Bathynomus maxeyorum Shipley, Brooks & Bruce

Shipley, Oliver N., Bruce, Niel L., Violich, Mackellar, Baco, Amy, Morgan, Nicole, Rawlins, Scott & Brooks, Edward J., 2016, A new species of Bathynomus Milne Edwards, 1879 (Isopoda: Cirolanidae) from The Bahamas, Western Atlantic, Zootaxa 4147 (1), pp. 82-88 : 86

Bathynomus maxeyorum Shipley, Brooks & Bruce sp. nov.

Figures 1 , 2

Material examined. All material from northeast Exuma Sound, Bahamas, western North Atlantic.

Holotype: ♀ (76 mm), 24°83.15’N, 76°37.88’W, depth: 672 m, temperature: 10.6 °C, 28 April 2014 (CMNH 38,598).

Paratypes: 3♂ (73–88 mm), ♀ (69 mm), 24°83.15’N, 76°37.88’W, depth: 672 m, temperature: 10.6 °C, 28 April 2014 (♂, NHMUK 2016.67 - 69; ♀, NHMUK, 2015.3278). ♀ (74 mm, non-ovig), 24°49.53’N, 76°23.38’W, depth: 769.2 m, temperature: 8.8°C, 28 April 2014 (MTQ 34369).

Diagnosis. Pleotelson with 7, broad, short spines with setation running along ~80% of the posterior margin. Uropodal lateral margins with 90–94% setation, and reduced setation on mesial margins of exopod and endopod.

Description of holotype female. Body 76 mm long, length 3 times that of width. Head with ridge above eyes discontinuous; clypeal area with distal margins slightly concave, apex broadly rounded.

Antenna 2 flagellum extending to within pereonite 3.

Pereopod 1 ischium with 2 posteroproximal robust setae, 5 robust setae on posterodistal margin; merus with 8, short robust setae on anterodistal angle, posterior margin with 2 robust setae in proximal row, and 4 robust setae in distal row; propodus length 2 times width, with 4 robust setae on posterior margin. Pereopod 2 ischium with 2 robust setae on posterior margin, and 3 robust setae on posterodistal margin; merus with 7 short robust setae on anterodistal angle, posteromedial margin with 3 robust setae in proximal row, and 3 robust setae in distal row; propodus with 3 robust setae on posterior margin. Pereopod 7 coxa distally attenuated and curved posteriorly.

Pleonite 3 extending up to pleonite 4.

Uropod extending slightly beyond pleotelson; peduncle with 2 robust setae; exopod and endopod with minutely scalloped lateral and distal margins; exopod lateral margin convex proximally, sinuate distally, with 0 robust setae along margin, setal fringe continuous (90–94%); medial margin straight; distomedial corner broadly rounded; distal margin sinuate, with 5 robust setae; distolateral corner sharply produced, subacute; endopod, lateral margin sinuate, with 3 robust setae; medial margin straight; distomedial corner rounded; distal margin straight, with 7 robust setae; distolateral corner not produced, not bifid.

Pleotelson broader than long, 1.5 times as wide as length, smooth (minute pores), longitudinal carina on dorsal surface conspicuous, with 7 distal and 1 lateral, short, straight prominent spines along distal margin, with setae between spines, central distal spine simple.

Etymology. This species Bathynomus maxeyorum sp. nov. is named after three generations of the Maxey family, who continue to facilitate and inspire the exploration and conservation of the world’s oceans.

Genetics. The newly designed primers yielded COI sequences 612 base pairs in length. Within Bathynomus giganteus specimens the maximum divergence among haplotypes was 1.2% with four of the twelve Bahamas haplotypes identical to Gulf of Mexico haplotypes. Bathynomus maxeyorum sp. nov. had four haplotypes, with a range of intraspecific divergence of 0.3–0.9%. There is not an established range of genetic divergence within or between species for cirolanids for mitochondrial COI that we could find in the literature. However, the divergence between the two species was much larger than the intraspecific divergence with a mean of 14.1% ( Table 1 ), strongly supporting B. maxeyorum sp. nov. as a distinct species from B. giganteus .

Sexual variation. Female specimens appear to have a more strongly extended uropodal exopods, which extend approximately 5% beyond the pleotelson posterior margin.

Remarks. Bathynomus maxeyorum sp. nov. belongs to the group of species termed “giants” by Lowry & Dempsey (2006). The other group within the genus was termed “super giants”, characterized by their large size with adults usually exceeding 20 cm. B. maxeyorum sp. nov. is a small species, (specimens gathered from 76 mm to 100 mm total body length including those not identified in type series), and can be identified by having 7 major pleotelson spines, with the median spine being simple, the prominent anterior pleotelsonic carina (rather than posteriorly prominent), with setation running along ~80% of the posterior margin and the shape and setation of the uropodal rami. The most similar species is B. obtusus ( Magalhães & Young 2003) , with both species showing distinctive setation on the uropodal exopod, which has robust setae only on the mid-distal lateral margin; B. obtusus differs in having the posterior part of the pleotelsonic carina more strongly defined, more elongate uropodal rami, and the uropodal exopod is slightly longer than in B. maxeyorum sp. nov. (0.86 as long as endopod vs 0.81; two specimens). Differences in uropodal robust setae cannot be compared as no counts were given for B. obtusus by Magalhães & Young (2003).

Figures

FIGURE 1. Bathynomus maxeyorum sp. nov. ♀ holotype (CMNH, 38,598). A, body, dorsal view; B, body, lateral view; C, pereon, lateral view; D, cephalon, anterior view; E, clypeal region, anterior view.

FIGURE 2. Bathynomus maxeyorum sp. nov. ♀ holotype (CMNH, 38,598). A, pleon and pleotelson, dorsal view; B, uropods, dorsal view; C, uropods, ventral view; inset: detail of peduncle region, dorsal view; D, pereopod 1; E, pereopod 2; F, merus.