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The Seashore

Dogfish (Scyliorhinus caniculus)

Dogfish (Scyliorhinus caniculus)

Mermaid's Purse: This is the egg sac for the dogfish and the young animal is visible inside.

Small shark with a short blunt nose. The skin is like sandpaper and mottled with dark spots. In fact it is sometimes known as the spotted dogfish. The adult can grow to a metre in length and lives on the bottom of the sea bed just off shore. On occasions they are washed up on the shore. It feeds on bottom dwelling invertebrates like molluscs and crabs. Between November and July it lays eggs amongst seaweeds. Known as Mermaid's Purses they are large bags with long strings at the corners that become entwined in the weed, anchoring them in the lower shore or sublittoral region.

Dogfish, like all the sharks and rays, do not have a bony skeleton but one made of the softer cartilage. The fins are muscular. The long tail gives propulsion. This is a heterocercal tail, as compared to the homocercal tail of most bony fish e.g. Blenny. This means that the Dogfish has a long lobe projecting back at the top of the tail, not present in the lower part. When swept from side to side it gives lift so while it is swimming it does not sink to the bottom. Bony fish may have a swim bladder, an air bag for buoyancy. Buoyancy is given to the Dogfish by yellow oils in the liver. The fins give stability and control diving. The skin of the Dogfish is covered by very tough placoid scales. These are like small teeth and their spine-like apex project backwards to give a superb protective coat like a heavy duty sandpaper. Male Dogfish have a long pair of claspers around the cloaca, the common exit for body waste and gametes. When he tries to mate he coils his body around the female until he can insert his claspers into her cloaca. The sharp placoid scales on the claspers point in the opposite direction to normal and these form a tight bond so that sperm can be ejected from one cloaca to the other. Dogfish feed on crustaceans by scooping them up in the jaws which hang down below the head. Sharp backward facing teeth grip the food, reminiscent of the placoid scales.

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