Polymixia lowei (in part), Smith-Vaniz et al., 1999: BAMZ 1997-159-006 (2, 170–180).

Holotype.—(Figs. 2A, 4A, 5A, B, 6A) BAMZ 1997-159-006, 173.0 mm SL, 224.0 mm TL, Bermuda, “outside Eastern Blue Cut” near the NW edge of the Bermuda Platform, 32°25′01″N, 64°56′43″W, 280 fathoms (512 m), hook and line (vertical long line), collected and donated by Craig Soares and Richard Allen, 1 August 1997.

Paratypes.—(Figs. 2B, 4B, 5C, 6B) FMNH 145004, 185.0 mm SL, 240.0 mm TL, collecting locality, date, depth, method, and collectors same as holotype. (Fig. 3) MCZ 174218, small juvenile, 20 mm SL, original catalog no. DeepEnd Consortium DPND 1320, original identification P. lowei, sample i.d. DP02-11Aug15-MOC10-SW-3D-017-N0, midwater trawl 10 m² MOCNESS, 3 mm mesh, depth range 0–1502.5 m, in water of maximum depth 2500 m, towed between 27°0′13″N, 88°29′39″W and 26°52′15″N, 88°31′22″W, 11 August 2015.

Diagnosis.—A species of Polymixia distinguished from all other species in the genus based upon the following unique characters. Measurements are based on adult specimens: eye diameter (EYD) larger than in other species of the genus, averaging 46.4% of POHL vs. 36%–44% in congeners; head longer relative to SL than in congeners; POHL averaging 25.4% of SL vs. 22%–24% in others; THL averaging 34.4% of SL; dorsal and anal fin heights greater than in other species of Polymixia, longest dorsal and anal rays averaging 20.6% and 15.2% of SL, respectively, vs. 14–17% and 11–13% in congeners; pectoral (PFL) and pelvic fins (VFL) relatively longer than in other species, averaging 22.1% and 15% of SL, respectively, vs. 18–20% and 12–13% in congeners. The first anal radial is distinctive: the main shaft of the radial is noticeably straighter as opposed to significantly curved in other species; the anterior process is relatively narrow and nearly horizontal with respect to the axis of the body (5° from axis vs. 10–25° in congeners) and makes an angle of about 38° with the main shaft, greater than the angles of approximately 25–32° seen in other species of Polymixia. In addition, the new species exhibits a unique pigmentation pattern on the dorsal, anal, and caudal fins. The pattern consists of narrow, dark black patches near the tips of the longest soft dorsal- and anal-fin rays (vs. much broader, dark gray patches in congeners), and a caudal-fin margin with a dark black, continuous fringe covering the distal fourth of almost all caudal-fin rays (vs. more diffuse patches only near the tips of the dorsal and sometimes the ventral caudal lobe in congeners). Like P. japonica and P. nobilis, the new species exhibits wedge-shaped rows of ctenus-like spines on its spinoid scales. This is contrary to the vertical rows of spines near the posterior edge of each scale in P. fusca, P. lowei, P. berndti, and P. longispina.

Etymology.—The new species is named in honor of Gloria E. Hollister (Anable), B.S., M.S. (1900–1988), pioneering ichthyologist, key member of the William Beebe bathysphere expeditions in Bermuda, world record holder for deep-sea descent by a woman, leader of tropical zoological expedition, Red Cross Blood Bank pioneer, and ground-breaking conservationist. Further details of her contributions are given in Appendix 1.

Molecular evidence.—Polymixia hollisterae was, until now, a true cryptic species. Its existence was not suspected when experts initially identified the holotype and Bermuda paratype as P. lowei (Smith-Vaniz et al., 1999), whereas the Gulf of Mexico paratype was initially identified as P. nobilis and later reidentified as P. lowei (T. T. Sutton, DeepEnd Consortium, pers. comm., 2020). Muscle tissues were removed by earlier investigators for DNA extraction from the right side of the holotype and the Bermuda paratype. The posterior third of the body and tail had been removed from the much smaller Gulf of Mexico paratype as a barcoding tissue sample (Fig. 3B).

The phylogeny of species clades in the genus Polymixia (Fig. 1) adopted here is the Bayesian analysis of mitochondrial and nuclear sequences by Borden et al. (2019: fig. 4). GenBank numbers for the sequences analyzed are given in that paper. The analysis by Borden et al. (2019) resolved a unique clade with high support values composed of three samples from Bermuda representing the new species, with relationships to other species clades as shown in Figure 1.

Two of the Bermuda samples from Borden et al. (2019) are represented in a distinct cluster in the neighbor-joining (NJ) identification ‘trees' for Polymixia from the Barcode of Life Database (BOLD Systems; Ratnasingham and Hebert, 2007), based on the mitochondrial COI locus (Borden et al., 2019: fig. S1). It is not possible based on present information to determine which of the two Bermuda samples in the BOLD cluster belongs to the holotype and which to the Bermuda paratype. A third sample in the barcode cluster is the Gulf of Mexico paratype of P. hollisterae (Fig. 3), with locality listed in BOLD as “United States.” The available barcode sequence of the Gulf of Mexico paratype is identical to that of one of the two Bermuda type specimens.

Description.—See Table 1 for measurements and meristics of both the holotype and the very similar Bermuda paratype. Damage to the body and fins of the Gulf of Mexico paratype precluded obtaining many of the meristic or morphometric data from the specimen.

Detailed measurements and meristics of the adult type specimens are given in Table 1. The holotype (SL 173 mm) and Bermuda paratype (SL 185 mm) have elongated and laterally compressed bodies (Fig. 2) with body depths of 37% and 36.4% of SL. The predorsal lengths are 46% and 45% of SL. The caudal peduncle lengths are 15% and 14.5% of SL. The caudal peduncle depths are 10.1% and 10.2% of SL. The THL are 34% and 35% of SL. The POHL are 25.1% and 25.7% of SL. The HDH are 32.3% and 34.9% of SL. Snout lengths are 23.3% and 21.6% of POHL or 17% and 16% of THL. This species has an extremely large eye compared to its congeners (Figs. 2, 4, 5B), 45% and 47% of POHL or 34% and 35% of THL. There are five large circumorbital bones, each with a laterally projecting flange bordering the orbit. Pores of the circumorbital sensory canal are seen immediately behind or beneath the flange (Figs. 5B, 6).

The Gulf of Mexico paratype is a small juvenile that is missing about one-third of its body and tail. Measurements estimated in ImageJ from the shipboard image (Fig. 3A), using the original record of its standard length (20 mm) for scale, are as follows: caudal peduncle length 16% of SL; caudal peduncle depth 2.5 mm (12.5% of SL); length of dorsal-fin base 46.5% of SL; length of anal-fin base 18% of SL. Measurements obtained directly from the existing specimen are as follows: body depth at dorsal origin 34% of SL; predorsal length 39% of SL; total head length 30% of SL; preopercular head length 19% of SL); head height 30% of SL; snout length 10.5% of POHL or 7% of THL; eye diameter 63% of POHL or 40% of THL.

Scales in P. hollisterae are of spinoid type, as in other species of Polymixia (Roberts, 1993: fig. 11). Flank scales anterior to the dorsal fin have their ctenus-like spines arranged in a wedge pattern, like those of P. japonica and P. nobilis (Kotlyar, 1993). In the juvenile paratype, the spines are few but in a triangular patch with anterior apex, interpreted as an incipient wedge. The snout is blunt and devoid of scales, while large scales are positioned on the opercle and preopercle. Scales are smaller on the ventral side of the fish, increasing in size dorsally. The largest scales are positioned on the trunk between the lateral line and the dorsal fin. Small scales are arranged along the base of the dorsal and anal fins. The lateral line is continuous, with 35 pored scales in both Bermuda type specimens. The last pored lateral-line scale is at the posterior margin of the hypural plate, just anterior to the caudal-fin rays. Scales do not extend onto the caudal-fin rays. Following Kotlyar (1982, 1993), the number of scales in a vertical row from the origin of the dorsal fin to the lateral line (his S1; S1S herein) is 5, the number of scales in an oblique row from the origin of the dorsal fin to the lateral line (his S2; S2S herein) is 10, and the number of scales in an oblique row from the lateral line to the origin of the anal fin (his S3; S3S herein) is 14. The measured distances (Table 1) corresponding to S1 (S1D herein) represent 31% and 30% of BD for the Bermuda types and 40% for the juvenile paratype, and for S2 (S2D herein) they represent 52% and 51% of BD for the Bermuda types and 44% for the juvenile paratype. The vertical distances from the origin of the anal fin to the lateral line (AO–LL) represent 59% and 57% of BD, respectively and 48.5% for the juvenile paratype.

This species, like other species of Polymixia (e.g., Kotlyar, 1993; Borden et al., 2019), has 29 total vertebrae, 10 of which are abdominal (defined as lacking a complete haemal arch) and 19 of which are caudal vertebrae (Fig. 4). Kotlyar (1982, 1984, 1991, 1993) counted abdominal vs. caudal vertebrae differently, apparently using the first haemal spine to mark the first caudal vertebra. Using Kotlyar's criterion, there would be 12 abdominals and 17 caudals in both Bermuda types. Ossified epipleural intermusculars (see Patterson and Johnson, 1995 for details) are borne on the ribs of vertebrae 9–12, and on the haemal spines of vertebrae 13 through about 19 (Fig. 4). Ossified epineural intermusculars are borne on the neural arches or spines of vertebrae 2 through about 19. Ossified epicentrals, the third set of intermusculars, first recognized in Polymixia by Patterson and Johnson (1995) and further described by Gemballa and Britz (1998), can be seen in the radiographs on several of the abdominal vertebrae, whereas on other vertebrae they most likely are ligamentous; a precise count even of the ossified ones is not possible in the radiographs. The homology of the single pair of large intermuscular bones borne on the first vertebra has been controversial. Patterson and Johnson (1995) argued that it was an epineural displaced ventrally, but Gemballa and Britz (1998) made a convincing case that it is an enlarged and heavily ossified epicentral. Its function remains unknown.

The first two caudal vertebrae in P. hollisterae (three in P. nobilis and at least one specimen of P. yuri: see details below in Comparisons) bear ribs in addition to having a closed haemal canal. The third caudal vertebra (vertebra 13) in P. hollisterae is the first one with a haemal spine, which is distinctively shaped in being expanded in the midline and prolonged into a needle-like posteroventral process (Fig. 4). According to some conventions (Kotlyar, 1993; Patterson and Johnson, 1995), this would be the first caudal vertebra. This enlarged haemal spine is associated with and lies immediately posterior to the enlarged first proximal anal radial, but they do not appear to be in contact with each other.

As in all species of Polymixia, neural spines are present on all vertebrae anterior to compound centrum PU1+U1 (Fig. 4). Four neural spines are anterior to the first proximal dorsal radial (Figs. 4, 7). Neural and haemal spines that interdigitate with dorsal and anal proximal radials have their distal third expanded antero-posteriorly (i.e., they are paddle- or oar-shaped; Figs. 4, 7). A trait that differs among individuals and among species is that six neural spines are counted in radiographs between the last dorsal-fin radial and the epurals in the holotype of P. hollisterae, but seven in the Bermuda paratype, while seven haemal spines occur between the last anal radial and the parhypural in both specimens (Fig. 4).

The mouth is large and terminal (Figs. 2, 5B, 6). The maxilla reaches to beneath the posterior margin of the orbit. The upper jaw lengths on the adult types are 73% and 71% of POHL, while the lower jaw lengths are 77% and 75% of POHL, respectively. On the juvenile paratype they are 71% of POHL for the upper and 84% for the lower jaw. On each maxilla there are two flat supramaxillae that abut but do not overlap each other and do not appear to be moveably articulated (Figs. 5B, 6), as is typical in Polymixia; the anterior one is about half the size of the posterior one. The dentition consists of tiny sandpaper-like teeth on the upper and lower jaws, vomer, and palatines. Teeth on the dentary of the adult types (Fig. 6) form a narrow, sinuous band with soft tissue on either side, different from those of the premaxilla, which form a broad band that extends to the outer margin of that bone. We were able to count about 30 pyloric caeca in the Bermuda paratype of P. hollisterae.

A pair of long hyoid barbels is present (Figs. 5B, C, 6), their posterior tips not quite reaching the pelvic-fin base. The barbels of the holotype as preserved are coiled (Figs. 5B, 6A) and cannot be measured accurately, but those of the Bermuda paratype are nearly straight, extend posteriorly (Figs. 5C, 6B), and are 55.6 mm in length (30% of SL). The small juvenile paratype's barbels are damaged but what remains of one is 2.9 mm long (14.5% of SL). Seven branchiostegal rays are present on each side in all species of Polymixia, although they are not easily counted in the specimens of P. hollisterae. In other species, the first three are very small and modified to support the hyoid barbel of the same side (Starks, 1904; Zehren, 1979; Ono, 1982; Grande et al., 2013; Borden et al., 2019). There are three large, spathiform branchiostegal rays per side arising from the anterior ceratohyal and one from the posterior ceratohyal in all species of Polymixia. The posterior margin of the preopercle is rounded, with no indentation along its lower posterior margin and no pointed posteroventral projection (Fig. 2), a condition similar to that in P. japonica (Borden et al., 2019).

The lengths of the dorsal-fin base (DFB) in the holotype and Bermuda paratype represent 42.7% and 40.9% of SL, respectively, and 46.5% of SL in the juvenile paratype. The origin of the dorsal fin, denoted by the base of the small first fin spine, is vertically above vertebra 8 (Figs. 4, 7D, E) in both adult specimens. The dorsal fin in the holotype has five spines and 32 soft fin rays (V, 32), all soft rays being branched and the last one doubled but counted as one (Figs. 2A, 4A). The Bermuda paratype has one fewer soft ray (Figs. 2B, 4B). Both it and the juvenile paratype also have five spines. The first dorsal-fin radial supports one small spine. Each subsequent fin spine is progressively longer than the one before. The longest soft ray is number two, measuring 19.5% of SL in the holotype, 21.5% of SL in the Bermuda paratype (Fig. 4), and 27.5% of SL in the juvenile paratype. The next five rays in both Bermuda types are also noticeably longer than more posterior rays, after which the remaining rays become just slightly shorter in length in a regular progression (Figs. 2, 4).

There are three supraneurals (Figs. 4, 7D, E), the first inserting anterior to the first neural spine, the second between the first and second neural spines, and the third between the third and fourth neural spines. Thus, there is no supraneural between neural spines three and four in either Bermuda type specimen. Dorsal-fin proximal radials are long, nearly straight, and anterior radials overlap significantly with the distal portions of neural spines (Figs. 4, 7D, E). Each of the first 15–17 proximal radials has an expanded lateral flange as well as a prominent anterior and posterior midline flange, whereas the more posterior radials are more rod-like. The interdigitation patterns in P. hollisterae and other examined species are summarized in Figure 8. In both Bermuda specimens, the first dorsal radial inserts between neural spines 4 and 5 and the second between neural spines 5 and 6. Beginning with the third and fourth radials, two radials insert between adjacent neural spines. Only one radial, the ninth, inserts between neural spines 9 and 10, followed by two each of the next ten radials inserting between adjacent neural spines. In both specimens, three radials insert between neural spines 15 and 16. The more posterior patterns of interdigitation differ slightly between the two specimens.

The lengths of the anal-fin bases (AFB) represent 16.9% and 16.3% of SL in the Bermuda types and 18% of SL in the juvenile type. The origin of the anal fin, denoted by the base of the first anal-fin spine, is directly below vertebra 18 (Fig. 4). The anal fin has four spines and 15 soft rays (IV,15) in both specimens. The juvenile paratype's anal-fin spines and rays are damaged. Each anal spine is progressively longer than the preceding one. All soft rays are branched and the last one is doubled and counted as one. The longest soft rays (the first and second) are each 13.8% of SL in the holotype, but subsequent rays are progressively shorter. The longest ray in the Bermuda paratype is 16.5% of SL. The distance between the vent and the anal-fin origin (AN–AF) is 8% of SL in the holotype and 6% of SL in the Bermuda paratype.

Anal-fin proximal radials slightly interdigitate with haemal spines. The proximal radials are less expanded than dorsal-fin proximal radials, except for the greatly enlarged first anal radial (Figs. 4, 9C, D, 10C, D), which has a general shape that is unique for the genus but also has a specific morphology that is diagnostic for the new species. This specific morphology was confirmed by CT scan in the Gulf of Mexico juvenile paratype (Fig. 11). In the new species, the main shaft of the first radial is nearly straight and inclined at an angle of about 43° to the vertebral column. The anterodorsal tip of the first radial ends just anterior to but does not contact the expanded first haemal spine of vertebra 13. The anterior process of the radial is more nearly horizontal than in other species, making an angle of about 38° with the shaft and about 5° with the vertebral column. In the angle between the two processes is a thin-walled, conical cavity that has a relatively greater volume than that of other species in the genus.

The caudal fin is forked and its fin-ray formula is vi 9,9 v, with two upper and two lower procurrent rays closest to the principal rays being segmented; principal rays include one unbranched ray in each lobe, the intervening principal rays being branched. This pattern is consistent with those of all species of Polymixia as far as known. The inner fin rays of the dorsal and ventral lobes frequently overlap distally (Figs. 2, 4, 5A). The caudal-fin skeleton is similar to that in other species of the genus (Grande et al., 2013), consisting of three epurals, six autogenous hypurals, and two uroneurals (Fig. 4). A full spine on preural centrum 2 is present as in other paracanthopterygians (Borden et al., 2013, 2019; Grande et al., 2013, 2018).

The pectoral fin has a splint at the base of its leading (anterodorsal) edge and 17 principal soft rays in all three types. The lengths of the soft rays increase from ventral to dorsal. The longest (dorsalmost) ray and the shortest (ventralmost) ray are segmented but not branched, while all other rays are branched. The pectoral fin is relatively long in the adult specimens, with PFL 21% of SL in the holotype, 24% of SL in the Bermuda paratype, and 15.5% in the Gulf of Mexico juvenile paratype. The pectoral fin is supported by five narrow radials contained within a narrow fleshy base (Figs. 2, 4, 5B) that is located directly ventral to vertebra 5 (Fig. 4). The distance between the pectoral-fin origin and the end of the hypurals (PF–CB) is 63% of SL in the holotype and 64% of SL in the Bermuda paratype.

The anterior tip of the pelvic girdle is directly ventral to vertebra 4, and the origin of the pelvic fin is ventral to vertebra 7 (Fig. 4). The distance between the pelvic-fin origin and the posterior end of the hypural plate (VF–CB) is 59% of SL in the holotype and 63% of SL in the Bermuda paratype. The number of pelvic-fin rays is 7, a number that is consistent among all species of Polymixia. The VFL is 14.5% of SL in the holotype and 15.5% of SL in the Bermuda paratype. The leading pelvic ray is segmented but not branched. The second pelvic-fin ray is the longest, and the remaining rays are progressively shorter. The pectoral fin extends as far posteriorly as does the pelvic fin. A similar condition is seen in P. japonica.

Coloration in alcohol.—Body coloration in alcohol in the Bermuda type specimens is not uniform, being light tan on the belly, progressively darker toward the lateral line, and further darkened to a medium brown mid-dorsally. The darkest body pigmentation (a dark brown) is on the upper scale-covered part of the head, beside the base of the dorsal fin, and on the upper part of the caudal peduncle (Figs. 2, 5A). Little or no pigmentation is present on the snout or paired fins, but the dorsal rim of the eye and orbit is pigmented black (Figs. 5, 6). No other external part of the eye or orbit is noticeably pigmented in alcohol. The most conspicuous pigmentation is present on the dorsal, anal, and caudal fins (Figs. 2, 5A) and sets this species of Polymixia apart from all others. Deep black pigmentation is present on the distal parts of the first five soft rays of the dorsal fin, forming a narrow dark patch at the anterodorsal tip of the fin. The first soft ray is the most heavily pigmented, with a slight reduction of pigmentation on rays 2–5. The pigmentation of rays 1–3 begins at the midpoint of each ray, while dark pigment is present on the distal third of rays 4–5. Pigmentation of the anal fin is very similar to that on the dorsal fin but more restricted in area, occurring on the first 4 soft rays, again forming a dark patch at the tip of the fin. Although these fin rays are slightly pigmented proximally, the pigment darkens to virtually black on the distal one-half or one-third of each ray. The proximal parts of the caudal rays appear brown to gray, but the distal ¹ 4 of the caudal-fin rays is pigmented with a black margin that occurs on all the principal rays of the fin, except for a few of the shortest rays /in the middle of the fin. This black fringe is not seen in congeners.

The Gulf of Mexico paratype is a small juvenile (Fig. 3) that lacks most of the distinctive pigmentation seen in the adult type specimens from Bermuda. The body and fins are mostly silvery in the shipboard image (Fig. 3A). Pigmentation of the anal fin appears to be mostly absent. The dorsal-fin spines and a few anterior dorsal-fin soft rays have a darker but not continuous pigmentation. The caudal fin is missing in the existing voucher (Fig. 3B) and difficult to see, but not obviously pigmented, in the ship-board image (Fig. 3A).

Habitat and distribution.—Polymixia hollisterae is known so far from only two localities. The adult specimens are from the NNW flank of the Bermuda Platform, which is an eroded remnant of an ancient oceanic volcano complex (Fig. 12). Both specimens were collected near or on the seafloor at the same place, on the same date, and at the same depth of approximately 280 fm or 512 m (Smith-Vaniz et al., 1999). This locality is different from the known distributions of the two other species of Polymixia that occur in Bermuda waters (Fig. 12). Specimens of P. lowei from Bermuda are so far only known (Fig. 12) from the southeastern flank of the platform (off the southeastern shore of Bermuda near Devonshire Bay and off the former Sonesta Beach) at depths of 150–250 fm (274–457 m). The single known specimen of P. nobilis from the Bermuda region was taken on the flank of Challenger Bank (Fig. 12), the closest submarine rise 24 km or 14 nautical miles southwest of the Bermuda islands, at a depth of 200 fm (366 m).

The small, juvenile Gulf of Mexico paratype was taken in a midwater trawl at an unknown depth between 0 and 1,502.5 m, in water with a maximum depth of 2,500 m. The middle of the trawl's track was about 232 km (130 nautical miles) SSE (bearing 157.5°) from the mouth of the main channel of the Mississippi River. The track was 16 km in length (8.7 nautical miles) and had a bearing of 191°.