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January 25, 2021

Discovery of a Colossal “Yokozuna” Slickhead:
An Active-Swimming Top Predator in the Deep Waters of Suruga Bay

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.

Discovery of a colossal slickhead (Alepocephaliformes: Alepocephalidae): An active-swimming top predator in the deep waters of Suruga Bay, Japan
Yoshihiro Fujiwara1†*, Masaru Kawato1†, Jan Yde Poulsen2, Hitoshi Ida3, Yoshito Chikaraishi1,4, Naohiko Ohkouchi1, Kazumasa Oguri1, Shinpei Gotoh5, Genki Ozawa1,6, Sho Tanaka7, Masaki Miya8, Tetsuya Sado8, Katsunori Kimoto1, Takashi Toyofuku1, Shinji Tsuchida1
5.Tokyo University of Marine Science and Technology
6.TechnoSuruga Laboratory Co.,Ltd.
†Co-first author
*Corresponding author

[Supplemental information]

The ecological pyramid is a diagram showing the total biomass of organisms in the order of trophic position. In many ecosystems, this diagram takes the form of a pyramid, as the total biomass decreases from primary producers to top predators.
Top predators are animals that are not predated by other animals. They are, thus, at the top of the food chain of the ecosystem. Top predators include lions in the savanna and killer whales in the epipelagic zone.
This is a baited camera system placed on the ocean floor by dropping it. It is equipped with systems to measure environmental factors such as salinity, water temperature, and depth. It also has a current meter and can be used to obtain information such as the diversity and population densities of predators and scavengers that are attracted to the bait.

Figure 1. Map of survey area  : Survey location


Figure 2. Narcetes shonanmaruae  (a) Side-view (b) CT image of bone structure  (c) Side-view of whole body  (d) Head side-view  (e) Row of teeth in upper jaw  (f) Gill raker  (g) A swimming Narcetes shonanmaruae captured by the bait camera.


Figure 3. Nitrogen isotope ratios (δ15Ν) and trophic positions (TP) of deep-sea fishes sampled in Suruga Bay  ●: Nitrogen isotope ratios of glutamic acid  ○: Nitrogen isotope ratios of phenylalanine  ■: Trophic positions estimated based on nitrogen isotope ratios of glutamic acid and phenylalanine 1 and 2: Frilled shark (Chlamydoselachus anguineus) 3 and 4: Smalltooth sand tiger (Odontaspis ferox)  5: Pacific sleeper shark (Somniosus pacificus)  6 and 8: Goblin shark (Mitsukurina owstoni)  7 and 9: Sharpnose sevengill shark (Heptranchias perlo)  10 and 12: Bluntnose sixgill shark (Hexanchus griseus)  11 and 14: Roughskin dogfish (Centroscymnus owstonii)  13: False catshark (Pseudotriakis microdon)  15 and 16: Narcetes shonanmaruae


(For this study)
Yoshihiro Fujiwara, Principal Researcher, Research Institute for Global Change(RIGC), Marine Biodiversity and Environmental Assessment Research Center (BioEnv), Deep-Sea Biodiversity Research Group (DeepBio), JAMSTEC
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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