Foraging behaviour of the common goby, Pomatoschistus microps was investigated in both the United Kingdom and Sweden, with the aim of establishing causes and consequences of prey choice and ontogenetic shifts in diet. Goby life-cycle could be clearly divided into two stages, where prey choice changed abruptly from meio- to macrofauna at a standard length of 30 mm. This diet-shift maximised net energy intake rates, as illustrated by a quantitatively validated optimal foraging model. Intrinsic mechanisms were of greater importance than extrinsic factors in driving this shift. Metabolism, the primary prey choice determinant, revealed canalised and predictable diet shifts in the face of variable prey availability. This was in strong contrast to the more usual determinants such as gape limitation or extrinsic factors, such as habitat shift, prey availability and predation risk. Post diet-shift gobies consumed a range of benthic macrofauna dependent on availability. This plasticity in prey choice suggested that foraging efficiency was at some level below that expected for specialist foragers. Translocation experiments provided support for the general assertion that learning and experience are mechanisms through which generalist foragers could improve their foraging efficiency. Ontogenetic changes in prey choice did not result in a trade-off between foraging efficiency and other ecological parameters, leading to a prediction, upheld by geometric morphometries, that there would be no change in morphology associated with this change in diet. Conditions precluding diet shifts, and the resulting consequences, were explored using mesocosm manipulations. Adult gobies prevented from feeding on macrofauna suffered reduced condition and fitness. Pomatoschistus microps is an ideal species for investigations into foraging behaviour and has provided valuable support for current foraging paradigms as well as novel insights into the causes and consequences of prey choice.

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