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Within tropical environments, short-term impacts of increased seawater temperature and pCO(2) on algae-herbivore interactions remain poorly understood. We investigated the isolated and combined 7-day effects of increased temperature (+ 4 degrees C) and pCO(2) (similar to 1000 mu atm) on the trophic interaction Ulva sp./Trochus histrio, by assessing: i) topshells' survival and condition index; ii) grazer consumption rates, nutritional composition and interaction strength expressed as a dynamic index. No survival differences were observed whilst body condition varied significantly. Topshells under high pCO(2) displayed poor performance, concomitant with lower consumption of macroalgae. Individuals exposed to increased temperature had better physical condition, thus stimulating herbivory, which in turn was negatively correlated with carbon and nitrogen contents. The dynamic index was temperature- and pCO(2)(-) interactively dependent, suggesting lower grazing pressure under single acidification. Despite some limitations inherent to a short-term exposure, this study provides new insights to accurately predict tropical species' phenotypic responses in a changing ocean.