Abstract

During November–March, blooms of Kai algae genera are commonly seen on rocks and cobblestones in the Nan River, providing habitat for hydropsychid larvae. This study attempted to determine a dietary relationship between the caddisflies and Kai algae by comparing gut contents of hydropsychid larvae between areas with and without Kai algae (Kai-blooming and Control sites). Fourteen specimens of Hydropsyche and Potamyia larvae were collected in the Kai-blooming and Control sites, respectively. Food items in the foreguts were classified as Kai algae (KA), other filamentous algae (OFA), diatoms (DT) and other items (OI). Although the main food type of larvae in both sites was Kai algae, the proportion of KA in larval foreguts from Kai-blooming sites was significantly higher than in those from Control sites (p < 0.05). In addition, larvae in the Kai-blooming area had a significantly lower proportion of OI than in the Control area (p < 0.05). Therefore, hydropsychid larvae tended to consume more Kai algae than diatoms and other filamentous algae during blooming periods. They tended to consume more Kai algae than would be expected by chance alone, even in the area with a small amount of Kai algae.