Regalecus russellii (Cuvier, 1816)

Rathnasuriya, Mahadurage I. G. & Hapuarachchi, Thennakoon H. I. S., 2025, First record of oarfish, Regalecus russellii (Actinopterygii, Lampriformes, Regalecidae), from Sri Lankan waters, Indian Ocean, Acta Ichthyologica et Piscatoria 55, pp. 145-150 : 145-150

Regalecus russellii (Cuvier, 1816)

1 , 2 , 3 ; Table 1

Specimen examined.

2024.05.01 NH , 2580 mm TL, off western coastal waters of Sri Lanka (6°32′14″N, 79°25′16″E) (Fig. 3 ), 27 October 2021 .

Description.

Total length 2580 mm. Other measurements, counts, and proportional measurements in Table 1 . Dorsal fin with 337 total soft dorsal rays; two dorsal crests (1 crest: 5 rays united by fin membranes; 2 crest: single ray free from other rays); preanus dorsal fin rays 77; pectoral fin rays 12; pelvic fin rays 1 stout, damaged; non-anal fin rays; caudal fin rays not evident; number of gill rakers on first gill arch 55 (12 + 43), elongated gill rakers with minute ridges projecting on the inner and outer margin (12 ridges on one side of longest gill raker); body long, deeply compressed, ribbon-like, tapered towards tail, without scales but covered with dermal tubercles with circular bases, either with or without conical or blunt point, tubercles are more prominent towards tail end and lower sides of body (especially lower edge of tail). Anal fin absent. Head small and head length 9.1 % in TL. Head profile almost vertical prior to dorsal fin origin. Maximum body depth about 68.1 % in head length. Mouth small, vertical, protractible and teeth absent. Single nostril presents on each side of snout. Dorsal fin origin well anterior to orbit; first five dorsal rays of first dorsal crest elongated (first four dorsal rays approximately 18 % in TL, fifth dorsal ray approximately 12 % in TL) and a single dorsal ray of second dorsal crest and subsequent rays relatively short; cristophore presents and supports to the first dorsal fin crest. Pectoral fins lanceolate, present low as horizontal line of anterior lower margin of preopercle, origin anterior to pelvic fin and horizontally oriented. Esophageal caecum ( Roberts 2017) extends towards end of body, and its body length 37 % in TL.

Coloration in fresh specimen.

The body of the fresh specimen is silvery-grey with amorphous oblique dark streaks and blotches distributed over the body. These dark streaks and blotches are distinct in the first third of the body. Forehead, mouth, preopercle, opercle (except upper front edge corner), and lower margin of head almost up to 16 % of body length, greyish black. Upper body margin and lower margin of body with crimson red luster especially in the first half of the body. Pupil dark and iris light. Dorsal fin and ventral fin crimson red, pectoral fin color less (Dorsal fin crest and ventral fin entirely dark crimson red color) (Fig. 1 A ).

Coloration when preserved.

The body of preserved specimen is yellowish brown with amorphous oblique dark streaks and blotches distributed over body. Forehead, front margin of mouth, and ventral region of head greyish black. Upper body margin beneath the dorsal fin yellowish-brown with darker in color. Dorsal, pectoral and ventral fin colorless (Fig. 1 B ).

Figures

Figure 1. Regalecus russellii (A) fresh specimen (B) preserved specimen from the off western coastal waters of Sri Lanka on 27 October 2021 (2580 mm TL).

Figure 2. Radiograph of Regalecus russellii, off western coastal waters of Sri Lanka on 27 October 2021 (2580 mm TL).

Figure 3. Map of Regalecus species recorded from the Indian Ocean.