure 3 ). That being said, while most specimens above 300 mm TL have pleotelson ratios ranging from 0.63–0.68, the largest specimens from Vietnam exceeding 400 mm TL ( ZRC 2024.0118 , ZRC 2024.0179 ) have ratios of 0.72 as well (Fig. 3 E View Figure 3 ). There is no correlation of pleotelson shape with sex. We also could not correlate pleotelson proportions with the kind of spines present along the margin. Those with more spines that have a relatively flat cross-section (Fig. 3 A, C, E, F View Figure 3 ) have ratios of 0.63–0.71, while those spines that have a more rounded cross-section (Fig. 3 B, D View Figure 3 ) range from 0.63–0.72.

The degree of within species variation observed in pleotelson shape and setation is slight in the Cirolanidae , and for most species in most genera, pleotelson shape is a prime taxonomic character in distinguishing species. This may not be the case for some cryptic species groups where the pleotelson is similar in form, but that reinforces the point of pleotelson uniformity. For example, the Cirolana ‘ parva - group’ is a well-known species group established by Bruce (1986) for 13 taxa, with seven characters used to differentiate species (including structures of the frontal lamina, pereopod 1, pleotelson and uropods). This species group currently contains 34 similar looking species worldwide ( Rodcharoen et al. 2016; Sidabalok and Bruce 2017; Jennings et al. 2020), and there remain many undescribed species. The degree of variation observed in B. jamesi specimens suggests that it may well be a species complex. To ascertain this, a much larger series of specimens collected from a wider geographical area will be needed, with the associated morphological and genetic studies done.

Bathynomus jamesi was first reported from Vietnam by Truong (2015) as “ Bathynomus kensleyi ” (cf. Huang et al. 2022; Huang and Bruce 2024). Truong (2015) based this record on a single specimen (sex not stated) measuring 260 mm collected by fishermen from the “ Trư ờng Sa area ” (the Spratly Islands) in the South China Sea. The repository for the specimen is not known. As his figures do not show the clypeal region in frontal view, form of the pereopod 7 coxa or convexity of the pleotelson, so we cannot be certain of its identity, and as such, we retain it under B. jamesi for the time being.

Bathynomus jamesi is one of the largest supergiants known. The largest is believed to be B. giganteus , with one specimen from Brazil supposedly reaching 500 mm in length ( Lowry and Dempsey 2006: 166). The two largest males of B. jamesi we have seen ( ZRC 2024.0118 , ZRC 2024.0179 ) measure 415 mm and 410 mm in length, respectively and weigh more than 2.6 kg each (Fig. 1 C View Figure 1 ). This makes B. jamesi the largest known supergiant species (and largest isopod) in the Indo-West Pacific.

One female specimen examined ( TMCD 3329, 288 mm) had the oostegites developed forming a brood pouch. There were about a dozen eggs inside the pouch, but the number of eggs is an underestimate as many had fallen out during collection and preservation.