1. Key points

◆

Researchers discover a new species, the largest of the family Alepocephalidae.

◆

A very high trophic position reveals the species a top predator in the deep waters of Suruga Bay.

◆

The diversity and roles of top predators in the bathyal zone urgently call for elucidation, since they influence the ecosystems and are vulnerable to environmental change.

2. Overview

Principal Researcher Dr. Yoshihiro Fujiwara, Research Technician Mr. Masaru Kawato, and other members of the Deep-Sea Biodiversity Research Group of the Marine Biodiversity and Environmental Assessment Research Center of JAMSTEC’s Research Institute for Global Change conducted a survey in the deep waters of Suruga Bay using Shonan maru, a training vessel owned by the Kanagawa Prefectural Marine Science High School (Figure 1). The researchers discovered Narcetes shonanmaruae (Figure 2), a new species of the family Alepocephalidae. This species was colossal for a slickhead, so it was given the common name “yokozuna iwashi” in Japanese after yokozuna, the title given to sumo wrestling champions.

This species was the largest of the Alepocephalidae family reported to date, as its total length and weight reached as much as about 140 cm and 25 kg. The researchers closely examined the morphologies of four individuals caught in Suruga Bay at depths of 2,171 m and deeper. They identified the fish as a new species, based on the combination of its number of vertebrae and scale lines in addition to characteristics such as the positional relation of the dorsal and anal fin, the developed rows of teeth on the upper and lower jaws, the relatively small head and eyes, and a large mouth. Further, the results of mitochondrial genome analysis indicated that this species was different from all fishes of the family Alepocephalidae whose gene sequences had been registered.

Alepocephalidae are generally deep-sea fishes that primarily feed on gelatinous planktons. However, the researchers determined that this species was piscivorous, based on traces of fish that were found in the contents of its stomach, in addition to the fact that it was caught using fish as bait. Moreover, the results of amino acid nitrogen stable isotope analysis (Figure 3) showed that this species had the highest known trophic position (in the ecological pyramid (*1)) in Suruga Bay, making it a so-called “top predator” (*2). In addition, a bait camera (*3) placed 2,572 m deep in the ocean was able to acquire footage of the fish swimming energetically. The enormous size and the high trophic position of the fish are possibly related to its wide-opening mouth and its ability to scavenge carrion, in addition to being a piscivore.

Top predators, such as this species, are believed to be extremely vulnerable to overfishing and environmental change. In addition, they generally have strong influences on their respective ecosystems. Changes in the number of individuals can significantly alter the structure and functions of the ecosystem. Therefore, for the purpose of conserving ecosystems and using them sustainably, it is urgent that we grasp the diversity of top predators in deep oceans and their roles in the ecosystem.

These results will be published on 25th January 2021 (Japan time) in Scientific Reports issued by the British science journal.

Title：

Discovery of a colossal slickhead (Alepocephaliformes: Alepocephalidae): An active-swimming top predator in the deep waters of Suruga Bay, Japan

Authors：

Yoshihiro Fujiwara^1†*, Masaru Kawato^1†, Jan Yde Poulsen², Hitoshi Ida³, Yoshito Chikaraishi^1,4, Naohiko Ohkouchi¹, Kazumasa Oguri¹, Shinpei Gotoh⁵, Genki Ozawa^1,6, Sho Tanaka⁷, Masaki Miya⁸, Tetsuya Sado⁸, Katsunori Kimoto¹, Takashi Toyofuku¹, Shinji Tsuchida¹

Affiliations：

1.JAMSTEC
2.AUSTRALIAN MUSEUM
3.KITASATO UNIVERSITY
4.HOKKAIDO UNIVERSITY
5.Tokyo University of Marine Science and Technology
6.TechnoSuruga Laboratory Co.,Ltd.
7.TOKAI UNIVERSITY
8.NATURAL HISTORY MUSEUM AND INSTITUTE, CHIBA

†Co-first author
*Corresponding author