Bathynomus jamesi (Kou, Chen and Li, 2017)

Huang, Ming-Chih, Kawai, Tadashi & Bruce, Niel L., 2022, A new species of Bathynomus Milne-Edwards, 1879 (Isopoda: Cirolanidae) from the southern Gulf of Mexico with a redescription of Bathynomus jamesi Kou, Chen and Li, 2017 from off Pratas Island, Taiwan, Journal of Natural History 56 (13 - 16), pp. 885-921 : 912-916

Bathynomus jamesi vs Bathynomus kensleyi

Two species of ‘super-giant’ Bathynomus have been described from the northern South China Sea region ; one is B . kensleyi and the other is B. jamesi . Lowry and Dempsey (2006) recorded B. kensleyi from south of Hong Kong (20.334°- 20.834°N, 115.500°– 116.250°E); their specimens clearly showed upwardly curved pleotelsonic spines, and B. kensleyi was the only ‘super-giant’ Bathynomus species known in the South China Sea at that time. Kou et al. (2017) described B. jamesi from 100 kilometres south-east of Hainan Island and believed that Bathynomus sp. from the Gulf of Aden ( Lowry and Dempsey 2006) was the same species as B . jamesi. All of these specimens of Bathynomus sp. and B. jamesi were immature or juvenile.

The COI sequences of our 10 sample individuals of B. jamesi are consistent with sequences of Kou et al. (2017) ( NCBI Acc. No. KX 417647 ), confirming that they are the same species as B. jamesi ( Figure 11 ). All material examined by Kou et al. (2017) was immature, and none of the individuals showed ‘upwardly curved pleotelsonic spines’. In contrast, the 10 mature individuals we collected all show the ‘upwardly curved pleotelsonic spines’ that is characteristic of B. kensleyi . However, as discussed in the ‘Remarks’ for B. jamesi , B. kensleyi differs noticeably from B. jamesi with regard to clypeus and pleon morphology, and also in the fine detail of the uropodal rami. It is clear that the mature adults of B. jamesi have upturned elongate pleotelson spines, and that three species now share this character: B. jamesi from the South China Sea; B. kensleyi from the Coral Sea in eastern Australia (see comments on B. kensleyi below); and B. lowryi from the Andaman Sea, Thailand.

Truong (2015) identified Bathynomus species found off the Spratly Islands in the South China Sea as B. kensleyi , with the characteristic 11 upwardly curved pleotelsonic spines, but this species shows the clypeus, pleon and uropodal characters of B. jamesi , and is here recognised as that species.

B. kensleyi has only one entry in NCBI (Acc. No. MN654915 ), from India ( Prasanna Kumar et al. 2020). A comparison shows that the DNA sequence of the COI from NCBI ( India sample) differs from each of the 10 individual sequences from the South China Sea . Direct comparison of the figures given by Prasanna Kumar et al. (2020) and Sankar et al. (2011) show that the specimen from India has been misidentified: it is neither B. jamesi nor B. kensleyi and may represent an undescribed species.

We used COI as the main basis for species identification, and 16S rRNA as an auxiliary mechanism for identifying the target species. As noted above, our recovered DNA sequences of B. jamesi for COI were not completely identical among individuals. Although the DNA sequence of COI displays individual differences, it is not a systematic variable ( Figure 11 ). These variations can be regarded as single nucleotide polymorphisms (SNPs). The SNPs were found at seven of 657 bp loci ( Figure 11 ). We interpret this as an individual, not taxonomic, variation because the ratio is low (the maximum is 3/657 on TMCD003332 (NCBI Acc. No. MW577651 )). The most mutable point appeared to be position 348, where A> T (in three individuals) and A> C (in two individuals) were both found. These results show that genetic diversity exists within the population, and that they have the potential to produce morphological variation and even new species.

Although the COI sequences of the 10 individuals are almost the same, there appear to be two phenotypes ( Table 1 ). With respect to the identification of Bathynomus in terms of the length of the flagellum, in most specimens it extends to pereonite 2, but in a few (such as TMCD003328 and TMCD003333) it can reach pereonite 3. Eight of our specimens were stout-bodied and two were slim-bodied ( Table 1 and Figure 9 ), the former being widest at pereonite 5 (vs pereonite 4) and having long, pointed pleotelson spines (vs stubby spines). Pleotelson length ranged from 1.51 to 2.71 times the width, with a large difference ( Table 1 ) among all 10 specimens. Due to the apparent difference in appearance, if the slim-bodied group is separated, it may potentially be regarded as a different species, while the stout-bodied type conforms to B. jamesi . There are differences in the number of pleotelson spines and RS as well ( Tables 1 and 4 ). COI and 16S rRNA sequences are reliable methods, but unfortunately, many molecular biologists are not familiar with the taxonomy of this group or how to identify its species. Incorrect identification creates many problems. The identification of B. kensleyi ( MN654915 ) from the Indian Ocean is incorrect ( Sankar et al. 2011), and it is improbable that B. doederleini , B. kensleyi , B. decemspinosus (from Parangipettai, India), and B. giganteus (from Chennai, India) occur in the Indian Ocean ( Prasanna Kumar et al. 2020) as no species of Bathynomus is known to have a trans-ocean distribution ( Magalhães and Young 2003; Lowry and Dempsey 2006; Sidabalok et al. 2020).

The misidentification of Bathynomus sp. by Sankar et al. (2011) and Prasanna Kumar et al. (2020) can be seen in both the morphology and DNA sequences of COI. The morphology of B. decemspinosus in terms of the central spine is significantly different from that of the holotype ( Sankar et al. 2011, p. 114, fig. 1 vs Shih 1972, p. 43, Plate IV); and B. doederleini , in terms of the pleotelson morphology and the length ratio of spines on the posterior margin, is also different. The holotype of B. doederleini has smaller spines on both sides of the central spines, but these characters are not present in the sample of Sankar et al. (2011 p. 114, fig. 2), Ortmann (1894, p. 193, lines 4–6) or of Lowry and Dempsey (2006, p. 177, fig. 10f), Figure 10f . It is difficult to confirm or reject the identification of B. kensleyi by photo ( Sankar et al. 2011, p. 114, fig. 3). Bathynomus decemspinosus has no registered DNA sequence data for comparison. Bathynomus doederleini has the most data for comparison (eg AB851912 , MZ723938 , and MK953514 ); there are at least six recorded positions of DNA nucleotides of various types. In B. kensleyi , in the DNA sequence of COI ( MN654915 ) and the data we analysed ( MW575454 ), there are nucleotide variants in 42 positions in the DNA sequence of COI. Finally, Prasanna Kumar et al. (2020) cited old references, claiming that B. giganteus was collected from in-shore waters of Chennai, India. Because DNA analysis is a very precise method of species identification, once the species is misidentified and submitted into the database, it can have very serious consequences.

Bathynomus kensleyi ( Lowry and Dempsey, 2006) was described from off the Great Barrier Reef, in the western Coral Sea . The type locality is ‘ South-east of Swain Reefs , 22.918°S ʹ 154.354°E, Coral Sea, 590–606 m depth’ . Additional locations in eastern Australia included off Flynn Reef and Lihou Reef (see Material examined) in the northern Great Barrier Reef . The species was also recorded from Taiwan, south of Hong Kong and the Philippines. The records from off Hong Kong and Taiwan and the specimens from Luzon, northern Philippines, have here been re-identified as B . jamesi. The two specimens of B. kensleyi from the Sulu Sea (AM P42711, P42712) differ from the holotype and other specimens from eastern Australia by a distinctly ovate pereon; deeply incised maxillipedal somite (in dorsal view); pleoteolson spines fine, more ‘needle-like’; pleon short, only 17% of total body length (compared to B. jamesi at 29%); pleonite 3 does not posteriorly overlap pleonites 4 and 5, and pleonite 4 does not extend posterior to pleonite 5; and uropodal exopod lateral margin strongly convex. The Sulu Sea specimens are also taken from a far greater depth than most Bathynomus , from the continental rise at 2500 metres. The Sulu Sea specimens represent an undescribed species of Bathynomus . At present we regard B. kensleyi as occurring only in the western Coral Sea ; the species is not present in the South China Sea or anywhere in the Northern Hemisphere.

Figures

Figure 9. Two types of Bathynomus jamesi: slender (right, TMCD003329) and stout (left, TMCD003326). Scale bars: 10 cm.

Figure 10. Alignment of partial DNA sequence of the cytochrome c oxidase I from the South China Sea Bathynomus spp. They are B. jamesi (NCBI Acc. No. MW575454), B. jamesi (KX417647) and B. doederleini (MZ723938).

Figure 11. Alignment of partial DNA sequence of the cytochrome c oxidase I from the 10 collected samples of B. jamesi (NCBI Acc. Nos. MW575424, MW575449, MW575455, MW580729, MW580730, MW575454, MW577650, MW577651, MW579548, MW577652 and reference sequence of B. jamesi KX417647.