Discussion More Glossosoma larvae moved to the thin algal mat, the thick algal-mat, or the stone substrates than to the control plates, indicating that the movement of Glossosoma larvae was induced by the algal biomass on the experimental plates placed upstream from their starting location. The present results suggests that Glossosoma larvae recognized the presence of the microalgae biomass upstream and significantly moved toward it. Moreover, the attraction of the larvae to the thick and thin algal mats was similar to their attraction to freshly collected natural river stones, indicating that the algal mats on the plates can modulate the behavior of Glossos oma larvae in a way similar to that of the algae growing on natural stones. Glossosoma larvae movements in the field are limited to approximately 5 cm h 1 (Kuhara, Nakano & Miyasaka, 2001), and the distance between the starting point on the initial plate and the final location of the larvae on the experimental plate was typically greater than 5 cm (5-15 cm). There- fore, under the conditions of our experiments, about half of the Glossos larvae would not have had enough time to move to the algal mat on the experimental plates within the 2-h time period. our night observations showed that the percen- tage of Glossosoma larvae that moved to the thick algal mat plates was significantly higher than the percentage that moved to the thin algal mat plates. The AFDM of the thick and thin algal mats indicated that the abundance of microalgae was much higher in the thick algal mats than in the thin algal mats. Thus, Glossosoma larvae might recog nize and respond to the abundance of microalgae moving more frequently to algal mats with higher biomass, and this is for two main reasons for the phenomenon: First, in aquatic ecosystems, chemi- cal cues are thought to be more important in determining the behavior of organisms during darkness because the organisms presumably would