Diplotaxodon dentatus Konings 2021, new species

Diplotaxodon dentatus, new species

Figs. 2, 3, 4; Table 1.

Holotype. SAIAB 209443 (male), 108.7 mm SL, collected by trawl off Thumbi East Island, Monkey Bay, Malaŵi, Africa at 73 m depth, J. R. Stauffer, 27 March 1985.

Paratypes. SAIAB 209441, 7, 96.9–119.7 mm SL, data as for holotype.

Diagnosis. The oblique mouth, a prognathic lower jaw, a small knob at the synthesis of the dentaries, the absence of distinct bars or stripes (Eccles & Trewavas, 1989), the closely-spaced teeth, and lack of beak-like premaxillaries (Trewavas, 1935; Turner, 1994) place this species in Diplotaxodon. Diplotaxodon dentatus has a smaller body depth (26.4–28.4 % SL) than D. altus (35.1–37.8% SL) (Stauffer et al., 2018), D. greenwoodi (34.2–36.5% SL) (Stauffer and McKaye, 1986), D. macrops (33.9–37.6 % SL), D. apogon (32.4–37.3 % SL), D. aeneus (34.3–37.1 % SL), and D. ecclesi (35.5 % SL) (Turner and Stauffer, 1998). There are more gill rakers on the first ceratobranchial in D. dentatus than in D. longimaxilla (23–27 vs. 16–21). Diplotaxodon dentatus has fewer teeth in the outer row of the left lower jaw (17–25) that are exposed (Fig. 3) when the mouth is closed and more widely spaced than those of D. limnothrissa (36–46), which are close together and embedded (Turner, 1994). Diplotaxodon dentatus has a shorter snout length (26.6–29.2 % HL) than D. argenteus (31.7–34.2 % HL), a longer post-orbital head length (39.0–41.7 vs. 37.7–38.8 % HL), a narrower cheek depth (10.6–15.2 vs. 15.3–17.0 % HL), generally more gill-rakers on the first ceratobranchial (23–27 vs. 20–23), and a greater distance between the anterior origin of the dorsal fin to the posterior insertion of the anal fin (50.3–53.1 vs. 49.6–51.2 % SL).

Description. Morphometric ratios and meristic data in Table 1. Body fusiform, slender, and laterally compressed, deepest at origin of dorsal fin (Figs. 2 & 3). Dorsal-fin origin posterior to operculum, above insertion of pectoral fin, dorsal-fin spines increasing to maximum length over 8-10 spines. Caudal peduncle relatively long, 4.8–5.6 times SL. Holotype with 16 abdominal and 18 caudal vertebrae (CT scan Fig. 3), typical of Diplotaxodon spp. Caudal fin emarginate. Anal-fin, below vertical through first dorsal-fin ray; anal-fin rayed section rounded not reaching to caudal-fin base. Pectoral-fin origin close behind gill slit; pectoral fin in mature males to anal-fin origin. Pelvic fin, short, and not to anal-fin origin. Ctenoid scales small, 32–38 in lateral line. Upper lateral line with slight upward curve anteriorly, to before posterior insertion dorsal fin; lower lateral line straight through mid-caudal peduncle; 0–1 pored scales posterior to hypural plate.

Head elongate (33.8–36.0 % SL), length greater than body depth, with lower jaw at oblique angle (Fig. 2 & 4). Eye large (horizontal eye diameter 29.9–33.9 % HL, vertical eye diameter 28.4–33.6 % HL), greater than pre-orbital depth (15.7–20.0 % HL). Snout smaller than orbit diameter, 26.6–29.2 % HL, with premaxillary pedicel to vertical through anterior orbit. Cheek shallow (10.6–15.2 % HL) with 2–3 scale rows. Mouth large and superior, lower jaw 38.0–41.2 % HL; teeth on lower and upper jaw caniniform, with space between them about width of tooth (17–25 in outer row of left lower jaw) and exposed (Fig. 4). Teeth on pharyngeal jaws unicuspid (Fig. 5). First ceratobranchial with 23–27 gill-rakers and first epibranchial with 6–8 rakers.

Coloration. Preserved males with dark snout. Flank dark dorsally, fading lighter ventrally. Dorsal fin with black lappets; caudal fin with 5–6 central rays black; anal fin with distal portion of membranes dark gray to black and without egg spots; pelvic fins black and pectoral fins clear. Head with black gular region. Coloration of preserved females similar to males, but lighter overall. Live coloration not recorded.

Remarks. In general appearance Diplotaxodon dentatus is most similar to D. argenteus and also shares the character of exposed teeth when the mouth is closed. When the morphometric and meristic data for D. dentatus were compared to that of the lectotype and paralectotypes of D. argenteus the first principal component (size variable) of the morphometric data explained 94.6% of the observed variance and the sheared second principal component explained 39.4% of the remaining 5.4%. Variables that had the highest loadings on the sheared second principal components of the morphometric data were distance between the posterior insertion of the dorsal fin and origin of the pelvic fin (0.55), distance between the anterior insertion of the dorsal fin and origin of the pelvic fin (-0.39), and caudal peduncle depth (0.33). The first principal component of the meristic data explained 45.8% of the variance. Variables with the highest loadings on the first principal components of the meristic data were number of gill-rakers on the ceratobranchial of the first arch (-0.56), pored scales posterior to the hypural plate (0.55), and number of gill-rakers on the epibranchial (-0.41). A plot of the second sheared principal component of the morphometric data versus the first principal component of the meristic data shows that D. dentatus is clearly separated from D. argenteus (Fig. 6).

Ct scans revealed that the stomachs of the type series were empty, however, based on the exposed teeth in the oral jaws and the unicuspid teeth of the pharyngeal jaws (Fig. 5) it is suggested that D. dentatus is a piscivore.

Etymology. Dentatus from Latin meaning toothed or having teeth alluding to the caniniform teeth on the outer margin of the dentary and premaxilla which are exposed when the mouth is closed.