Porogadus dracocephalus, Schwarzhans & Møller, 2021

Schwarzhans, Werner W. & Møller, Peter R., 2021, Revision of the ‘ dragon-head’ cusk eels of the genus Porogadus (Teleostei: Ophidiidae), with description of eight new species and one new genus, Zootaxa 5029 (1), pp. 1-96 : 30-34

Porogadus dracocephalus n. sp.

Figs. 3 , 11–12 , 44 , 49 , Tab. 1–7

Porogadus trichiurus (not Alcock, 1890): Norman 1939: 77.

? Porogadus miles (not Goode & Bean, 1885): Machida & Amaoka 1990: 66, fig. 3. Material examined (8 specimens): Holotype: MNHN 2002-3875, 144 mm SL, 09°00’S 159°49’E, 1169–1203 m, Solomon 1 expedition, Sta. cpl1754, 26 September 2001; paratypes: BMNH 1939.5.24.1453, 131 mm SL, John Murray Expedition station 1933-34, sta. 26, Gulf of Aden, 12°29’N 50°51’E, 2312 m, 11 October 1933; BMNH 1939.5.24.1455, 130 mm SL, John Murray Expedition 1933-34, sta. 118, off Zanzibar, 04°05-17’S 41°10-11’E, 1789 m, 17 January 1934; MNHN F-470, Madagascar exp. sta. 126, 153 mm SL, off Madagascar, 17°50’S 43°07’E, 1475–1530 m, collected by Dr. A. Crosnier, 16 January 1975; MNHN 2021-0161 (ex MNHN 2002-3875), 122+ mm SL,same data as for Holotype; NSMT-P 63890, 133 mm SL, Okinawa Trough, 25°25’N 124°57’E, 2133 m, 3 m ORE Beam Trawl, 25 April 2002; NSMT-P 98894, 320 mm SL, collected by S. Kojima, 4 m ORE Beam Trawl, North slope of Ryukyu Trench, 24°51’N 127°25’E, 4466–4634 m, 18 May 2005; tentatively assigned specimen: BMNH 1939.5.24.1454, 177 mm SL, John Murray Expedition 1933-34, sta. 62, Arabian Sea, 22°53-56’N 64°56’E, 1893 m, 18 November 1933.

Diagnosis. Precaudal vertebrae 17–18; long gill rakers on first gill arch 13–15; HL:HD 1.97–2.08; distance of pelvic base to beginning of anal fin 0.72–0.98 times head length; all head spines strong, present on ethmoidal, lacrimal, prefrontal, interorbital, supraorbital, sphenotic, 5th infraorbital, supratemporal, inner and outer posttemporal, inner and outer preopercular rims; opercular spine sharp, strong, extruding; maxilla without scales or a single line of scales along upper margin; lower lateral line pores until beginning of anal fin 19–21; vomer with narrow dentition patch (1–2 rows of teeth); palatines with narrow dentition patch (2–5 rows of teeth); otolith with single, uniform colliculum; OL:OH = 1.4–1.5; OL:TCL = 1.9–2.0.

Description. Meristics: precaudal vertebrae 18 (17–18), 1 (0–1) last vertebrae without ribs; pectoral-fin rays 18 (16–18); D/V = 5 (5–7); D/A = 24 (24–28); V/A = 18 (17–20); long gill rakers on first lower gill arch 13 (13–15, 19 in tentatively assigned specimen). Gill rakers in holotype on lower first gill arch with seven plate-shaped rakers, followed by a series of 13 long rakers. The lower five of those intercepted by single plate shaped rakers. Upper gill arch with a plate shaped rakers followed by three short rakers intercepted by plate shaped rakers and then additional two plate shaped rakers.

Morphometrics: in % of SL: HL 16.5 (16.5–17.6); maximal HD 8.3 (8.1–8.9); HD through center of eye 5.7 (5.2–5.7); bony interorbital width 4.3 (3.4–4.3); snout length 5.5 (5.5–5.9); upper jaw length 9.5 (9.5–10.1); predorsal 16.5 (16.5–18.2); preanal 28.6 (28.6–30.6); prepelvic 13.1 (13.1–14.9); prepectoral 17.2 (16.1–17.8); pectoral length (9.0–10.1). Relations: HL:HD = 1.97 (1.97–2.08); HL to snout length 3.01 (2.87–3.02); preanal to predorsal 1.73 (1.60–1.76); predorsal to prepectoral 0.96 (0.96–1.08).

Slender fish with long tapering tail and long, pointed snout. Maximal size of fishes investigated 320 mm SL. Head long and very slender, with flat dorsal profile, with strong spines as follows: ethmoidal (1), lacrimal (1–3 spines, increasing with fish size), prefrontal (1), interorbital (3), supraorbital (2–3), sphenotic (3–4), 5th infraorbital (1–3), supratemporal (2), inner and outer posttemporal (4–5), inner preopercular rim (3–4), outer preopercular rim (1–4, variable in strength). Opercle with sharp, strong, extruding spine. Eye small located in strongly asymmetric orbit. Maxilla extending far beyond eye, strongly widened posteriorly and with distinct supramaxilla. Infra-/postorbital and mandibular-preopercular pores wide, no or up to 2 pores on occiput along inner edges of temporal spines. Head squamation on opercle, cheeks, occiput, frontal, snout, and in front of eyes, maxilla without scales or, in the largest specimen ( Fig. 12A ) with single, short row of scales along upper margin of maxilla in front of supramaxilla. The largest specimen (NSMT 98894) is remarkable for a complete preservation of body scales, which were used for Fig. 3 . Opercle with two to three distinct large neuromasts behind preopercular edge; opercular flap small, with few radial ridges along lower margin in largest specimen. All three lateral line rows well visible. Upper lateral line row with 10–13 pores; lower lateral line row with 18–21 pores until beginning of anal fin; central lateral line row long, lower and central lateral line rows fading behind beginning of anal fin.

Dentition. All teeth tiny and cone-shaped. Vomer with a v-shaped dentition patch with 1–2 rows of teeth anteriorly; palatines with a long and narrow patch with 2–5 rows of teeth in the middle part. Premaxilla tooth patches not fused anteriorly; ca. 4 teeth rows anteriorly and 1–2 rows posteriorly. Dentary tooth patches not fused anteriorly; ca. 3 teeth rows anteriorly and 1–2 rows posteriorly. Median basibranchial tooth patch short, moderately wide, in one case with pair of tiny lateral tooth patches.

Otolith morphology (n = 2). Size up to 4.35 mm in length (NSMT 98894); OL:OH = 1.4–1.5; OH:OT c.2.6. Thin, oval, moderately elongate otolith, anteriorly and posteriorly rounded. Dorsal and ventral rims regularly curving. All rims smooth. Inner face nearly flat, smooth, with short, centrally positioned sulcus; OL:TCL = 1.9–2.0. Sulcus with shallow, undivided, uniform, oval colliculum. Dorsal field with broad, indistinct depression; ventral field with faint ventral furrow. Outer face smooth, with weak central umbo.

Coloration. Live coloration not known. Color of preserved specimens medium to reddish brown; head slightly darker than body, opercle distinctly darker.

Discussion. Porogadus dracocephalus is similar to P. miles with which it may geographically overlap off Japan and elsewhere in the western Pacific. A single specimen from the John Murray Expedition from the Arabian Sea (BMNH 1939.5.24.1454) is only tentatively placed in P. dracocephalus because of its unusually high number of long rakers on the first gill arch (19 vs 13–15), which nevertheless could be just within the expected range of variability. In all other characters, this specimen resembles the type-series well. A single large specimen from the Ryukyu Trench of southern Japan (NSMT-P 98894) differs from the other type specimens in the larger size (320 mm SL vs 130–170 mm SL) and the complete preservation of body squamation. It thus represents a rare instance where head and body squamation can be studied in full extend. Unfortunately, it has an open mouth which prohibits certain measurements, e.g., head length and predorsal and preanal lengths. However, the spines on the head, the restricted squamation on the maxilla, the number of lower lateral line pores, vomer and palatine dentition and last but not least the otolith morphology confirms the attribution to P. dracocephalus . In this respect it needs to be noted that not all specimens of P. miles recorded from off Japan by Machida & Amaoka (1990) were available for review, in particular not the southernmost specimen from the Ryukyu back arch basin (ZUMT 54316) and the unusual deep catch of ZUMT 54315. The very detailed head drawing of ZUMT 54316 shows complete squamation on cheeks, opercle and top of head, but no scales on the maxilla, which would be consistent with P. dracocephalus but not P. miles . It is therefore possible that Machida & Amaoka (1990) also contain records of P. dracocephalus which were then regarded as representatives of P. miles .

Porogadus dracocephalus differs from all other species of Porogadus in the distance from the pelvic fin base to the beginning of the anal fin being shorter than the head length (ratio 0.72–0.98) whereas it is of equal or considerable larger length in all other species (ratio 0.98–1.31). However, this is often a difficult character to reliably measure, particularly in fishes with open mouth. The external differences of P. dracocephalus to P. miles are all subtle or overlapping, e.g., number of lower lateral pores until beginning of anal fin 18–21 (vs 23–26), pectoral-fin rays 16–18 (vs 18–21), preanal 28.6–30.6 % of SL (vs 30.0–36.0), vomer with 1–2 rows of teeth (vs 2–5 rows), and palatines with 2–5 rows of teeth (vs>5 rows). The most reliable and secure character for distinction therefore is the otolith, when preserved, which in P. dracocephalus is relatively compressed (OL:OH = 1.4–1.5) and bears a short, undivided colliculum while in P. miles it is more elongate (OL:OH = 1.55–1.9) and always shows two, clearly separated ostial and caudal colliculi. Porogadus dracocephalus differs from P. longiceps and P. mendax n. sp. in the lower number of lower lateral pores until the beginning of the anal fin of 18–21 (vs 22–24), the preanal length (28.6–30.6 % of SL vs 30.6–37.0 %), the narrow vomer with 1–2 rows of teeth (vs wide with 3–9 rows, rarely 2 rows in specimens of P. longiceps <150 mm SL), the narrow palatines (2–5 rows of teeth vs 5–11 rows, except 2–5 in specimens of P. longiceps <260 mm SL). From P. longiceps it differs additionally in the otolith showing a uniform colliculum (vs two separate colliculi or at least a v-shaped indentation at the collum). Porogadus dracocephalus differs from P. trugidus in the dentition of the vomer (narrow with 1–2 rows of teeth vs wide with 4–5 rows) and the palatines (2–5 rows of teeth vs 5–6 rows), the more intense spines along the inner and outer rims of the preopercle, the reduced squamation on the maxilla (no scales or a single short row along the upper margin of the maxilla in front of the supramaxilla vs rear part of the maxilla with many scales represented as scale pockets in the specimens studied), and the sulcus of the otolith with a single, undivided colliculum (vs two distinctly separated ostial and caudal colliculi). For distinction from P. caboverdensis see above.

Distribution. Porogadus dracocephalus is widely distributed in the Indian Ocean from the Arabian Sea to the Mozambique Channel, but further south is replaced by P. miles . There are no records (yet) from the eastern Indian Ocean. Several records from off southern Japan (Ryukyu Chain) and off the Solomon Islands result in an apparent disjunctive distribution pattern. Porogadus dracocephalus may potentially overlap in geographic distribution with P. miles along the outer reaches of its distribution, particularly in the western Pacific. Most specimens studied of P. dracocephalus of sizes from 130–170 mm SL are from relatively shallow depths from about 1200 to 2300 m. The only confirmed large specimen (320 mm SL; NSMT-P 98894) was caught significantly deeper at 4466–4634 m. This depth is deeper than any of the confirmed occurrences of P. miles and could indicate that P. dracocephalus is migrating into great depth when growing older.

Etymology. Name composed of draconis (Latin) = dragon and cephalus (Latin) = head, referring to the dragonhead shape of the heads of this species.

Figures

FIGURE 3. Lateral line system in Porogadus. Photograph of fish and drawings of opercular neuromast is from P. longiceps, drawings of scales are from P. dracocephalus n. sp.

FIGURE 11. Porogadus dracocephalus n. sp.; 11A–B, NSMT-P 98894, paratype, 320 mm SL, 24°51’N, 127°25’E, 4466–4634 m; 11C–E, MNHN 2002-3875, holotype, 144 mm SL, 09°00’S 159°49’E, 1169–1203 m.

FIGURE 12. Porogadus dracocephalus n. sp.; 12A–B, NSMT-P 98894, paratype, 320 mm SL, 24°51’N, 127°25’E, 4466– 4634 m, head drawing; 12C, BMNH 1939.5.24.1453, paratype, 131 mm SL, 12°29'N, 50°51'E, 2312 m, head drawing; 12D–E, MNHN 2002-3875, holotype, 144 mm SL, 09°00’S 159°49’E, 1169–1203 m, head drawing; 12F–G, NSMT-P 63890, paratype, 133 mm SL, 25°25’N, 124°57’E, 2133 m, head drawing; 12H–I, NSMT-P 98894, paratype, 320 mm SL, 24°51’N, 127°25’E, 4466–4634 m, otolith (H inner face, I ventral view); 12J, BMNH 1939.5.24.1453, paratype, 131 mm SL, 12°29'N, 50°51'E, 2312 m, otolith (inner face).

FIGURE 44. Geographic distribution of the species of the Porogadus miles group.

FIGURE 49. Depth versus distance from shelf plots for specimens of the Porogadus miles group (upper panel) and the Porogadus gracilis group (lower panel).