Japanese Bullhead Shark, Japanese Horn Shark.
Stout body. Large square head with prominent ridges above eyes. Two very high dorsal fins, each with a prominent spine at its origin. First dorsal fin very high with a pointed or acutely rounded apex. First dorsal origin slightly anterior to pectoral fin insertion. Second dorsal fin origin level with pelvic fin insertion. Pectoral fins much larger than first dorsal. Pelvic fins about the same size as second dorsal fin. Relatively small anal fin. Large, triangular caudal fin with deep subterminal notch.
Dorsal coloration tan or dark brown, with a subtle, thin, pale, vertical bars or saddles. Fins may be dusky in areas but mostly unmarked.
Maximum length 120cm. Size at birth approx. 18cm.
warm-temperate and sub-tropical seas. Prefers rocky coral reefs and areas of kelp from 6-37m.
Northwestern Pacific. The Japanese horn shark is found around Japan, Korea, northern China, and Taiwan.
The Japanese bullhead shark is probably of little interest to fisheries, but is caught as bycatch by gillnet fisheries (Tanaka 2006) and possibly other fisheries in its range (Compagno 2001, S. Tanaka and K. Nakaya pers. obs. 2007). In northern Japan, and possibly elsewhere, its inshore habitat is threatened by marine pollution and coralline flats. Algae are declining in abundance, which will affect this species’ prey items; Batillus, top shells sea urchins, and in turn may impact populations of H. japonicus (S. Tanaka pers. obs. 2007).
It is also a very popular aquarium species in Japan (S. Tanaka and K. Nakaya pers. obs. 2007).
Citations and References
Tanaka, S. & Nakaya, K. 2009. Heterodontus japonicus. The IUCN Red List of Threatened Species 2009: e.T161714A5486896. https://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T161714A5486896.en. Downloaded on 30 November 2020.
Oviparous. Females lay pairs of eggs among rocks or kelp at depths of 8-9 m between March and September; mainly March to April in Japan. In one year, 6-12 spawnings may occur.
Feeds on molluscs, small fishes and sea urchins.
Hunts nocturnally. Female Japanese bullhead sharks are known to nest in small groups.
Reaction to divers
Easy to approach, remaining completely motionless unless molested, at which point Japanese bullhead sharks usually retreat deeper under cover or slowly swim away to a quieter resting spot.
In Japan, this species is regularly spotted off the south Coast of Honshu Island. I have seen numerous Japanese bullhead sharks at dive sites in Chiba and on the Izu Peninsula. Likely, it is relatively easy to find all around Honshu where diving is organized.
Big Fish Expeditions runs Japanese Shark Diving Expeditions for English speaking divers to various dive sites around Honshu each year. So far, bullhead sharks have always been encountered. Other species seen include hundreds of banded houndsharks, Japanese wobbegongs, Japanese angelsharks, and numerous other sharks and rays.
Although the Japanese bullhead shark is also recorded from China and Korea, visibility along the Pacific coast of mainland Asia is notoriously bad so these areas are rarely dived.