Physiculus megastomus, Tang, 2021

Tang, Chi-Ngai, 2021, Description of a new codling species of Physiculus from Taiwan (Gadiformes: Moridae), Zootaxa 5052 (1), pp. 105-116 : 106-114

Physiculus megastomus sp. nov.

Figs. 1‒6 , Tables 1‒2

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Gadella jordani (non Böhlke & Mead, 1951): Chiang et al., 2014: 46, fig. 3.

Holotype. NMMB-P 35678, 307.2 mm SL, 344.0 mm TL, female with mature ovaries, off northern Taiwan, by hook-and-line, purchased from Keelung Fish Market , 2 May 2020; COI: MZ 873335 .

Paratype. NMMB-P 35679, 143.6 mm SL, 162.6 mm TL, off Chenggong , Taitung, eastern Taiwan, depth ca. 800 m, by hook-and-line, 16 May 2021; COI: MZ 873336 .

Diagnosis. A Physiculus species distinct from all congeners by possessing a very large mouth, posterior end of maxilla extending well behind the level of the posterior margin of orbit, its length 15.3‒15.9% SL and 57.8‒ 60.7% HL; both jaws with caniniform teeth; snout, suborbital area, and gular region fully scaled. It can be further distinguished from its congeners by the following combination of characters: light organ small, 5.5‒6.7% InV-af; light organ located near the mid-point of InV-af, its anterior margin located at 46.9‒50.7% InV-af; anus located closer to the anal-fin origin, its anterior edge located at 74.2‒83.1% InV-af; gill rakers on upper limb of first gill arch 5; scales in longitudinal series ca. 114‒122; scale rows between first dorsal-fin base and lateral line 10‒11.

Description. The following data are given for the holotype with paratype information in parentheses when different. First dorsal-fin rays 9, first ray short and obscure; second dorsal-fin rays 74 (69); pectoral-fin rays 24 (left)/24 (right); anal-fin rays 80 (74); pelvic-fin rays 5; scales in longitudinal series ca. 122 (114); scale rows between first dorsal-fin base and lateral line 10/11 (10/10); total gill rakers on upper limb 5/5, all developed (except paratype with 1 rudimentary raker on right side); total gill rakers on lower limb 10/12 (12/11), including 2/2 (1/0) rudimentary rakers; vertebrae 14+40 (39) = 54 (53); pyloric caeca 12.

Head length 26.1% (26.5) SL; predorsal length 28.0% (28.7); prepectoral length 26.9% (27.6); prepelvic length 20.7% (20.8); preanal length 33.8% (32.7); preanus length 29.1% (30.1); first and second dorsal-fin bases 6.5% (6.7) and 59.8% (59.2), respectively; anal-fin base 63.0% (65.2); snout length 27.5% (24.3) HL; interorbital width 34.0% (32.6); first dorsal-fin height 26.8% (27.9).

Body moderately elongate, robust anteriorly, gradually compressed and narrowing to a narrow caudal peduncle and caudal fin. Body depth 19.8% (21.4) SL. Caudal peduncle narrow, its depth and length 6.7% (7.2) and 23.4% (27.5) HL, respectively. Head large and broadly rounded, its width 15.7% (15.9) SL; snout wide with a round anterior outline; eye relatively small, its diameter 15.9% (19.1) HL; mouth very large and terminal, upper jaw extends well behind a vertical through the posterior margin of eye, its length 60.7% (57.8) HL; upper jaw slightly overhanging the lower jaw. Teeth large and caniniform. Upper jaw with 3 rows of slightly curved teeth (2 rows in paratype), the middle row largest (outermost row largest in paratype; Fig. 5 ), the outermost and innermost rows slightly shorter. Lower jaw with 3 rows (2 in paratype) of teeth, the outermost row small ( Fig. 5 ), the inner rows larger, subequal in length (only one inner row in paratype, distinctly larger than outer row). Vomer and palatine toothless. Chin barbel present but short, its length 4.0% (10.5) HL.

Two dorsal fins. First dorsal-fin origin slightly behind a vertical through the insertion of pectoral fin; second dorsal-fin origin right behind the end of first dorsal fin, rays generally uniform in height. Anal-fin origin anterior to the level of second dorsal fin, rays generally uniform in height. Pectoral fin moderate, its length 15.2% (16.4%) SL and 58.2% (61.8) HL, rounded posteriorly, with lower rays gradually shorter, posterior end to a vertical through ca. the 7th‒8th ray of the second dorsal fin. Pelvic fin long and elongated, its length 14.3% (22.1) SL and 54.5% (83.6) HL; when appressed, its tip extends slightly behind the anal-fin origin (well behind the anal-fin origin); pelvic fin thoracic, inserting anterior to the level of pectoral-fin base. Caudal fin small, well separated from dorsal and anal fins, rounded and symmetrical posteriorly.

Scales cycloid, small, longitudinally oval-shaped. Body fully scaled. Head almost fully scaled; snout, suborbital area fully covered with small scales; gular region (throat) fully scaled ( Fig. 4B ); branchiostegal membrane naked. Dorsal and anal fins totally naked. Pectoral-fin base scaled, slightly extending onto the fin. Caudal-fin base scaled. Lateral line not completely continuous, continuous tubed scales do not extend beyond the posterior end of the first dorsal fin.

Ventral light organ very small, the distance between its anterior and posterior margins 3.3% (6.7) HL, 2.9% (6.7) in InV-af; located near the mid-point of InV-af ( Fig. 4B ), InV-LO 46.9% (50.7%) InV-af, LO-an 17.9% (22.6) InV-af; anus large and rounded, surrounded by black naked skin, situated closer to the origin of anal fin than interventral line, InV-an 74.2% (83.1) InV-af; genital papilla located right behind the anus.

Coloration. When fresh ( Fig. 1 ), body dark brown, anterior trunk and head darker. Lips of upper and lower jaws black. Maxilla white, with a small, irregular but faint black blotch at the posterior end. Tongue and palatine lining dusky ( Fig. 3 ). Throat and branchiostegal membrane totally black. Barbel black (pale reddish-orange in paratype). Fin color of holotype: all fins dark reddish-brown. Dorsal, anal, and caudal fins with a faint but broad black margin. Fin color of paratype: all fins reddish-orange. Base of dorsal and anal fin darker and dusky. Caudal fin with a black edge.

When preserved ( Fig. 2 ), body and head overall black. Black pigment on lips and in oral cavity remains. Fin color of holotype: dorsal and anal fins dark brown, paler posteriorly. Second dorsal and anal fin with a darker margin. Pectoral fin black. Pelvic fin dusky, paler posteriorly. Caudal fin dusky, with a broad dark brown margin. Fin color of paratype: dorsal, anal, and pectoral fins dusky black near the fin base, pale white posteriorly, but with an indistinct black margin. Pelvic fin pale white. Caudal fin pale white with a black edge.

Etymology. The specific name megastomus means large-mouthed in ancient Greek which is the combination of “ mega ” (large) and “ stoma ” (mouth), referring to its unique large mouth among congeners.

Distribution and habitat. Known only from off northern and eastern Taiwan, based on the type specimens and Chiang et al. (2014) (see Remarks). The paratype (NMMB-P35679) was captured with hook-and-line by a fisherman over a sandy bottom at ca. 800 m depth. This species is assumed to inhabit deep sandy bottoms, down to ca. 800 m.

Comparisons. According to the keys to Physiculus provided by Paulin (1989) and Shcherbachev (1993), the diagnostic characters of Physiculus megastomus sp. nov. do not match any other species. Among its congeners, it is most similar to Physiculus nigrescens Smith & Radcliffe, 1912 [in Radcliffe, 1912], which occurs from the Philippines to Australia ( Paulin, 1989). They share a more posterior light organ (InV-LO>30% InV-af); a short continuous tubed lateral line, not extending beyond the origin of the second dorsal fin; and similar number of second dorsal-fin rays ( Table 2 ). However, P. megastomus can be readily distinguished from P. nigrescens in having a longer maxilla (15.3‒15.9% SL in P. megastomus vs. 13.5‒14.1% in P. nigrescens ; Table 1 ); light organ situated closer to the mid-point of InV-af (InV-LO 46.9‒50.7% InV-af vs. 39.3‒45.5%); more gill rakers on upper limb of first gill arch (5 vs. 3‒4); both jaws with 2‒3 rows of caniniform teeth (vs. villiform teeth on both jaws; Smith & Radcliffe, 1912).

Physiculus megastomus sp. nov. can be distinguished from most of its congeners by its 5 gill rakers on the upper limb of first gill arch, including rudiments (vs. usually 2‒4 in most congeners), except Physiculus caboverdensis González, Triay-Portella & Biscoito, 2018 , Physiculus marisrubri Brüss, 1986 , Physiculus maslowskii Trunov, 1991 , Physiculus nematopus Gilbert, 1890 , and Physiculus talarae Hildebrand & Barton, 1949 , which also possess 4‒5 rakers ( Table 2 ). However, P. megastomus can be distinguished from them by its relatively posteriorly situated light organ (InV-LO 46.9‒50.7% InV-af vs. 16.0‒36.9%). Among these, P. megastomus is closest to P. caboverdensis in having caniniform teeth on both jaws. Physiculus megastomus can be readily separated from P. caboverdensis in having relatively more second dorsal-fin and anal-fin rays (69‒74 and 74‒80 vs. 57‒67 and 63‒69 in P.caboverdensis , respectively); smaller light organ (its length 5.5‒6.7% InV-af vs. 6.8‒13.3%); and longer maxilla (15.3‒15.9% SL vs. 12.7‒14.6; Table 1 ).

Furthermore, the relatively large mouth and long upper jaw are diagnostic characters of P. megastomus and likely unique among congeners, the length of upper jaw (maxilla) 15.3‒15.9% SL and 57.8‒60.7% HL. However, only Physiculus coheni Paulin, 1989 has a maxilla length, 28.7% SL ( Paulin, 1989), exceeding that of P. megastomus . This proportion is misleading because, based on the original description ( Paulin, 1989), the holotype of P. coheni has lost the distal portion of its body and possesses a large regenerated caudal end. Therefore, it is not appropriate to standardize the maxilla length with its incomplete body length. The maxilla length was standardized by its head length instead. As a result, the maxilla length of P. coheni is 50.8% HL, which is distinctly shorter than that of P. megastomus (57.8‒60.7% HL).

Remarks. The holotype is a large female with ripe ovaries and the paratype is likely a subadult without developed gonads. They differ from each other significantly in several morphometric proportions, dentition, and appearance: pelvic fin in holotype extends slightly behind the anal-fin origin when appressed, its length 14.3% SL and 54.5% HL (vs. extends well behind the anal-fin origin, 22.1% SL and 83.6 % HL); barbel length 4.0% HL (vs. 10.5% HL); three rows of caniniform teeth on both jaws (vs. two rows); dorsal, anal, and pelvic fins are dark reddish-brown (vs. reddish-orange). Except for the aforementioned characters, other morphometric and meristic characters are consistent between the two type specimens ( Tables 1–2 ). We performed DNA barcoding analysis and constructed a phylogenetic tree ( Fig. 6 ) to further confirm the identification of the new species. The COI sequences of the two specimens are 100% identical and reveal a single lineage with 100% bootstrap support. With high similarities in morphological and molecular data, the aforementioned morphological variations are likely ontogenetic.

Molecular data reveal Physiculus megastomus is closest to an undescribed species from Taiwan, Physiculus sp. 3 (sensu Koeda & Ho, 2019). The two species are sister to each other with 100% bootstrap support ( Fig. 6 ). The genetic distance between the COI sequences of the two species is 3.3%, as calculated by the K2P model ( Kimura, 1980). More specimens and sequences are needed to confirm the taxonomic status of Physiculus sp. 3 . Therefore, Physiculus sp. 3 is not described herein.

Chiang et al. (2014: 46, fig. 3) labeled a photograph as Gadella jordani ( Böhlke & Mead, 1951) from eastern Taiwan. However, the specimens in their figure show the following characters: presence of a chin barbel; all fins reddish-orange; mouth large and maxilla extends well beyond the level of the posterior edge of orbit; rays of second dorsal and anal fins generally uniform in height. Their specimen was most likely P. megastomus sp. nov. The specimen was examined by Dr. H.-C. Ho, but it is badly damaged due to preservation (Ho, pers. comm.) and is not included in the present study.

Figures

FIGURE 1. Fresh coloration of Physiculus megastomus sp. nov. A. Holotype, NMMB-P35678, 307.2 mm SL, picture inverted from the right side of body. B. Paratype, NMMB-P35679, 143.6 mm SL. Scale bar = 2 cm.

FIGURE 2. Preserved condition of Physiculus megastomus sp. nov. A. Holotype, NMMB-P35678. B. Paratype, NMMB- P35679. Scale bar = 2 cm.

FIGURE 3. Black pigmentation in oral cavity of the holotype of Physiculus megastomus sp. nov. A. On palatine lining. B. On tongue.

FIGURE 4. Dorsal and ventral views of the anterior body of the holotype of Physiculus megastomus sp. nov. A. Predorsal region, an arrow indicates the origin of the dorsal fin. B. Ventral view of the head and abdomen, three arrows indicate the pelvicfin origin, light organ, and anal-fin origin, from left to right.

FIGURE 5. Dentition and gill arch of Physiculus megastomus sp. nov. A–B. Dentition of upper and lower jaws of the holotype, respectively. C–D. Dentition of upper and lower jaws of the paratype, respectively. E. The first right gill arch of the holotype, two arrows indicate the position of the uppermost and lowermost rakers of the upper and lower limbs. Scale bar = 2 mm (upperright corner of each picture).

FIGURE 6. The maximum likelihood phylogenetic tree constructed by the best-fitting Tamura-Nei + Γ + I substitution model, based on COI sequences of selected species of Physiculus from Taiwan. The tree with the highest log likelihood (-2383.42) is shown. Two undescribed species, Physiculus sp. 2 and sp. 3 (sensu Koeda & Ho, 2019), were included. Gadus morhua, L. rhacina, M. moro and S. australis were selected as outgroups. Bootstrap proportions below 50% not shown.