The Small-headed Clingfish (Apleton dentatus) can grow to 4cm.
A very small flattened fish with a triangular head. A single dorsal fin at the rear of the body opposite the anal fin. Pelvic fins fused into a suction cup. Variable bright colours, often an olive green with a white band across the head when looking down from above and a stripe through the eye.
Two-spotted Clingfish (Diplecogaster bimaculata) has a broader head.
Small crustaceans (probably)
South and west of the British Isles. No BMLSS records from the east coast.
The Aquarium Project