Preference by restricted habitat in the gills of fishes is a well-known
phenomenon among ectoparasites, especially copepods and monogenea. Factors
that could explain this preference are not yet clear. An approach to the
solution of this problem has been proposed by J Paling, who showed that the
spatial distribution of glochidia is determined by the differential action
of water flow through gill arch. A similar conclusion was reached by J Davey.
When microhabitat preference is studied in species with similar ecological
requirements, ecological instead of mechanical phenomenon are apparent. The
presence of a single species of Monogenea in the gill filaments of the
teleost Cheilodactylus variegatus Valenciennes, 1833 allows us to
test the hypothesis of habitat restriction caused by ecological processes.