The Mediterranean-Atlantic genus Gobius L. (type G. niger L.) is redefined with listing and comparison of the 14 species currently assigned to it. A G. auratus complex of species is discussed with special reference to the real identity of the former species originated by Risso. Features of a further new species, Gobius couchi, from Helford, Cornwall, and Lough Ine, Co. Cork, are compared with those of its nearest relatives, G. auratus Risso, 1810, G. luteus Kolombatovic, 1891, and another new offshore form from the English Channel.

A systematic description of G. couchi is given, including details of head lateral-line canals and sensory papillae, and also of the karyotype (Appendix). A summary of biology covers geographical distribution, habitat and associated intertidal fishes, diet, parasites, reproduction, age-composition, and size. The electrophoretic properties of selected functional proteins (haemoglobins, muscle-myogens, and lactate-dehydrogenase isozymes) are described and contrasted with those in other available neighbouring species (G. paganellus, G. niger, and Thorogobius ephippiatus). Alternative terminology for LDH patterns is discussed, and the role of protein studies considered for gobioid classification.

The specific status of G. couchi relative to the Mediterranean G. auratus is reviewed, and meristic divergence between the two compared with that found among widely separated populations of G. niger which exhibit less geographical variation in these criteria. Mechanisms for allopatric speciation are discussed, and it is concluded that ancestral populations of the two forms became parted possibly at the time of the Roman regression (corresponding to the Mindel glaciation) when the Mediterranean may have been closed-off from the Atlantic. Rates of evolution shown by European gobiids are considered in the light of Pleistocene and postglacial opportunities for reproductive isolation.