Photonectes mirabilis Parr, 1927

Prokofiev, Artem M. & Klepadlo, Cynthia, 2019, Two new species of Photonectes with blue luminous tissue on body, and a re-examination of P. mirabilis (Teleostei: Stomiidae), Zootaxa 4590 (2), pp. 270-282 : 276-279

Photonectes mirabilis Parr, 1927

Figs. 6–8

Photonectes mirabilis: Parr, 1927: 111 , fig. 59 (original description); Regan & Trewavas, 1930: 123, fig. 117B; Morrow & Gibbs, 1964: 505; Gibbs & Barnett, 1990: 335; Klepadlo, 2011: 202, 207 (in key).

Material examined. IOM 3581 , 83 mm SL, North Atlantic , 35° 29' N, 51° 54' W, 900– 740 m, FRV Atlant, 8 August 1976 , trawl no. 201. USNM 256912 , 52 mm SL, Central Pacific , 0° 18–24´N, 150° 11–12´W, 0–375 m depth, R/ V Wecoma, NORPAX, 9 December 1977 ; USNM 270613 , 2 (19.1–21.6 mm SL), Bahamas Islands , 25° 25´N, 78° 04´W, 0–200 m depth, R/ V Cape Florida, cruise 18l–13, sta. JEC 84-22, 20 October 1984 .

Note: All the USNM specimens are in very poor condition now (dry and extremely brittle from the ethanol, skin and jaws strongly damaged); it is thus virtually impossible to check most of characters of these specimens. Nevertheless, the structure of the mental barbel confirms identification of the specimen USNM 256912 as P. mirabilis . No apparent barbels left in the specimens USNM 270613; however, these specimens were identified when fresh by late Dr Robert Gibbs, so we accept them. The present redescription is based mainly on the IOM specimen 83 mm SL. The holotype of P. mirabilis (YPM 2077) is in such bad condition now that any manipulation of it is impossible. The characters from the original description of Parr (1927) that differ from that observed in the specimens examined are given in brackets.

Diagnosis. A species of Photonectes unique within the genus in combination of the presence of several bulbs and bulblets and a dorsal branch of the mental barbel and of the blue luminous tissue on body. Pattern of blue tissue (a pair of longitudinal lines between IP photophores widely separated from the longitudinal row of spots or continuous line above PV photophores on each side of body) showing no similarity with other “blue-tissue” species. In addition, no other Photonectes species possessing three pairs of luminous spots inside mouth is characteristic for P. mirabilis .

Description. Body elongate, tapering toward caudal fin ( Fig. 6A ). Snout 1.25 times shorter than eye. Morphometric values (IOM specimen only), in % of SL: head length 15.7; snout length 2.4; eye diameter 3.0; least interorbital width 4.0; barbel length 10.8; upper jaw length 13.3; postorbital organ length 3.0; ditto, greatest depth 0.9, entire black surrounding area 4.2; predorsal, preanal and prepelvic distances 78.3, 78.3 and 66.3, respectively; dorsal- and anal-fin base length 12.7 and 13.9, respectively; pelvic-fin length 12.1; greatest body depth 12.1; least depth of caudal peduncle 2.4. Opercle almost straight dorsally, its posteroventral corner somewhat extended, rounded; gill filaments extending slightly beyond gill cover. Gill filaments shortened in upper half of ceratobranchial of first arch (twice shorter than lowermost filaments of ceratobranchial-1). Color of skin turns to rusty brown after more than 40 years of storage.

Dorsal-fin rays 16–17; anal-fin rays 18–20; pelvic-fin rays 7; pectoral fins absent. Black fleshy skin extends onto bases of dorsal and anal fins. Pelvic fins reaching to vent. Caudal fin forked, with lower lobe longer. Vertebrae 50. Photophores: IP 9–10; PV 24; IV 33 –34; VAV 11–12 [last 2 over anal-fin base, not raised]; AC 11; IC 57; OV 21–24; VAL 10–11 [last 3 over anal-fin base]; OA 31–34; BR 8–9. Anteriormost eight IP photophores smaller than remaining ones, closely spaced, with distances between them not exceeding diameter of organ; remaining one or two IP photophores closely spaced, separated from anterior group of eight organs by a gap equal to 2.5 diameters of organ. Distance between anterior tip of isthmus and first pair of IP photophores equal to combined length of anteriormost seven IP photophores. Secondary photophores very small, scattered over head and body, most densely clustered along dorsum, absent on fin-rays. Postorbital organ elongate, narrowly oval (nearly 4 times its own length), equal to or greater than eye in length. Antero-ventral corner of jet-black tissue encircling the photogenic gland of postorbital organ extends anteriad to a vertical through the center of eye ( Fig. 6B ).

Blue luminous tissue on body represented by a longitudinal row of roundish spots ( Fig. 6A ) [longitudinal line] on each side of body situated above the interspaces between PV photophores (twice as close to PV than to OV series). This row originates above 1 st organ of PV and extends backward to 17 th (left side) or 20 th organ (right side) of PV; anteriormost spots partly confluent to form short streaks, but well separated from each other behind 7 th PV photophore. A pair of longitudinal stripes of blue luminescent tissue present inward of anterior group of eight IP photophores, these stripes being completely and widely separated from the longitudinal rows above PV photophores. No other elements of blue luminous tissue on flanks or ventral surface of body. A bluish light organ in front of the eye, below level of upper eye margin, nearly round, diameter nearly 25% of eye. Three roundish spots of pale blue (whitish?) luminescent tissue on each mandibular ramus (3 times closer to mandibular symphysis than to lower jaw articulation), and a diffuse patch in posteriormost portion of each ramus. Round spot of pale blue (whitish?) tissue on branchiostegal membrane above the uppermost photophore of BR ( Fig. 6F ). Three pairs of light organs on inner side of lower jaw ( Figs. 6C, 6D ): first pair small, on either side of symphysis; second pair larger, behind bases of 2 nd and 3 rd teeth; third pair as large as second one, behind bases of 5 th and 6 th teeth [largest, behind bases of 6 th and 7 th teeth]. Narrow streaks of pale (whitish?) apparently luminescent tissue along anal-fin rays (absent along dorsal-fin rays); small white luminescent spots scattered along anal-fin base.

Jaw dentition heterogeneous; premaxillary teeth weakly biserial, with outer teeth much shorter ( Fig. 6E ). Premaxillary with 7–8 + 3 caniniform teeth; inner premaxillary teeth distinctly longer than those on maxillary and dentary. At least some of inner maxillary teeth possessing small barbs at tips (tips of many teeth broken). Maxillary with 4–11 erect caniniform teeth (3 rd and 4 th longest), following by a comb-like row of small inclined needle-like teeth (12–15 in number). Dentary teeth about 23–25 in number, consisting of long and short canines. Vomer with 2 + 2 teeth, outer ones being distinctly longer. Palatine with 2 teeth on each side.

Barbel moderate in length (69.2% of head length) [slightly greater than head in length], containing several bulbs of different size and position ( Figs. 6F , 7 ): moderately small round bulb on dorsal surface just behind base of barbel, separated from the subsequent large oval (main) bulb by a distance equal to the length of main bulb (16.7% of full barbel length), stem of barbel densely pigmented from its base to base of main bulb [two light caeruleanblue spots on stem]. Main bulb bearing a long terminal appendage (two-thirds of full barbel length, 66.7%) ending in a small bulblet. Short dorsal branch ending by a small round bulb separate from terminal appendage 3 times closer to the end of main bulb than to the end of terminal appendage. Small raised bulblet present on dorsal side of appendage midway between main bulb and dorsal branch of appendage [two bulblets, one on dorsal side and one on ventral side, just in front of the level of the former]. Dorsal surface of terminal appendage between main bulb and dorsal branch darkly pigmented, stem of dorsal branch heavily pigmented; otherwise, terminal appendage unpigmented.

Remarks. Our specimens well agree with original description of Parr (1927) except the blue line above PV organs represented by longitudinal row of spots (continuous line according descriptions of Parr (1927) and Morrow & Gibbs (1964)). Barbel structure is almost the same as described for the holotype except that its length is somewhat shorter and the small bulblet on the ventral side of the barbel’s appendage between the main bulb and dorsal process is absent in the 83-mm specimen (uncertain for the smaller specimens). We treat all these differences as individual variations. The 83-mm SL specimen is largest known specimen (the holotype attained 60 mm SL at time of description). However, it is not clear, if the aforementioned differences are connected with growth or not.

Photonectes cornutus was described by Beebe (1933) from a single juvenile specimen, 19 mm SL. Gibbs & Barnett (1990) synonymized this species with P. mirabilis . However, the blue tissue was not reported for P. cornutus . Furthermore, in P. cornutus the small bulb on the stem proximal from the main bulb is situated on a long stalk, which is probably longer than small dorsal branch on the appendage, so that the barbel of P. corntus possesses two dorsal branches of which the proximal one is better developed (though distal branch might be broken at tip according to Beebe’s figure) ( Beebe, 1933: Fig. 6 ). Since two other species of Photonectes with dorsal branch of barbel and lacking blue luminous tissue were described subsequently ( Klepadlo, 2011; Prokofiev, 2014), the synonymy of P. cornutus and P. mirabilis cannot be warranted with certainty. The holotypes of both these species are in such bad condition now, that collection manager was afraid to touch them (Susan Jewett, personal communication). Thus, the taxonomic status of P. cornutus remains uncertain and we treat this species as species inquirenda. We did not see any specimens agreeing with original description of P. cornutus .

Distribution. Most of the known specimens were collected in the Western North Atlantic between 24° and 35° N; however, one specimen (USNM 256912) comes from the Equatorial Central Pacific. Although this specimen is in very poor condition now ( Fig. 8A ), the structure of its mental barbel ( Fig. 8B ) leave no doubts as to its identification.

Figures

FIGURE 6. Photonectes mirabilis, IOM 3581: (A) habitus; (B) postorbital organ (anteroventral extension of jet-black tissue encircling the photogenic gland arrowed); (C, D) light organs inside mouth (arrowed) (C: right side; D: left side; scar-like remnants of jet-black surrounding tissue retained when photogenic tissue torn off); (E) premaxillary dentition (teeth of outer row arrowed); (F) mental barbel and adjacent region: (1) main bulb, (2) distal appendage, (3) dorsal branch, (4) luminous spot above the uppermost photophore of BR. Scale bars: (A) 5 mm; (B, E) 2.5 mm; (C, D, F) 1.5 mm.

FIGURE 7. Mental barbel of Photonectes mirabilis, IOM 3581. Scale bar: 1.5 mm.

FIGURE 8. Photonectes mirabilis, USNM 256912, Central Pacific: (A) habitus; (B) mental barbel. Scale bars: (A) 10 mm; (B) 0.75 mm.