J, Fish Biol. zyxwvuts (1976) 9, 67-74 Increased oxygen consumption in blennies (Blennius pholis L.) exposed to their mirror images zy P. WIRTZ* AND zyxwv J. DAVENPORT N.E.R.C. zyxwvuts Unit, Marine Science Laboratories, Menai Bridge, U.K. (Accepted zyxwv 11 May 1975) The sight of its mirrored image increases the oxygen consumption of Blennius pholis. This is due to an increased metabolic rate and an increased activity. For schooling species, zy the opposite effect has been reported in the literature and has been termed the ‘group effect’. The B. pholis response is discussed in relation to species specific preferences of the social environment. The hypothesis is put forward that deviations form the preferred social environment cause an activation of the individual. I. INTRODUCTION A number of studies have been concerned with the question of whether the energy expenditure, measured in terms of oxygen consumption, of a group of fish is different from the sum of the energy expenditures of the individuals, if the latter are isolated from each other, (Schuett, 1933; Shlaifer, 1938, 1939, 1940; Motwani zyx & Bose, 1957; Shtefan, 1958; Alekseyeva, 1959; Delco & Beyers, 1963; Ryzhkov, l968,197O).In all of these studies the oxygen consumption of the group was found to be lower than that of the sum of the isolated individuals, a phenomenon termed ‘the group effect’ by Schuett (1933). Ryzhkov (1968) was even able to demonstrate a group effect for fish egg respiration. For the eggs it would appear that a chemical factor causes the depression in energy expenditure, but in adult fish visual contact with a conspecific has been found to result in a group effect (Shlaifer, 1939; Delco & Beyers, 1963). These workers also demonstrated that the presentation of a mirror had the same effect as the sight of a conspecific. All of these investigations were conducted with naturally gregarious fish; in the experiments reported here we have used specimens of the blenny, Blemius pholis L. which is not a gregarious fish but occupies a home range in which it defends the space surrounding its position (Gibson, 1967a, 1968). 11. MATERIALS AND METHODS COLLECTION AND MAINTENANCE OF FISH Specimens of B. pholis were collected from intertidal pools near Aberffraw on the west coast of Anglesey, U.K., using the anaesthetic quinaldine (Gibson: 1967b). The fish used in these experiments were juveniles according to the classification of Qasim (1 957) and varied in weight from 1.63458 zyxwvu g. For at least 1 week before the start of experiments,the animals were kept in a large tank which contained inverted plastic cups for shelter. During this *Present address: Max-Planck Institut fiir Verhaltensphysiologie, D 8131 Seewiesen, Abteilung Wickler, W. Germany. 67