Oarfishes, representing the family Regalecidae, are poorly known from the tropical Indian Ocean and are of exceptional interest in terms of their distribution. This study presents the first detailed morphological description of Regalecus russellii (Cuvier, 1816) based on the single specimen (2580 mm total length) collected from Sri Lankan waters on 27 October 2021. The morphological characters of the presently reported specimen well matched the key identification features of R. russellii reported from the eastern and western Pacific Ocean, and the Atlantic Ocean. This report of R. russellii from the Indian Ocean provides vital information on their biology and distribution.

Diversity, Indian Ocean, mesopelagic fish, morphological description, Regalecidae

Oarfishes, representing the family Regalecidae, are well-known sea creatures connected with numerous mythological beliefs, but a fish family is little known about their biology and distribution. The family Regalecidae is characterized by an extremely slender, laterally compressed, and very elongated ribbon-like body and is recognized for being the longest bony fish in the world (Olney 2002; Roberts 2012). The family is classified into two genera, Regalecus Ascanius, 1772, and Agrostichthys Phillipps, 1924, and there are three known species i.e., Agrostichthys parkeri (Benham, 1904), Regalecus glesne Ascanius, 1772, and Regalecus russellii (Cuvier, 1816). Species of Regalecus are oceanic fishes that occur from surface waters to greater depths in temperature and tropical waters, and are rare in nature (Wiley et al. 1998; Roberts 2012, 2017; Lee et al. 2023). The limited sightings, incomplete notes on sightings, limited number of voucher specimens, misidentifications or uncertain identifications, and incomplete specimens lead to inadequate information on the diversity, taxonomy, ecology, and distribution of oarfish (Regalecus) species (Wiley et al. 1998; Roberts 2012; Lee et al. 2023).

Two species of Regalecus occur in all three oceans, Pacific, Atlantic, and Indian. In the Pacific Ocean, two species show contrasting distribution, where numerous records of R. russellii were documented from the North Pacific and all the records of R. glesne were concentrated in to the South, except one R. russellii record from northern New Guinea. In contrast, such a pattern is not evident in the Atlantic Ocean. While R. glesne has been documented exclusively in the North Atlantic and Mediterranean, both species are known to occur in the South Atlantic (Psomadakis et al. 2008; Roberts 2012; Feeney and Lea 2018; Lee et al. 2023). Moreover, two species exhibit distinct habitat preferences related to water temperature and ocean currents: R. glesne is typically associated with colder waters, whereas R. russellii is commonly found in warmer regions (Schmitter-Soto 2008; Roberts 2012). Regalecus russellii was originally described as Gymnetrus russelii Cuvier, 1816 from Vizagapatam (Visakhapatnam), India from the Bay of Bengal (Fricke et al. 2025); however, the reports are few and rare from the Indian Ocean. Few records of Regalecus are scattered over the Indian Ocean and only two specimens have been identified to species level as R. russellii (see Roberts 2012). The limited observations of Regalecus from the Indian Ocean highlight the importance of the current report. In the present study, we report the first record of R. russellii from Sri Lankan waters of the northern tropical Indian Ocean, along with a detailed morphological analysis of the specimen.

A single, fresh, complete specimen of Regalecus russellii (2580 mm total length) (Fig. 1) was caught by the surface tuna gillnet operated by multiday fishing boat from the off western coastal waters (6°32′14″N, 79°25′16″E) of Sri Lanka on 27 October 2021 and landed at Beruwala Fishery Harbor. The specimen was preserved in the 5% formaldehyde and deposited in the National Faunal Repository, Colombo National Museum, Sri Lanka (Repository Number: 2024.05.01NH).

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).

The morphological methodologies follow Psomadakis et al. (2008) and morphometric measurements were collected to 1 mm using calibrated tape and Vernier caliper. The specimen weight was measured to the nearest 1 g using a digital weighing balance.

The specimen was radiographed by dividing it into six overlapping sections using a static X-ray machine (Dongmun-DIG 650 Single Phases) (Fig. 2) at the Faculty of Veterinary Medicine and Animal Science, University of Peradeniya. The X-radiograph images of all six sections were processed to create the full image using image processing software (Photoshop). The X-radiograph images were used to count the vertebrae and dorsal count and fin rays arrangement of the specimen.

Figure 2. 

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

This study presents the first comprehensive report on Regalecus russellii from the Indian Ocean and provides an account of oarfishes reported from the region. The first documentation of Regalecus species was recorded from Visakhapatnam on the eastern coast of India from Eastern Indian Ocean in 1788 by Patrick Russell (1726–1805) (Russell 1803); this juvenile specimen drawing served for the description of Gymnetrus russelii in 1816. However, Russell’s specimen was not retained after drawing. The drawing and description of the specimen were inadequate to identify species beyond the genus level. There have been many Regalecus species records from the Indian Ocean, although no single specimen has been accepted to use as a neotype for R. russellii from the Indian Ocean (Roberts 2012).

To date, only two confirmed R. russellii specimens have been recorded from the main part of Indian Ocean. There are no confirmed records of R. glesne from the region, with the exception of Gymnetrus hawkenii Bloch 1795, described from Goa on the Indian coast of Arabian Sea. This species record is currently synonymized with Regalecus glesne Ascanius, 1772. However, spawning grounds of both species of Regalecus occur in extreme western end of Indian Ocean, on the southern coast of South Africa, and R. glesne in the eastern end of the Indian Ocean, on the coast of Western Australia (Roberts 2012). According to Roberts (2012) the adults and early life stages of Regalecus from south coast of South Africa could be part of populations from the South Atlantic.

Seasonal patterns in Regalecus strandings and accidental caught records have been observed in a few areas, such as Gulf of Mexico coast of Florida; however, these events have not been clearly linked to environmental factors. In contrast, stranding/ accidental catch reports from Australian waters have shown a clear association with storm events. The few records of Regalecus from the Indian Ocean are inadequate to observe any patterns of stranding/ accidental catch but the species recorded are closely associated with upwelling zones of the Indian Ocean (Roberts 2012).

The association of Regalecus with upwelling zones is probably due to the surface cold water rich in biological productivity in these areas which can serve as a feeding ground with optimum environmental conditions close to surface waters (Barber 2001). The present specimen was found from shallow waters off the western coastal waters of Sri Lanka, overlapped with the coastal upwelling region (Yapa 2009; De Vos et al. 2014). There were number of mesopelagic fish species records including family: Trachipteridae from western and southern coastal upwelling zones of Sri Lankan waters during the summer monsoon (Krakstad et al. 2018; Rathnasuriya et al. 2019; Pers. comm. U. Liyanage).

Most records of Regalecus species are accidentally caught in fishing gears during their operations and are often discarded without proper documentation (Roberts 2012). There have been many reports of oarfish (Regalecus spp.) landings by fishermen at various sites along the Indian and Sri Lankan coastal waters. Many of these accounts have not been published in peer-reviewed publications or made publicly accessible to the scientific community (Pers. comm. K.V. Akhilesh; Pers. comm. U. Liyanage). Advancing technology, along with the widespread accessibility of cameras and smart phone cameras, have significantly improved the frequency of recordings in recent years within the Indian Ocean region. Citizen science programs and knowledge dissemination platforms (e.g., social media, apps, websites) play a crucial role in enhancing public awareness and encouraging the recording of rare or poorly known marine species (Dickinson et al. 2012; Kosmala et al. 2016). Such programs are essential to understand the marine biodiversity and to support conservation strategies, particularly in understudied tropical waters.

We are grateful to fishermen who collected and preserved the specimen. We sincerely thank Ms. Rifdha Riswan for her assistance in collecting, transporting, and supporting the study. The authors also express their gratitude to Mr. Lenin De Silva and the staff of the Faculty of Veterinary Medicine and Animal Science, University of Peradeniya for their support in radiographing the specimen. We thank Mr. S.S. Gunasekara for his assistance in developing the map used in this study.