Studies based on functional approach seek to understand the ecological roles developed by species as well as their interactions with the environment in which they are inserted. The hypothesis tested was that functional richness and diversity of molluscan community will be higher at the most complex macroalgal habitat. The study was carried out at Casqueira river estuary (Rio Grande do Norte, Brazil). Three species of macroalgae were collected—Gracilaria domingensis (Kützing) Sonder ex Dickie 1874, Gracilaria cuneata Areschoug 1854, and Solieria filiformis (Kützing) P.W.Gabrielson 1985—and the composition of seven functional traits of the mollusk fauna associated to the algae was characterized in 22 categories. The highest values of functional richness and diversity were for macroalgae with greater habitat complexity (G. domingensis). Some functional traits were influenced more by macroalgal morphology, like 'life way,' feeding strategy, body size, and larval development. Thus, we show that the greatest richness and functional diversity of the communities is related to sites with more complex habitat, sites with more shelter and refuge, and food. This highlights the importance of the quality of habitat for shellfish communities and shows that it can be assessed from the use of a functional approach. [ABSTRACT FROM AUTHOR]