Common names
Japanese Eagle Ray.
Binomial
Myliobatis tobijei
Synonyms
Mobula tobijei, Myliobatis tobije.
Identification
A relatively small eagle ray with a very wide kite-shaped disc and large protruding head. Disc width 1.6-1.8 x length. Rostral lobe short, fleshy, broadly rounded. Eyes slightly anterior to pectoral fin bases. Small hornlike knob present above eyes on adult males. Spiracles large, positioned laterally, barely visible from above. Nasal curtain short and wide with a long fringe along posterior margin, without deep central notch.
Pectoral fins have mildly convex anterior margins, concave posterior margins, tightly rounded or angular apices and angular free rear tips. Disc entirely smooth, lacking denticles or thorns. Pelvic fins large and long, extending well beyond disc margin. One small dorsal fin with a broadly rounded apex and a short free rear tip, positioned well back on tail, close to caudal sting. Tail long, tapering to caudal sting then filamentous to tip. Tail length 1.1-1.5 x disc width when intact. 1-2 caudal stings usually present.
Colour
Dorsum yellowish to olive-brown, often with irregular dark brown blotches. Ventrum white with dusky or dark posterior disc margin and pectoral apices.
Size
Maximum disc width 102cm. Disc width at birth 20cm.
Conservation Status
VULNERABLE
The Japanese eagle ray is taken as bycatch in numerous ground fisheries throughout its range including Trawl, gillnet and set net fisheries. In Ariake Bay, Japan, it has been intentionally reduced by 94% due to a predator control program. Although no species specific catch data exists, the overall Japanese eagle ray population is suspected to have been reduced by 30-49% in the last two decades alone.
Habitat
Subtropical to temperate seas. Primarily inshore in sandy or muddy bays. Usually shallower than 60m but recorded at 333m in Japan.
Distribution
Northwest Pacific. From Sakhalin Island in Russia to central China including Korea and Japan.
Reproduction
Viviparous. 1-10 pups per litter.
Diet
The Japanese eagle ray consumes a varied diet of crabs, clams, and demersal small fishes.
Behavior
In Japan, Japanese eagle rays are sometimes seen resting in shallow bays during the day. They are most commonly seen during the northern summer months implying a north south or perhaps vertical migration.
Reaction to divers
Mostly shy around scuba divers but Japanese eagle rays sometimes ignore divers unless closely approached.
Diving logistics
Both coastlines of central Izu are reputedly good spots to see Japanese eagle rays in the summer.
I found at least 4 of these rays in March while shore diving at Inatori; a small village north of Shimoda on the east side of the Izu Peninsula.
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Similar species
Purple Eagle Ray – Myliobatis hamlyni Distinguished by uniformly purplish-brown to olive-brown dorsum. Possibly sympatric with M tobijei at the northern extent of its range.