The common blenny is a small, shallow water fish. It is identifiable by large, protruding eyes and wide lips, slimy skin with no scales and a long dorsal fin separated by a notch into two unequal sections. Between the first and second ray of the dorsal fin, there is an indistinct dark/red spot. The pectoral fins are fan shaped and transparent with visible green spines.  The colour of each individual varies from dark brown/greenish or blotched to blackish according to their habitat. Males can change almost completely black during mating season. 

This fish is most commonly found in rockpools during low tides. If a common blenny cannot locate a rockpool, it will often take refuge hiding in damp patches under stones or exposed seaweed. In these conditions, the common blenny can survive for many hours out of water. It can also breathe air during this exposure. Once the tide returns, the fish will emerge to feed on barnacles, small crabs and other crustaceans. 

It has been reported that some common blennies display homing behaviour when it comes to rockpools. These individuals have been seen to return to the same rockpool during each low tide. The common blenny can live up to 15 years, however, it is more likely to live about 5 years.