Description
The body and head of the Cornish jack are elongated; the head is nearly twice as long as high, smooth and depressed in front. The snout is rounded and almost as wide as the head. The mouth is terminal, with the upper jaw slightly longer than the lower, both bearing a single row of small, pointed teeth. The eyes are very small and placed in the front third of the head. The gill openings are small and inclined at an angle. The dorsal and anal fins are set well back on the body; the caudal fin is relatively small and forked with rounded lobes. There are 21–30 fin rays in the dorsal and 38–51 rays in the anal fin. The anal fin of the male differs from that of the female in having longer rays and a pronounced concavity in the anterior half.
The scales are small, numbering 85–100 in the lateral line. The coloration is gray above, lighter silvery white below, often with a bronze or yellow sheen. The juveniles are darker in color, being grayish blue or brown. The meristic characteristics of the body (such as the number of scales, fin rays, and vertebrae) vary by geographic location. The largest members of the family Mormyridae, Cornish jacks attain a maximum known length of 1.5 m and a weight of 15 kg.
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