A New Cryptic Species of Polymixia (Teleostei, Acanthomorpha, Polymixiiformes, Polymixiidae) Revealed by Molecules and Morphology
A new and previously cryptic fish species, Polymixia hollisterae, new species, proposed common name “Bermuda Beardfish,” is described from two voucher specimens collected in 1997 at a depth of about 280 fm or 512 m on the NW slope of the Bermuda Platform, and a third voucher collected by a midwater trawl in the north-central Gulf of Mexico. The species belongs to the paracanthopterygian acanthomorph genus Polymixia, family Polymixiidae, order Polymixiiformes, and is named after pioneering ichthyologist, ocean explorer, and conservationist Gloria E. Hollister (Anable). It is distinguished from all other species in the genus by its greater dorsal and anal fin heights (19% and 14% of SL, respectively), its extremely long pectoral and pelvic fins (22% and 14% of SL, respectively), its relatively large eye diameter (34% of total head length), and its morphologically distinctive first anal-fin radial. It has a unique pigmentation pattern on the dorsal, anal, and caudal fins consisting of narrow, dark black patches near the tips of the longest few soft dorsal- and anal-fin rays, and a caudal-fin margin with a dark black fringe covering the distal fourth of the fin. Based on an earlier molecular study that revealed it to be a cryptic species, P. hollisterae, new species, is closely related to P. japonica, but it differs by 20–33 nucleotides in a total alignment of 4,983 nuclear and mitochondrial sites. The new species shares a distinctive scale morphology with P. japonica and P. nobilis (the type species); an intermediate number of dorsal-fin rays (V,31–32), like P. japonica (V,31–34); a distinctive preopercle shape with P. japonica, P. lowei, and P. berndti; and a low number of pyloric caeca with P. lowei and P. berndti. Pored lateral-line scales number 35, an intermediate number compared to reported counts of other species (31–39). The three supraneurals are sigmoid-shaped like those of P. nobilis, while dorsal-fin proximal radials interfinger with neural spines in a unique pattern. Body shape, as studied with measurement proportions and multivariate morphometrics, is distinguished by a relatively large head, large eyes, long jaws, and a more streamlined pre-dorsal body profile than its congeners. Further research and additional collecting will be needed to ascertain the geographic distribution and conservation status of the new species.
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 + 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).
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 = 1 cm.
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 = 5 mm (estimated from the specimen's original standard length).
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 = 1 cm.
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 = 1 cm.
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 = 1 cm. Note the sinuous band of teeth on the dentary in both specimens, and the broad band of teeth on the premaxilla.
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).
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.
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.
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.
Drawing of the first anal-fin radial of the second paratype of Polymixia hollisterae, new species, MCZ 174218, north-central Gulf of Mexico, from μCT 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.
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).
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.
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.
Contributor Notes
Associate Editor: W. L. Smith.