ONTOGENIC CHANGE IN THE DIET OF APLODACTYLUS-PUNCTATUS (PISCES, APLODACTYLIDAE) - AN ECOPHYSIOLOGICAL EXPLANATION

Aplodactylus punctatus is a temperate herbivorous fish that changes from an omnivorous to a herbivorous diet and increases its ability to assimilate algae as it grows. To investigate whether this dietary shift is related to size-specific differences in energetic demands imposed by metabolism and the amount of assimilated energy, oxygen consumption (V(O2)) was determined experimentally in 12 specimens ranging in size from 62 to 545 g. V(O2) increased allometrically with body size from 8.41 to 55.95 mg O2 individual-1 h-1. Individual energetic requirements were 2.8 to 33.7 kJ d-1. The assimilated energy was estimated, taking into consideration: (1) the energetic value of the most important alga in the diet (Lessonia trabeculata); (2) size-specific differences in assimilation rates for fish fed on this alga; (3) size-specific differences in throughput time and in the amount of food in a full gut. Comparison of the energy required and the assimilated energy revealed that fishes of < 22 to 29 cm total length had a negative energetic balance when consuming algae exclusively. This may explain the reliance of small A. punctatus on more easily-digested invertebrates. The largest individuals can meet their energetic demands by consuming algae alone, apparently because of their higher assimilation capability. In A. punctatus, changing energetic requirements and capacities for algal assimilation may be responsible for the observed ontogenetic change in diet.