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 Author Huang, Ming-Chih Author Kawai, Tadashi Author Bruce, Niel L. text Journal of Natural History 2022 2022-08-09 56 13 - 16 885 921 http://dx.doi.org/10.1080/00222933.2022.2086835 journal article 127590 10.1080/00222933.2022.2086835 62431510-0c41-43f2-b9c0-5a75c31679bc 1464-5262 7012152 Bathynomus yucatanensis sp. nov. ( urn:lsid:zoobank.org:pub: 399605D3-356E-402D-9ED1-E36A649A3F1B ) Material examined Holotype . Male obtained from an aquarium as noted above (voucher ‘number TMCD003335 , NCBI Acc. No. MZ 354630 for COI and MZ 042927 for 16S rRNA), the TL 257 mm , CL 129 mm , and wet weight 550 g ( Table 1 ). Reportedly caught in a baited cage at 600–800 m depth on 19 April 2017 in the Gulf of Mexico off the Yucatán Peninsula. Diagnosis Clypeus with straight lateral margins. Antennal flagellum extending to within pereonite 3. The distal part of the coxa of pereopod 7 is broad. Uropodal exopod not extending beyond pleotelson: endopod with distolateral corners slightly produced. Length:width ratio of pleotelson approximately 0.8:1; number of pleotelsonic spines 11 or 13, short, or straight. Description of male (TMCD003335) Body ( Figure 13 (a)) 260 mm in TL, 2.6× times longer than wide. Head ridge above eyes discontinuous ( Figure 13 (b)); clypeal region with distal margin distinctly concave, apex narrowly rounded ( Figure 13 (c)). Flagellum of antennula longer than peduncle, more than 48 articles (lacking terminal part of both antennulae). Antenna article 1 very short, article 2 about 1.5 times longer than 1, articles 3–4 bearing neither exopod nor seta ( Figure 15 (h)),article 5 extremely short ( Figure 15 (i)); flagellum longer than peduncle, extending to within pereonite 3 ( Figures 13 (d) and 14(a)), composed of approximately 55 articles (near-terminal segmentation unclear). Mandible with broad tridentate incisor. Palp extending beyond cutting edges. Figure 12. Phylogenetic tree based on the DNA sequences of cytochrome c oxidase I (COI). The sequences were aligned using Clustal Omega, and the tree was constructed by the neighbour-joining method. Numbers at branches indicate bootstrap values. The sequence of Cirolanidae ( Atarbolana exoconta , KX 782999) COI was used as the outgroup. Evolutionary analyses were conducted in MEGA 11. Mandible palp not reaching the incisor margin ( Figure 17 (d)). Maxilla with long setae ( Figure 17 (c)); lateral lobe with 9 keratinised spines on exopod, 3 RS on endopod ( Figure 17 (e)). Maxillipedal palp ( Figure 17 (a)) with broad articles bearing plumose setae on lateral margins and simple setae on medial margins, all articles wider than their articulating junctions, and terminal article triangular; maxillipedal endite with 5 equally coupling setae ( Figure 17 (b)). Pereopod 1 ( Figure 15 (a)) with ischium bearing 3 posteroproximal RS and 3 RS on posterodistal margin; merus bearing 3 RS on an anterodistal angle, 3 RS in a proximal row on posterolateral margin, and 3 RS in a distal row; propodus twice as long as wide, with 5 RS on posterior margin. Pereopod 2 ( Figure 15 (b)) with ischium bearing 3 RS each on posterior and posterodistal margins; merus with 7 short setae on an anterodistal angle, 3 RS in a proximal row along the posteromedial margin, and 3 RS in a distal row; and propodus with 5 RS on posterior margin. Pereopod 7 basis 2.5 times as long as greatest width, superior margin convex, inferior margin with 5 palmate setae; ischium 0.7 times as long as basis, inferior margin with 14 RS (4 clusters of 1 + 3 + 6 + 4), superior distal angle with 12 RS, inferior distal angle with 6 RS; merus 0.5 as long as ischium, 2.1 times as long as wide, inferior margin with 6 RS, superior distal angle with 9 RS, inferior distal angle with 8 RS; carpus 0.6 times as long as ischium, 1.6 times as long as wide, inferior margin with 5 RS (as 1 + 2), superior distal angle with 13 RS, inferior distal angle with 9 RS; propodus 0.7 times as long as ischium, 2.4 times as long as wide, inferior margin with 6 clusters of RS (as 3 clusters of 2), superior distal angle with 4 slender setae, inferior distal angle with 1 RS; dactylus 0.5 times as long as propodus. Figure 13. Bathynomus yucatanensis sp. nov. Holotype (voucher no. TMCD003335). (a) Body, dorsal view; (b) cephalon, anterior view; (c) clypeal region, ventral view; (d) cephalon, lateral view. Scale bars: 1 cm. Figure 14. Bathynomus yucatanensis (voucher no. TMCD003335). (a) Body, lateral view; (b) pereion, lateral view; (c) pleotelson, dorsal view; (d) uropod, dorsal view; (e) uropod, ventral view. Scale bars: 1 cm. Coxa of pereopod 7 distally broadened and slightly upcurved posteriorly ( Figures 14 (a,b)). Penial processes separated by 5% of sternal width ( Figure 16 (f)). Figure 15. Bathynomus yucatanensis (voucher no. TMCD003335). (a) Pereopod 1, mesial view; (b) pereopod 2, mesial view; (c) pereopod 2 merus, posterolateral margin, (d, e) pereopod 7; (f) antennula; (g) antennal peduncle article 4; (h) antennal peduncle; (i) antennula peduncle article. Scale bars: 1 cm. Pleon comprises approximately 20% of body length ( Figures 13 (a) and 14(a)). Posterolateral angles of pleonites 3–5 reach almost the same level posteriorly ( Figure 14 (a,b)). We did not find an appendix masculina on the pleopod. Uropods ( Figure 14 (c)) not extending to posterior margin of pleotelson. Peduncle ventrolateral margin with 3 ventral RS ( Figure 14 (e)). Exopod with smooth lateral and distal margins ( Figure 14 (d)); with 7 RS ( Figure 14 (d,e)) along the lateral margin, straight medial margin, and distomesial corner rounded; convex distal margins both lacking setae, and distolateral corner not produced, distolateral corner subacute. Endopod ( Figure 14 (d, e)) lateral margin straight with 9 RS, medial margin straight, distomesial angle rounded, distal margin straight with 15 RS, distolateral angle slightly produced, subacute. Figure 16. Bathynomus yucatanensis (voucher number TMCD003335). (a) pleopod 1, ventral view; (b) pleopod 1, dorsal view; (c) peduncle, ventral view; (d) pleopod 2, ventral view; (e) pleopod 3, ventral view; (f) penes, ventral view. Scale bars: a, b, d–f = 1 cm; c = 0.5 cm. Figure 17. Bathynomus yucatanensis (voucher no. TMCD003335). (a) Maxillipedal palp; (b) maxillipedal endite; (c) maxilla; (d) mandible; (e) lateral lobe of maxilla. Scale bars: a, d, e =1 cm; b, c = 2 mm. Figure 18. Bathynomus yucatanensis (voucher no. TMCD003335). Body length 260 mm. (a) Dorsal view; (b) ventral view. Each square is 1 cm on a side. Pleotelson ( Figure 14 (b,c)) approximately 0.5 times as long as wide, smooth except for minute pores; with inconspicuous longitudinal carina on dorsal surface, running anterior from pleotelsonic spines; vestigial spines and posterior margin with 10 prominent spines and 1 pair of vestigial spines ( Figure 14 (c)), all long and straight with no setae between them, central distal spine simple. Figure 19. Alignment of partial DNA sequence of the cytochrome c oxidase I from the Gulf of Mexico Bathynomus spp. They are B. yucatanensis (NCBI Acc. No. MZ354630 ), B. giganteus ( MG229639 ) and B. maxeyorum ( KT963292 ). Figure 20. Alignment of partial DNA sequence of the 16S rRNA from Bathynomus spp. They are B. yucatanensis (NCBI Acc. No. MZ042927 ), B. giganteus ( MG229479 ), B. jamesi ( KX417643 ) and B. doederleini ( MK953514 ). Colouration. Dorsal surfaces of body creamy yellowish; pleotelson pale and dark yellow; ventral sides of pereopodal coxae, pleotelson, and uropods also creamy yellow, and pleopods dark rose ( Figure 18 ). Habitat Captured with B. giganteus , presumably similar to B. giganteus . Distribution Off the Yucatán Peninsula of Mexico , and in the Gulf of Mexico ; approximate depth range 600–800 metres. Etymology. The epithet is an adjective derived from the name of the nearest land mass to its common locality, the Yucatán Peninsula. The Japanese name: Enosuigusokumushi. Molecular biology. Amplified PCR products of 489 bp from 16S rRNA and 599 bp from COI, respectively, were obtained from the holotype B . yucatanensis ( TMCD003335 ). The sequence data have been uploaded to DDBJ/EMBL/ GenBank ( Accession Nos. MZ042927 for 16S rRNA, MZ354630 for COI. In a few COI experiments, the variational sequence was obtained as MZ742155 ). Although the phylogenetic tree ( Figure 12 ) shows B. yucatanensis as closest to B. giganteus ( MG229639 ), our COI sequence for the former differed in at least 35 places from that of a specimen of B. giganteus registered in NCBI ( MG229639 ) ( Figure 19 ). The alignment of 16S rRNA nucleotides among Bathynomus species in the Gulf of Mexico is also shown in Figure 20 . COI and 16S rRNA sequencing revealed that the two individuals of B. giganteus were different species. After comparing them with the NCBI database, we found that one was B . giganteus , but the other did not correspond to any known species. Comparisons with descriptions of existing species showed that it was a new species. Due to the different sequences of the two genes (COI and 16S rRNA) ( Figures 19 and 20 ), coupled with differences in morphology ( Table 3 ), we identified it as a new species. Remarks Two Bathynomus species are known from the Gulf of Mexico , B. giganteus and the recently described B. maxeyorum . Compared to B. giganteus , B. yucatanensis has more slender body proportions and is shorter in total length than B. giganteus , and the pereopods are more slender. The antennal flagellum extends to pereonite 3 in B. yucatanensis vs reaching pereonite 2 in B. giganteus ); pereonite 3 is widest (vs pereonite 5: the pereion shape of B. yucatanensis is an inverted triangle vs ovate in B. giganteus ( Bruce 1986 )) . The pleotelson spines of B. yucatanensis are more slender than those of B. giganteus . The coxal plates in B. yucatanensis are pale in comparison to those of B. giganteus . Compared with B. maxeyorum , the most distinctive feature is the number of pleotelson spines (11 spines in B. yucatanensis vs 7 in B. maxeyorum ), the uropod exopod distolateral corner is not produced in B. yucatanensis vs produced in B. maxeyorum , and in B. yucatanensis the clypeus anterior margins are concave and the lateral margins straight vs anterior margins straight and lateral margins concave in B. maxeyorum ( Shipley et al., 2016 ) ( Figure 1 (e)). These morphological characters have been used as diagnostic characteristics and keys to identify and describe species ( Bruce 1986 ; Bruce and Bussarawit 2004 ; Lowry and Dempsey 2006 ; Kou et al. 2017 ). The colour of the lateral margin of the pereion, width of pereopods, longitudinal median carina on pleotelson, and proximal width of the pleotelson spines clearly differ between B. giganteus and B. yucatanensis ( Table 3 ). These morphological characters have not previously been used as diagnostic characters, but differences in these morphological features are consistent with the results of molecular analyses in the present study ( Figures 10–12 ).