The bullhead is the only freshwater member of the family Cottidae that is native to the UK. It is a small fish, with a large mouth (4), large pectoral fins, prominent eyes (5) and a wide flattened head; hence the common names 'bullhead' and 'miller's thumb' (4). They are brownish in colour with mottling or barring, and pale undersides (6). During the spawning period males become black in colour with a white-tipped dorsal fin, and females become plump (4).