Abstract
Ecology of plant or animal communities requires knowledge of the biotope structure of these organisms. In the case of parasitic communities, the organs of a host constitute heterogeneous biotopes. Fish gills are an example of this, but their heterogeneity is not often considered. The gills of the mullet Liza ramada are such a biotope for several ectoparasites. Parasite density is an important factor in the study of fish-parasite interactions, but cannot be determined if the colonized surface area of the gills is not known. The number of primary filaments, and therefore the surface area potentially colonized, is subject to variation with age. Assessment of the colonizable area raises specific problems of estimation. A new method taking into account surface areas of primary filaments and using image-processing techniques is proposed. Models related to the increase of filament number and colonizable gill area as functions of the fish fork length are proposed. The increase of primary lamellae number with fish length can be fitted by an exponential-type model and the accompanying increase in size of the colonizable gill area by a polynomial-type model.