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Peritrichs usually settle on the external surface and gills of crustaceans. However, their physiological effect on the host has not been studied. Respiratory rate is a susceptible index for some factors that affect an organism. To test if ciliates attached to the crustacean gills have a physiological effect, we measured crustacean oxygen consumption in a closed system. Crayfish Cambarellus (Cambarellus) montezumae were collected in Lago Xochimilco in Mexico City and were isolated in chambers. Two peritrich species, Cothurnia variabilis (32.5%) and Epistylis gammari (67.5%), were observed on the gills. The metabolic rate was evaluated by oxygen consumption with a polarimetric oximeter. Statistical analyses demonstrated that a density of ciliates attached to the gills of the crayfish above ~50 individuals increased oxygen consumption with a positive correlation to epibiont density. We conclude that C. variabilis and E. gammari ciliate epibionts attached to the gill of the crayfish Cambarellus (Cambarellus) montezumae cause an increase in metabolic rate. [ABSTRACT FROM AUTHOR]