One swellshark with a unique appetite

The Japanese swellshark or sometimes called the Blotchy swellshark, (Cephaloscyllium umbratile) is a common species of catshark, belonging to the family Scyliorhinidae. They are wide ranging in the Pacific Ocean. They may inhabit shallow waters down to 656 feet and favor rocky reefs. Like other members of its genus, the Japanese swellshark can inflate its abdomen with water or air in an effort to deter potential predators.


Family: Scyliorhinidae – Catsharks

Genus: Cephaloscyllium 

Species: umbratile


Phylum– Chordata

Class– Chondrichthyles


Common NameGround Sharks

Family– Scyliorhinidae

Common NameCatsharks




Average Size and Length: Mature males have been measured at 98 cm/3.2 feet. They can grow longer than 3.2 feet.

Current Rare Mythical Sightings: American ichthyologists David Starr Jordan and Henry Weed Fowler described the Japanese swellshark or the Blotchy swellshark in a 1903 volume of Proceedings of the United States National Museum, based on a 98 cm/3.2 feet long stuffed dry skin originally obtained from Nagasaki, Japan. They gave it the specific epithet umbratile (from the Latin umbratilis, meaning “shaded”) and assigned it to the genus Cephaloscyllium.

The dried skin of the holotype could not be located when shark expert Stewart Springer prepared his 1979 review of the catsharks, and in its absence, he synonymized C. umbratile with C. isabellum on the basis of “inconclusive morphometric differences”. Some authors followed Springer’s judgment while others, particularly in Japan, preferred to keep referring to C. umbratile. The taxonomy of this species was further confused by the application of the name C. umbratile to a similar but smaller species sharing part of its range. This second species, once referred to as “pseudo-umbratile” by Leonard Compagno, has since been identified as C. sarawakensis. Recently, the holotype was found again, and in 2008 Cephaloscyllium umbratile was re-described as distinct from C. isabellum by Jayna Schaaf-Da Silva and David Ebert. (Springer, S. (1979). “A revision of the catsharks, Family Scyliorhinidae.” NOAA Technical Report NMFS-Circ. 422: 1–152).

Teeth and Jaw: The mouth is large. The mouth forms a broad arch, and lacks furrows at the corners. The small teeth have a central cusp flanked by a smaller cusplet on both sides. There are around 59 tooth rows in the upper jaw and 62 tooth rows in the lower jaw.

Head: The Japanese swellshark differs from the Droughtsboard shark by having a longer snout. The snout is also rounded. The head is short, broad and flattened. There are large nostrils divided by short, triangular flaps of skin in front. The small, horizontally oval eyes are placed high on the head and equipped with rudimentary nictitating membranes. A tiny spiracle lies closely behind each eye.

Denticles: The skin is thick and sparsely covered by large, well-calcified dermal denticles; each denticle has a diamond-shaped crown with three horizontal ridges.

Tail: The caudal fin is large and broad, with the upper lobe longer than the lower and bearing a prominent ventral notch near the tip.

Demographic, Distribution, Habitat, Environment and Range: The Japanese swellshark can be found in the western Pacific Ocean in Japan. They are wide ranging through even China and Taiwan. Little is known about their habitat, but research suggests that they may inhabit shallow water, as well as could possibly be found at depths between 295-656 feet. They are benthic in nature and favor rocky reefs.

Diet: The Japanese swellshark or Blotchy swellshark is opportunistic in its feeding habits, and is known to consume numerous types of fishes and invertebrates, including an unusually high diversity of cartilaginous fishes. One recorded shark had more than 10, 20 cm fish and 15, 15 cm squid in her stomach at one time.

Predominantly piscivorous, the Japanese swellshark is known to prey upon hagfish and at least 50 species of bony fishes, including fast-swimming types that inhabit open water; significant prey species include the mackerel Scomber japonicus, the sardine Sardinops melanostictus, the filefish Thamnaconus modestus, and the hakeling Physiculus japonicus, this is quite unique for a shark this size. Also, unusual, the Japanese swellshark also feeds on at least 10 species of cartilaginous fishes, including lantern sharks, catsharks (particularly the cloudy catshark, Scyliorhinus torazame, and its eggs), the electric ray Narke japonica, and skates (including their eggs).

The Japanese swellshark also cannibalizes smaller members of its own species.

Horie, T. & S. Tanaka (2002). “Reproduction and food habits of the Japanese swellshark, Cephaloscyllium umbratile (family Scyliorhinidae) in Suruga Bay, Japan“. Journal of the School of Marine Science and Technology, Tokai University. 53: 89–109

Aesthetic Identification: The Japanese swellshark is similar, and formally synonymous with the Droughtsboard shark; however, the Japanese swellshark has a more mottled pattern. They have a pale brown dorsal surface with dense dark brown mottling and regular saddles separated by lighter reddish-brown areas. There are scattered small white and dark brown spots. The ventral side is lighter and mostly unspotted. The dorsal and caudal fins are mottled and spotted. The pectoral and pelvic fins are mottled and spotted above and light below. The anal fin is fin is mottled. Behind the spiracle are five pairs of gill slits, which are short and become progressively smaller posteriorly. Like their other family members, the Japanese swellshark has an inflatable stomach. It has a thick body. The second dorsal fin is much smaller than the first dorsal fin. The dorsal fins have rounded apexes and are positioned far back past the pelvic fins. The pectoral fins are moderately large and wide. The anal fin is nearly as large as the first dorsal fin and placed slightly ahead of the second dorsal fin.

Biology and Reproduction: They are oviparous. The species is thought to be relatively prolific, as the ovary contains numerous ova at various stages of development. Females lay egg cases two at a time. Adult females have a single functional ovary, on the right, and two functional oviducts. There is no well-defined breeding season and reproduction occurs year-round. The eggs hatch after approximately one year.

Females have been documented producing eggs even after years without male contact, suggesting that they may be able to store sperm. (Masuda, M.; S. Kametsuta & M. Teshima (1992). “Female swell sharks (Cephaloscyllium umbratile) producing fertile eggs after a long separation from male sharks“. Journal of Japanese Association of Zoological Gardens and Aquariums. 34 (1): 1–3.)

Males and females attain sexual maturity at the size of 86–96 cm/2.8-3.1 feet and 92–104 cm/3-3.4 feet. After they reach maturity, the growth rate is low after they reach sexual maturity.

When the embryo is 11 cm/4.3 inches long, the external gills have been lost, the dermal denticles have begun to develop, and light brown saddles are present. Newly emerged sharks measure 16–22 cm/6.3–8.7 inches long. Hatchling sharks grew in length by up to 0.77 mm (0.03 in) per day. (Tanaka, S. (1990). “Age and growth studies on the calcified structures of newborn sharks in laboratory aquaria using tetracycline.” NOAA Technical Report NMFS 90: 189–202.)

Each egg case is shaped like a purse, and are relatively large and thick, measuring around 12 cm/4.7 inches long and 7 cm/2.8 inches across. The capsule surface is smooth with lengthwise striations, and opaque cream in color with yellow margins. Long, coiled tendrils extend from the four corners of the capsule.

Known parasites of the Japanese swellshark include the nematode Porrocaecum cephaloscyllii, and the leech Stibarobdella macrothela.

Behavioral Traits, Sensing and Intelligence:  The Japanese swellshark is capable of rapidly inflating its body with water or air (on the surface) as a defense against predators. This will also allow the shark to wedge itself inside a rocky crevice, becoming extremely difficult to remove.

Japanese Swellshark Future and Conservation: There is not enough data to evaluate. They do seem to do well in captivity. It is caught as bycatch in commercial bottom trawls, though its population does not seem to have suffered from fishing activity.

They adapt readily to captivity and has reproduced in public aquariums.

Japanese Swellshark Recorded Attacks on Humans: Not a threat to humans.