Trophic ecology of two benthivorous fishes in relation to drift and benthos composition in a pristine Serra do Mar stream (Rio de Janeiro, Brazil)

The objective was to explore how and when prey availability in the form of drift and benthos are exploited by two phylo-genetically distant but morphologically similar fish species namely Characidium vidali and Pimelodella lateristriga, the only two benthivorous species co-occurring in a pristine Serra do Mar stream (Southeastern Brazil). We simultaneously quantified drift, benthos and the food of the two fish species. Benthos was quantified once a day whereas drift and fish diets were quantified over diel cycles (six times per day) in December 2006 and February, April, July and October 2007. The major components of drift and benthos were Simuliidae, Chironomidae and Baetidae with an additional contribution of Elmidae to benthos. Drift and benthos components showed little differences in size across samples with 2-4 mm long invertebrates representing = 80% of the total. Both drift rates and benthos densities were surprisingly low and showed practically no difference between day and night (drift) across months. Pimelodella fed more actively during the night whereas Characidium fed more actively during the day but the two species fed essentially on the dominant, small sized Simuliidae, Chironomidae and Baetidae. However, Pimelodella showed a markedly broader feeding niche including a higher prey diversity (i. e., 21-30 taxa across months relative to only 12-15 taxa fed upon by Characidium). Apparently, eco-morphological constraints and species-specific foraging behaviors eventually induce species-specific feeding patterns and as a consequence, feeding overlap and therefore potential competition between these two species even if benthos density was low, appeared weak all year round.