Macrognathus pentophthalmos

Pethiyagoda, Rohan, Silva, Anjana, Maduwage, Kalana & Kariyawasam, Lalith, 2008, The Sri Lankan spiny eel, Macrognathus pentophthalmos (Teleostei: Mastacembelidae), and its enigmatic decline, Zootaxa 1931, pp. 37-48 : 38-43

Macrognathus pentophthalmos (Gronow, in Gray, 1854)

Figure 1

Mastacembelus pentophthalmos Gronow , in Gray, 1854: 172.

Neotype. NMSL FF 137 (174 mm SL) Sri Lanka, Kalu River at Modara Ela, Kalutara 06º35’N, 79º58’E, elevation 2 m (coll. P.E.P. Deraniyagala, 21.03.1927).

Other material. MCZ 166239 (195 mm SL) Sri Lanka (coll. P. E. Pieris, 1924). NMSL FF 196 (2 ex., 138.8 mm, 221.6 mm SL) Sri Lanka, Kelani River at Kelaniya 06º51’N, 80º33’E, elevation 2 m) (coll. P.E.P. Deraniyagala, 11.10.1926).

Diagnosis. Macrognatus pentophthalmos is distinguished from its closest congener, Macrognathus aral , as follows: dorsal spines 14–16 (vs. 18–22 in M. aral ); pre-dorsal length 43.3–46.8% of SL (vs. 35.5–40.8%); pre-anal length 60.0–64.6% of SL (vs. 53.7–58.4%); and pre-dorsal vertebrae 17–19 (vs. 12–16); and having the vertebra corresponding to the pterygiophore supporting the last dorsal spine in advance of that supporting the first anal spine (vs. vice versa in M. aral , see Fig. 2 A,C). Further, it is distinguished from M. guentheri , its only other Indian congener, by having 24 pairs of rostral tooth plates (vs. tooth plates absent); 14–16 (vs. 28– 30) dorsal spines; 50–52 (vs. 65–70) dorsal-fin rays; 46–47 (vs. 70–72) anal-fin rays; and a striped (vs. mottled) body coloration ( Fig. 3 ).

M. aral M. pentophthalmos M. guentheri Description. For general appearance see Fig. 1 . Morphometric and meristic data for three specimens are provided in Table 1 . Body elongate, elliptical in cross-section anteriorly, strongly compressed posteriorly. Head laterally compressed; snout acute; rostrum fleshy. Rostrum ventrally concave, lined with a series of 24 paired tooth-plates. Lower lip short. Anterior nostrils obscure (probably due to poor preservation); posterior nostrils small, elongate. Eyes small, flat. Mouth small, angle of gape 1½ eye-diameters anterior to posterior nostrils. Teeth numerous, small, pointed. Opercle scaly, its opening large, oriented ventrally, extending dorsally only to level of upper third of pectoral-fin base. Preopercle smooth, without spines.

Pectoral fin small, with 20–22 rays. Dorsal fin with 14–16 sharp spines (the penultimate one longest) and 50–52 rays, its origin in advance of anal fin. Anal fin with 3 spines and 46–48 rays. Caudal-fin rays 14 or 15. Caudal fin not confluent with dorsal and anal fins. Three anal-fin spines, the second one longest, the last almost subdermal.

Scales cycloid, minute, uniform. Lateral line complete, with about 220 scales, extending from above origin of opercular flap to base of caudal fin, about 24 scale rows above and 30 scale rows below it. Pre-caudal vertebrae 33–35; caudal vertebrae 37–39; pre-dorsal vertebrae 17–19.

Coloration: in preservative, background colour light brown, belly lighter than dorsum (specimens somewhat bleached), a faint whitish lateral stripe barely discernible in MCZ 166239 ( Fig. 1 A). A series of 4–9 dark-brown ocelli edged in white, subequal to eye, visible on base of dorsal fin. A smaller ocellus present at posterior base of anal fin in MCZ 166239 ( Fig. 1 A).

Deraniyagala (1932) provided the following detailed description of the coloration of this species: “Colours [not] subject to much variation in Ceylon. Dorsally a reddish brown or olive with a median light stripe and a yellow lateral band from eye to tail above the lateral line. An indistinct row of light spots between these two light bands, which fade with age. In some young there is a faint light band from pectoral to tail. Sides greenish yellow ventrally salmon pink, yellow or white. Single fins usually dusky brown, dorsal with three to nine yellow or orange rimmed ocelli along its base above which is a longitudinal yellow stripe. The number of ocelli is not influenced by age, sex or locality. Anal often a uniform orange, or yellow, or with a yellow longitudinal band occasionally with one or two ocelli on its base. Caudal with short bars and spots of yellow. Pectorals orange or yellow.”

Distribution. Macrognatus pentophthalmos has been recorded throughout the lowlands of Sri Lanka ( Fig. 4 ).

Figures

FIGURE 1. Lateral views of Macrognathus pentophthalmos. A, MCZ 166239, 195 mm SL (courtesy of Museum of Comparative Zoology, Harvard University). B, NMSL FF 137, 174 mm SL.

FIGURE 2. Radiographs of A, Macrognathus pentophthalmos, NMSL FF 137, 174 mm SL; B, M. aral, ZMB 1420 (lectotype), 214 mm SL (courtesy of Museum für Naturkunde, Berlin); C, M. aral, WHT 7510 (topotype), 154 mm SL.

FIGURE 3. Lateral views of Indian species of Macrognathus. A, M. aral, WHT 7510 (topotype), 154 mm SL. B, M. guentheri, WHT 7644, 156 mm SL.

FIGURE 4. Former distribution of Macrognathus pentophthalmos in Sri Lanka, based on Senanayake, 1980 (black dots); Willey, 1910 (green dot); Deraniyagala, 1932 (blue triangles); and material in the National Museum of Sri Lanka, Colombo (red dots), indicating also the 500 m and 1,000 m contours.

Tables

TABLE 1. Measurements (as a proportion of head length or standard length) and meristics of Macrognathus pentophthalmos (3 ex., NMSL FF 137, NMSL FF 196, 138.8 – 221.6 mm SL); M. aral (18 ex., WHT 7510, 130 – 181 mm SL); and M. guentheri (3 ex. 156 – 253 mm SL).

Measurements (% of HL)	Range Mean	SD	Range	Mean	Range	Mean
Eye diameter	10.3–13.8 11.7	1.1	9.9–11.0	10.7	8.9–9.9	9.4
Nostril-eye distance Internarial width	7. 7–10.4 8.7 5.8–7.3 6.7	0.9 0.4	8.2–9.7 6.3–6.7	9.1 6.5	7.5–9.8 7.9–8.9	8.4 8.6
Head depth	26.1–29.7 27.7	0.9	25.6–27.0	26.3	25.6–28.5	26.8
Extended head length (% of SL)	117.1–130.3 124.2	3.6	117.8–125.7	121.8	119.1–123.3	120.1
Head length	16.1–18.2 16.9	0.5	16.4–18.4	17.3	12.8–14.5	13.8
Total length Extended pre-dorsal length	106.1–107.7 106.8 31.1–38.0 34.4	0.5 1.9	106.8–108.6 39.2–43.5	107.7 41.0	105.9–110.6 23.3–26.3	107.5 24.5
Pre-dorsal length Post-dorsal length	35.5–40.8 38.3 56.8–63.3 60.0	1.7 1.7	43.3–46.8 51.5–58.4	45.0 54.6	20.7–23.6 74.3–79.5	21.7 76.5
Pre-anal length	53.7–58.4 55.8	1.3	60.0–64.6	61.9	48.6–54.5	52.5
Dorsal-fin breadth Anal-fin breadth	30.4–37.6 32.5 31.0–33.9 32.6	1.6 0.8	32.7–33.1 31.9–34.0	33.0 33.0	37.4–38.8 40.2–41.7	38.0 40.9
Pectoral-fin length	5.5–6.7 6.0	0.3	5.4–6.8	6.1	4.9–5.2	5.0
Caudal-fin length Meristics	6.8–8.0 7.3	0.3	6.9–8.4	7.6	5.4–6.8	6.3
Rostral toothplates	22–27		24		-
Dorsal spines Dorsal-fin rays	18–22 47–52		14–15 50–52		28–30 65–70
Anal spines	2–3		3		3
Anal-fin rays Pectoral-fin rays	47–57 17–20		46–47 20–21		70–72 20
Caudal-fin rays	14–15		14–15		12