Pachycara angeloi Thiel, Knebelsberger, Kihara & Gerdes 2021, sp. nov.

Pachycara angeloi Thiel, Knebelsberger, Kihara & Gerdes, sp. nov.

Angelo´s eelpout

(Figures 3–6; Table 1)

Holotype. ZMH 26363, male, 208 mm SL, RV ‘ Pourquoi Pas?’, INDEX2016 /1 cruise, station I16_12RO, voucher specimen code I16_158, Kairei Vent Field, Central Indian Ridge, 25°19´S, 70°02´E, ROV´VICTOR´, 2419 m depth, 15 January 2016.

Paratypes. Four specimens; ZMH 26364, male, 191 mm SL, RV´Pelagia´, INDEX 2018 cruise, station I18_ 73RO, voucher specimen code I18_1240, New Vent Field 1, Southeast Indian Ridge, 27°15´S, 72°43´E, ROV`ROPOS´, 2906 m depth, 28 November 2018. ZMH 26365, male, 112 mm SL, RV´Pelagia´, INDEX 2018 cruise, station I18_95RO, voucher specimen code I18_1301, New Vent Field 2, Central Indian Ridge, 25°28´S, 69°56´E, ROV`ROPOS´, 2633 m depth, 09 December 2018. SMF 38801, male, 244 mm SL, RV´Pelagia´, INDEX 2019 cruise, station I19_27RO, voucher specimen code I19_1547, Edmond Vent Field, Central Indian Ridge, 23°52´S, 69°36´E, ROV´ROPOS´, 3275 m depth, 10 November 2019. SMF 38802, male, 143 mm SL, RV´Pelagia´, INDEX 2019 cruise, station I19_83RO, voucher specimen code I19_1726, New Vent Field 3, Southeast Indian Ridge, 25°50`S, 70°08´E, ROV´ROPOS´, 2919 m depth, 29 November 2019.

Diagnosis. A species of Pachycara according to the definition of Anderson (1994), distinguished from its congeners by the following combination of characters: scales absent; pelvic fins absent; lateral line configuration mediolateral; dorsal fin origin associated with vertebrae 7–9 with no free predorsal pterygiophores; vertebrae 27–28 + 57–59 = 85–87; dorsal-fin rays 78–80; anal-fin rays 58–62; pectoral-fin rays 13–15.

Description. Counts and measurements are provided in Table 1. Body elongated; trunk robust, tail laterally compressed along its entire length (Fig. 3). Scales absent, skin and flesh firm, in one of the smallest individuals covered with a thick layer of mucus (Fig. 4).

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*On epurals + on upper hypural plate + on lower hypural plate.

Head ovoid, more rounded when fresh (Fig. 4) than after preservation (Fig. 3). Snout relatively long. Eye small, circular, not entering dorsal profile of head. Gill slit oblique, extending ventrally below the level of lower pectoral base. Opercular lobe at dorsal margin of gill slit developed. Mouth subterminal and oblique; upper lip continuous across snout; lower lip without lateral lobe. Upper jaw protruding, extending posteriorly to middle of eye. Nostril tube relatively short, not reaching upper lip. Gill rakers 2+10, short and blunt.

Pectoral fin origin below body midline. Posterior margin of pectoral fin rounded. Ray tips of pectoral fin not exserted. Pelvic fin absent.

Teeth in vomer, jaws, and palatine relatively large, conical, sharp and many with tips bent backwards. Vomerine teeth in irregular patch; premaxillary teeth in 2 irregular rows anteriorly, dentary teeth in 2–3 irregular rows anteriorly, blending into single, short posterior row in both jaws; palatine teeth in 2 irregular rows anteriorly, blending into single, posterior row.

Head pores moderately large (Fig. 5). Nasal pores 2, first nasal pore set just in front of nostril tube, the other set dorsoposteriorly. Suborbital pores 6–7, arising from ventral ramus of bone chain under eye. Preoperculomandibular pores 8, 4 arising from dentary, 1 from anguloarticular and 3 from preopercle. Postorbital pores 2, located at positions 1 and 4. Interorbital and occipital pores absent.

Body lateral line with well-developed mediolateral branch starting from above upper end of gill slit and extending laterally to end of tail (Fig. 3). In addition, a very short predorsal row of neuromasts in front of dorsal fin origin and a few neuromasts in a weak dorsolateral row (Fig. 5).

Parasphenoid wing not reaching mid-height of trigeminofacialis foramen, and relatively broadly articulated with pterosphenoid (Fig. 6). Frontal and parasphenoid not separated by pterosphenoid. Palatopterygoid series well developed. Branchiostegal rays 6.

Dorsal fin origin above middle of pectoral fin (Fig. 5); lower tip of pterygiophore of first dorsal-fin ray associated with vertebra 7–9 (Fig. 6). Last precaudal vertebra associated with dorsal-fin ray 19–21. Free pterygiophores not present (Fig. 6). Caudal fin with 2 epural, 4–5 upper and 4–5 lower hypural rays.

Pyloric caeca and pseudobranch filaments not developed.

Fresh color (Fig. 4). Head and body beige-brown. Branchial and oral cavity light; peritoneum black. Smaller specimens, lighter and more transparent.

Preserved color (Fig. 3). Head and body brownish. Branchial and oral cavity light; peritoneum black.

Etymology. The species is named after the first name of Ângelo Miguel de Oliveira Mendonca, the husband of the third author.

Distribution. Pachycara angeloi sp. nov. is known from the following five locations in the Indian Ocean: Edmond, Kairei and a new vent field, Central Indian Ridge; two new vent fields, Southeast Indian Ridge. Ecological information and abiotic data for assemblages where this species was collected can be found in Gerdes et al. (2019a, b).

DNA barcode. Full COI barcodes (652 bp) were obtained for the holotype ZMH 26363 and the four paratypes ZMH 26364, 26365 and SMF 38801, 38802 of Pachycara angeloi sp. nov., and uploaded to GenBank (https://www. ncbi.nlm.nih.gov/genbank/) with the following accessions: ZMH 26363: MW 888718, ZMH 26364: MW 888716, ZMH 26365: MW 888717, SMF 38801: MW 888715, SMF 38802: MW 888714. Pairwise genetic distances between specimens range from 0.0–0.3% sequence divergence (0–2 bp). Thus, intra-specific variation was low and does not reflect different vent sites.

Additional relevant voucher information, photos, DNA barcodes, used primer pairs and trace files were uploaded to the Barcode of Life Data Systems (BOLD; www.boldsystems.org) (Ratnasingham & Hebert 2007), project “Indian Ocean Hydrothermal Vent Fauna—INDEX”—subproject “Indian Ocean Hydrothermal Vent Megafauna” in the public dataset “INDEX_Megafauna_2021_1”.

Remarks. The new species is included in the genus Pachycara based on the characters defined by Anderson (1994).

Pachycara angeloi sp. nov. is different from all other described Pachycara species in having 85–87 total vertebrae (vs. 92–125 in all other species). Thus, the new species expands the lower limit of total number of vertebrae of the genus Pachycara (92 according to Corbella & Møller 2014) down to 85.

Further detailed comparisons with the five Pachycara species known from the Indian Ocean and the four Pachycara species associated with hydrothermal vents were performed based on the morphological characters provided by Anderson (1989), Anderson & Bluhm (1997), Anderson et al. (2016), Biscoito & Almeida (2004), Geistdoerfer (1994), Møller (2003), Møller & King (2007) and Shinohara (2012). Different characters between the new species and each of all nine congeners are indicated in bold in Table 2. Below are counts for the respective characters for the species are given in brackets.

In comparison with all nine congeners, the new species has lower numbers of total vertebrae (85–87 vs. 93– 123), caudal vertebrae (57–59 vs. 67–91), dorsal-fin rays (78–80 vs. 86–115) and anal-fin rays (58–62 vs. 70–95).

The following five species out of all nine congeners have higher numbers of precaudal vertebrae than P. angeloi sp. nov. (27–28): P. caribbaeum (31–33), P. cousini (29), P. saldanhai (31–34), P. shcherbachevi (29–32) and P. thermophilum (31), whereas the following three congeners have lower numbers of precaudal vertebrae: P. andersoni (24–26), P. priedei (22–23) and P. rimae (26). P. arabica (27) has overlapping precaudal vertebrae counts with the new species.

Moreover, P. saldanhai, P. shcherbachevi and P. thermophilum differ from the new species in having a pelvic fin (vs. absent in the new species), higher numbers of pectoral-fin rays (18–19 in P. saldanhai, 16–17 in P. shcherbachevi, 17–18 in P. thermophilum vs. 13–15 in the new species) and higher numbers of gill rakers (15–18 in P. saldanhai, 16 in P. shcherbachevi, 15–16 in P. thermophilum vs. 12 in the new species). In comparison with the new species, P. saldanhai and P. shcherbachevi have a different lateral line configuration (ventral & mediolateral). Pachycara caribbaeum and P. cousini differ from the new species in having fewer caudal-fin rays (10 in P. caribbaeum and P. cousini vs. 11–12 in P. angeloi sp. nov.) and more gill rakers (13 in P. caribbaeum, 15 in P. cousini vs. 12 in P. angeloi sp. nov.). Pachycara caribbaeum differs also from the new species in having a pelvic fin (vs. no pelvic fin) and P. cousini also has more pectoral-fin rays than P. angeloi sp. nov. (17 vs. 13–15).

P. andersoni and P. priedei differ also from the new species in having more pectoral-fin rays (18–20 in P. andersoni, 16–17 in P. priedei, vs. 13–15 in P. angeloi sp. nov.), more gill rakers (16–17 in P. andersoni, 14 in P. priedei vs. 12 in the new species) and different lateral line configurations (a ventral and mediolateral line in P. andersoni and P. priedei vs. a single mediolateral lateral line in the new species). Additionally, P. priedei has fewer caudal-fin rays (10) than P. angeloi sp. nov. (11–12).

In comparison to the new species, P. rimae and P. arabica have fewer caudal-fin rays (10 and 9 vs. 11–12). Additionally, P. arabica differs from the new species in having more pectoral-fin rays (19 vs. 13–15) and gill rakers (17 vs. 12) as well as a different lateral line configuration (a ventromediolateral line vs. a single mediolateral lateral line). Pachycara rimae is also different from P. angeloi sp. nov. in having a pelvic fin (vs. no pelvic fin present in the new species), fewer caudal-fin rays (9 vs. 11–12), gill rakers (10 vs. 12) and suborbital pores (5 vs. 6–7).

This paper describes the 29 th species of Pachycara, which is the 5 th to be described from specimens collected only from chemosynthetic environments and the 6 th known from the Indian Ocean.