The blennies, one of the larger families of fishes, number about 300 species worldwide. In Britain’s cool climate there are only four true blennies – the shanny, tompot, butterfly and Montagu’s.
Most blennies are small. The largest of the four, the tompot, is 30cm (12in) long, with a weight of just over 5oz (140g). All have a long dorsal fin running from the head to the tail fin, the first half made of slender, flexible spines. They also have large pectoral fins, but their pelvic fins consist of two long finger-like rays on each side. Their jaws have a single row of fine teeth. Many blennies have fleshy, tentacle-like flaps of skin on the head, above the eyes.
Telling them apart
The British blennies are easy to tell apart from one another.
The tompot is a tubby little fish with w/A strong spines in the first part of the dorsal fin; above each eye it has a large fringed skin flap. The fish is reddish-brown with darker bars across its sides. The butterfly also has fringed skin flaps above its eyes and there is a smaller flap on the back in front of the dorsal fin. This species has a white-ringed black eye-spot on the dorsal fin (it looks similar to the eye-spot on many butterfly and moth wings). The dorsal fin is high, but deeply notched. Large and stout, like the tompot, the butterfly is greenish brown with five to seven dark bands down its sides.
Montagu’s is the smallest British species; it too has a skin flap on the head – running across the forehead between the eyes. The free edge is fringed, and a series of small tentacles runs back along the nape. The fish is greeny brown with small white or pale blue spots on the head and front of the body; the fringe on the head is yellow to orange in adult males.
The shanny, the most common of the four, is widely distributed all round the coast. It’s the only one without a skin flap.
All blennies are shallow-water fishes. The shanny and Montagu’s blenny are found on the shore in tide pools, and in the case of the shanny, under stones or overhanging weed. The shanny is not confined to rocky shores, although it is most common on them. It lives in pools on muddy and sandy shores alongside breakwaters, pier pilings or groynes – provided green filamentous algae is growing on the hard surfaces.
The tompot lives on rocky coasts, but is only rarely found above the low tide mark. It conceals itself in crevices or under ledges l-12m (3-40ft) below low tide levels.
The butterfly blenny, less confined to rocky areas, is quite common on shelly sea beds and on sand and mud, but it needs shelter to hide in – loose stones, empty mollusc shells or even man-made debris.
Blennies lay their eggs in rock crevices or on the underside of rocks. The male guards the eggs until they hatch, spending a lot of time fanning them with his fins to ensure they get enough oxygen for survival.
The size and shape of the blennies’ teeth and jaws suggest that the food they eat is mostly small. Shannies feed on barnacles, small crabs and sandhoppers. Young shannies and Montagu’s blennies of all ages nip off the appendages of barnacles when they are protruded from the protective shell. Tompot and butterfly blennies eat small crustaceans, worms and small fish.