Polymixia hollisterae, Grande & Wilson, 2021

Grande, Terry C. & Wilson, Mark V. H., 2021, A New Cryptic Species of Polymixia (Teleostei, Acanthomorpha, Polymixiiformes, Polymixiidae) Revealed by Molecules and Morphology, Ichthyology & Herpetology 109 (2), pp. 567-586 : 569-581

Polymixia hollisterae , new species urn:lsid:zoobank.org:act:475AE39F-7A68-4519-870E-3228F9893AD8

Order Polymixiiformes Rosen and Patterson, 1969

Family Polymixiidae Bleeker, 1859

Genus Polymixia Lowe, 1836

Proposed common name: Bermuda Beardfish

Figures 2–6 , Table 1

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 , 32825 0 01 00 N, 64856 0 43 00 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 m 2 MOCNESS, 3 mm mesh, depth range 0–1502.5 m, in water of maximum depth 2500 m, towed between 2780 0 13 00 N , 88829 0 39 00 W and 26852 0 15 00 N, 88831 0 22 00 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 (58 from axis vs. 10–258 in congeners) and makes an angle of about 388 with the main shaft, greater than the angles of approximately 25–328 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 PU 1 þ 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 oarshaped; 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 438 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 388 with the shaft and about 58 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 157.58) 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 1918 .

Comparisons

Molecular and morphological characters.— The molecular results of Borden et al. (2019) demonstrate that Polymixia hollisterae is distinct from all other species of Polymixia . Although phylogenetically closest to P. japonica and an unnamed species from Australia ( Fig. 1 ; Borden et al., 2019: fig. 4), P. hollisterae differs genetically from P. japonica by 20– 33 nucleotides in the total alignment of 4,983 sites ( Borden et al., 2019). Polymixia hollisterae is also genetically distinct from the type species P. nobilis and from P. lowei by 57–63 and 42–48 nucleotides, respectively ( Borden et al., 2019: fig. 7). The latter two species also occur in Bermuda waters. The new species differs the most genetically from the P. berndti species complex (phylogenetically the earliest branching clade in the genus, known from the Pacific and Indian Oceans) by 80–93 nucleotides out of 4,983 ( Borden et al., 2019).

Borden et al. (2019) discussed four morphological characters, originally proposed by Kotylar (1993), for which Borden et al. (2019) examined possible phylogenetic signal among species of Polymixia . First, Polymixia hollisterae shares with P. nobilis and P. japonica a distinctive and presumably derived scale morphology in that the ctenus-like spines are arranged in a wedge pattern along the posterior margin of the scale. The cteni of all other species of Polymixia are arranged in vertical rows along the posterior margin.

Second, the new species differs from P. nobilis in the number of dorsal-fin rays, and is more similar to P. japonica in this respect. Polymixia nobilis exhibits 33–38 dorsal-fin rays, while P. japonica has 30–35 and P. hollisterae has 31–32 rays in the dorsal fin. Kotlyar (1993) considered the condition found in P. japonica to be an intermediate number of rays, whereas P. lowei (26–32) and P. berndti (26–31) have the lowest counts. The counts in P. hollisterae overlap both with the intermediate range of P. japonica and the lower range of P. lowei and P. berndti .

Third, similar to P. japonica , P. lowei , and P. berndti , but different from the presumably more derived condition in P. nobilis , the new species shares a similar preopercle shape ( Kotlyar, 1993; Borden et al., 2019). In these species, the posteroventral margin of the preopercle is rounded, never extending to a point as it is in P. nobilis .

Fourth, according to Kotlyar (1993), the species Polymixia nobilis , P. yuri , and P. sazonovi are distinct from all other species in having a very high number of pyloric caeca (more than 100). Using counts reported by Kotlyar (1993), the number of pyloric caeca in the other known species of Polymixia is 65 or fewer. Our examination of the paratype of P. hollisterae (FMNH 145004) revealed a pyloric caeca count of about 30, somewhat fewer than that of P. japonica (48–65) and within the range of both P. lowei (27–30) and P. berndti (28–48).

In addition to the four characters discussed above, the coloration in alcohol distinguishes P. hollisterae from all other known congeners. The closest similarity is probably to P. japonica , in which there is a broad, dark, pigmented area on the distal half of the anterior dorsal-fin soft rays, broader and not as dark (in alcohol) as that in P. hollisterae . Polymixia japonica has a much fainter and more diffuse pigmentation on the anal fin and a more diffuse and much paler pigmentation usually restricted to the tip of the dorsal lobe and sometimes the tip of the ventral lobe of the caudal fin.

Supraneurals.— The supraneurals in Polymixia ( Figs. 4 , 7 ) are shaped like mountaineering ice picks with the pick’s point facing posteriorly within the dorsal body musculature and the shaft corresponding to the pick’s handle. The shaft of the first supraneural inserts anterior to the first neural spine, the second between the first and second neural spines, and the third supraneural between neural spines three and four. There is no supraneural between the second and third neural spines in any specimen of any of the species examined radiographically. The supraneurals differ somewhat among examined species ( Fig. 7 ). For example, the shaft of the first supraneural has a sigmoid curvature in P. nobilis ( Fig. 7A–C, F ) and P. hollisterae ( Fig. 7D, E ) but has a posteriorly concave curvature in P. lowei ( Fig. 7G–I ) and P. japonica ( Fig. 7J–L ). Supraneurals in P. lowei do not extend as far ventrally between neural spines as they do in the other species examined ( Fig. 7 ).

Dorsal radial interdigitation with neural spines.— Dorsal-fin proximal radials interdigitate with neural spines, the overlap between spines and radials being greater in P. hollisterae and slightly less so in P. japonica , and lesser in the other species examined ( Fig. 7 ). There are also differences among the species in the number of radials between adjacent neural spines, with distinctive patterns moving along the vertebral column. All species examined have one radial between neural spines 4 and 5 ( Figs. 7 , 8 ). However, in almost all specimens examined of P. japonica , there are two radials between spines 5 and 6, 6 and 7, and 7 and 8, then one between spines 8 and 9 and one between spines 9 and 10. In the other examined species, there is a single radial between spines 5 and 6. In both specimens of P. hollisterae , there is one radial between each of spines 4 and 5 and 5 and 6, followed by two each between spines 6 and 7, 7 and 8, and 8 and 9, and one between spines 9 and 10 ( Figs. 4 , 7 ). These sequences ( Fig. 8 ) can be summarized as P. japonica 1-2-2-2-1-2-, P. hollisterae 1- 1-2-2-2-1-, P. nobilis 1-1-2-2-2-2- (in most specimens), and P. lowei 1-1-2-2-1-2- (in the two specimens from Bermuda that we examined radiographically). The pattern is more variable in P. berndti ( Fig. 8 ), with four of the specimens examined showing 1-1-2-2-2-1-, but with variant patterns of 1-1-2-2-2- 2- (in two), 1-0-2-2-2-1-, 1-1-1-2-2-2-, and 1-2-1-2-2-2- (in one each). Thus P. hollisterae differs in the pattern of interdigitation of the anterior dorsal-fin proximal radials from P. japonica , P. nobilis , and P. lowei , and from about half of examined specimens of P. berndti . In all examined species, the interdigitation pattern of more posterior dorsal radials shows more individual variation, but there is no specimen of any species with an identical pattern overall to that of either specimen of P. hollisterae ( Fig. 8 ).

First haemal arch.— The first caudal vertebra is often defined as the anteriormost one with a closed haemal arch; this is vertebra 11 in all specimens of the genus Polymixia for which we could verify the presence or absence of a closed arch. Defined this way, there are ten precaudal centra and 19 caudal centra in all specimens where the character could be assessed. However, in most species, the anterior two caudal vertebrae (vertebrae 11–12), thus defined, support well-developed ribs and lack a haemal spine. In P. nobilis (five specimens; e.g., Fig. 9A ) and P. yuri (one specimen), the anterior three caudal vertebrae (vertebrae 11–13) have a complete haemal arch but no spine.

First haemal spine.— In all species, the first haemal spine is shorter than subsequent spines and expanded in the midline ( Fig. 9 ). It lies just posterior to the dorsal tip of the first anal-fin proximal radial and differs slightly in shape among examined species. In P. hollisterae , P. japonica , P. lowei , and P. berndti , the first vertebra with a well-developed haemal spine is vertebra number 13. Only in P. nobilis and the single specimen of P. yuri available to us is the first haemal spine found on vertebra 14. This is the case for the three examined specimens of P. nobilis from the type locality of Madeira, as well as the single dried skeleton from the Canary Islands and the single known specimen of P. nobilis from Bermuda ( Fig. 9A ).

First proximal anal-fin radial.— In all species of Polymixia , the first proximal anal-fin radial as seen in radiographs, clearedand-stained specimens, and a single dried skeleton inserts just anterior to the first haemal spine. The radial and spine approach each other closely but do not appear to be in contact ( Fig. 9 ). The first anal proximal radial has a characteristic shape in Polymixia and it also has distinctive features in each of the examined species ( Figs. 4 , 9 , 10 ).

The main shaft of the radial varies in curvature; it is nearly straight in P. hollisterae , moderately curved in P. japonica and P. nobilis , and strongly curved in P. lowei ( Figs. 9 , 10 ). The shaft of the radial is even more strongly curved in P. berndti (not shown here). The angle that the main shaft makes with the vertebral column varies ( Fig. 9 ) from a high value of about 508 in P. nobilis , through intermediate values of about 428 in P. lowei , 408 in P. hollisterae , and 358 in P. japonica , to a low of about 258 in P. berndti . In addition, the anterior process of the radial differs in the angle it makes with the main shaft. The largest angle among examined species (about 388) occurs in P. hollisterae , in which the anterior process lies close to the ventral body wall and is only 58 from being parallel to the body axis ( Fig. 4 ), whereas in other species the anterior process is directed anterodorsally to varying degrees ( Figs. 9 , 10 ).

Morphometrics.— Altogether, 34 landmarks for each of 27 specimens representing five species of Polymixia were digitized and used for morphometric analysis. The principal components analysis based on Procrustes coordinates yielded PC1 through PC4 explaining 28.3%, 19.2%, 11.7%, and 9.7% of the variance in the data. Only the first two principal components were needed to separate the two adult specimens of P. hollisterae from the other four species ( Fig. 13 ), with PC 2 giving the greatest separation and specimens of the other four species occupying mostly non-overlapping regions of the morphospace. The most extreme scores on PC 2 are for the holotype of P. hollisterae and one of our specimens of P. lowei from Bermuda, although P. hollisterae scores higher and P. lowei lower on PC 1.

Morphometric differences among the species are illustrated by the wireframe drawings ( Fig. 13 ). These results illustrate that there are important differences among most of the examined species in the relative size of the head, eyes, and jaws, and also in the dorsal outline of the body. Polymixia hollisterae has a relatively large head, large eyes, and long jaws, and in addition its pre-dorsal body outline is more streamlined than in most other species ( Fig. 13 ).

Body proportions.— Similar differences are seen in body proportions (measurement ratios), which are shown in Figure 14 and were calculated from the landmark pixel locations (point to point). Compared to the other four species analyzed morphometrically, P. hollisterae has the greatest average eye diameter and snout length as a ratio of preopercular head length, as well as the greatest ratio of preopercular head length to standard length, and conversely it has the smallest ratio of pectoral fin to caudal base distance, pelvic fin to caudal base distance, and pelvic fin to anal-fin origin distance, all as ratios of standard length ( Fig. 14 ). Among proportions not analyzed morphometrically, P. hollisterae also has the greatest average ratio in the lengths of the longest dorsal ray and longest anal ray, as well as in the lengths of the pectoral and pelvic fins to standard length. Overall, P. hollisterae appears to be the most streamlined of all species of Polymixia , suggesting that it is among the fastestswimming species of the genus.

Figures

FIG. 2. The type specimens of Polymixia hollisterae, new species, from Bermuda. (A) Holotype, BAMZ 1997-159-006, 173 mm SL. (B) Paratype, FMNH 145004, 185 mm SL. Scale bars ¼ 1 cm.

FIG. 3. The second paratype of Polymixia hollisterae, new species, from the north-central Gulf of Mexico, MCZ 174218, 20 mm SL (formerly DeepEnd Consortium specimen DPND 1320). (A) The image taken aboard ship shortly after collection in 2015, photo by Danté Fenolio, DeepEnd, used with permission. (B) The existing specimen, lacking the posterior one-third of the body and tail owing to tissue sampling for DNA. Scale bars ¼ 5 mm (estimated from the specimen’s original standard length).

FIG. 4. X-radiographs of the type specimens of Polymixia hollisterae, new species, from Bermuda. (A) Holotype, BAMZ 1997-159-006, 173 mm SL. (B) Paratype, FMNH 145004, 185 mm SL. Scale bars ¼ 1 cm.

FIG. 5. Closeup views of Polymixia hollisterae, new species. (A) Caudal region and median fins of the holotype, BAMZ 1997-159-006, 173 mm SL. (B) Lateral view of the head of the holotype. (C) Ventral view of the head showing the hyoid barbels of the Bermuda paratype, FMNH 145004, 185 mm SL. The base of each barbel contains three small, modified branchiostegals (Ono, 1982). Scale bars ¼ 1 cm.

FIG. 6. Oblique views of the head of the type specimens of Polymixia hollisterae, new species. (A) Holotype, BAMZ 1997-159-006, 173 mm SL. (B) Bermuda paratype, FMNH 145004, 185 mm SL. Scale bars ¼ 1 cm. Note the sinuous band of teeth on the dentary in both specimens, and the broad band of teeth on the premaxilla.

FIG. 1. Phylogeny of species and species groups in the genus Polymixia based on Bayesian results using nuclear and mitochondrial loci from Borden et al. (2019). The new Bermuda species is a member of the P. japonica species group, sister to P. japonica þ Polymixia cf. P. japonica. The triangle representing each species has its vertical edge proportional to the number of genetic samples studied and its left apex located at the earliest split among all of its samples (compare with Borden et al., 2019: fig. 4).

FIG. 7. X-radiographs of supraneurals and proximal dorsal radials in several species of Polymixia. (A–C, F) Polymixia nobilis. (A) FMNH 142335, 243 mm SL, Madeira. (B) FMNH 142336, 266 mm SL, Madeira. (C) FMNH 142337, 175 mm SL, Madeira. (F) ANSP 124292, 290 mm SL, Bermuda. (D) Polymixia hollisterae, new species, holotype, BAMZ 1997-159-006, 173 mm SL, Bermuda. (E) Polymixia hollisterae, new species, paratype, FMNH 145004, 185 mm SL, Bermuda. (G–I) Polymixia lowei. (G) BAMZ 1989-047-003, 210 mm SL, Bermuda. (H) BAMZ 1989-047-006, 226 mm SL, Bermuda. (I) USNM RAD 2126927. (J–L) Polymixia japonica, Sea of Japan. (J) FMNH 63860-1, 153 mm SL. (K) FMNH 63860-2, 144 mm SL. (L) FMNH 63860-3, 146 mm SL. Abbreviations: dr, dorsal-fin proximal radials (numbered); ds, dorsal-fin spines (numbered); dsr, dorsal-fin soft rays (numbered); ns, neural spines (numbered); sn, supraneurals (numbered).

FIG. 8. Summary of the patterns of interdigitation of dorsal proximal radials between neural spines in selected species and specimens of Polymixia. The number of radials between adjacent neural spines is shown beginning with neural spines 4 and 5 and ending at or before neural spines 22 and 23. Data are taken from whole-fish radiographs except for one cleared and stained specimen of P. nobilis.

FIG. 9. X-radiographs of anal-fin radials and associated vertebrae in selected species of Polymixia, all from Bermuda except for (B) which is from the Sea of Japan. (A) P. nobilis, ANSP 124292, 290 mm SL. (B) P. japonica, FMNH 63860-1, 153 mm SL. (C) Polymixia hollisterae, new species, paratype, FMNH 145004, 185 mm SL. (D) Polymixia hollisterae, new species, holotype, BAMZ 1997-159-006, 173 mm SL. (E) Polymixia lowei, BAMZ 1989- 047-003, 210 mm SL. (F) Polymixia lowei, BAMZ 1989-047-006, 226 mm SL. Abbreviations: ap, anterior process of first anal-fin proximal radial; ar, anal-fin proximal radial (numbered); as, anal-fin spines (numbered); asr, anal-fin soft rays (numbered); cc, conical cavity of first anal radial; ehs1, expanded first haemal spine; hi, probable healed injury of haemal spines; ms, main shaft of first anal-fin radial.

FIG. 10. Drawings of first anal-fin proximal radials in selected species of Polymixia (compare with the radiographs in Fig. 9). (A) P. nobilis, ANSP 124292, 290 mm SL, Bermuda. (B) P. japonica, FMNH 63860-1, 153 mm SL, Sea of Japan. (C) P. hollisterae, new species, paratype, FMNH 145004, 185 mm SL, Bermuda. (D) P. hollisterae, new species, holotype, BAMZ 1997-159-006, 173 mm SL, Bermuda. (E) P. lowei, BAMZ 1989- 047-003, 210 mm SL. (F) P. lowei, BAMZ 1989-047-006, 226 mm SL. Compare with the radiographs in Figure 9. Abbreviations: see caption for Figure 9.

FIG. 11. Drawing of the first anal-fin radial of the second paratype of Polymixia hollisterae, new species, MCZ 174218, north-central Gulf of Mexico, from lCT scan data. Note the nearly straight main shaft and the nearly horizontal anterior process, like those of the Bermuda holotype and paratype (Fig. 10C, D). These diagnostic traits confirm morphologically its correct identification to species. Abbreviations: see caption for Figure 9.

FIG. 12. Bathymetric map of the Bermuda Platform, Challenger Bank, and adjacent submarine slopes, with collecting localities for three species of Polymixia. ‘‘P. sp. nov.’’ denotes Polymixia hollisterae, new species, caught ‘‘outside Eastern Blue Cut’’ off the NW edge of the Bermuda Platform. The more frequently caught Polymixia lowei has only been taken in two areas off the SE shore of Bermuda, and the single known specimen of Polymixia nobilis was taken on the slope of Challenger Bank, 24 km (14 nautical miles) southwest of the Bermuda Islands. Base map by L. Doughty (BAMZ).

FIG. 13. Results of the Principal Components Analysis of Procrustes coordinates for 34 landmarks from 27 adult specimens belonging to five species of Polymixia. (A) Wireframe representation of the 34 landmarks digitized for the analysis superimposed on an image of the holotype of Polymixia hollisterae, new species. (B) Graph of component scores for PC 1 and PC 2, showing evidence of the distinct body form of Polymixia hollisterae, new species. The four wireframe cartoons illustrate the variation along both principal axes in the analyzed species.

FIG. 14. Comparison of average proportions among adult specimens of five species of Polymixia, calculated using lengths derived from the morphometric landmark coordinates. Averages for each proportion and species are colored orange if they are the highest ratio among the five species and blue if they are the lowest in the comparison. Polymixia hollisterae, new species, has the greatest number of extreme values (12: 8 highest, 4 lowest) of the five species.