Effects of light cycle on circadian feeding activity and digestive physiology in Haliotis discus hannai

The abalone Haliotis discus hannai is a typical nocturnal animal. It generally feeds and moves at night but evades light by hiding in the shade during the day. However, the effects of different light cycles on the feeding physiology and potential molecular mechanisms responsible for such effects in H. discus hannai have not yet been reported. We examined the feeding patterns of abalones under different light (L)/dark (D) cycles (0 L:24D, 12 L:12D, 24 L:0D) using an infrared camera and analyzed the relationships between circadian digestive enzyme activities, gene expression levels of neuropeptide Y (NPY) and its receptor (NPYR), which can promote appetite, and feeding rhythms. Feeding in abalones, including searching for food and, food capture and intake, was examined by observation of videos. The percentage of abalones feeding under different light/dark cycles had two peaks, at 20:00?22:00 and 02:00?04:00. The ingestion rate was significantly higher in abalones maintained under 12 L:12D compared with 24 L:0D conditions. The percentage of feeding individuals and food intake were significantly lower in the ?daytime? compared with at ?night? irrespective of the light/dark cycle, suggesting the existence of an independent internal timing mechanism and the possible involvement of circadian clock genes in the regulation of periodic feeding activity in abalones. In abalones maintained under 12 L:12D and 24 L:0D cycles, protease, cellulase, and ?-amylase activities started to increase from 18:00, 2 h before the first peak of food intake. Expression levels of NPY peaked at 20:00 and expression levels of NPYR increased again at 04:00, suggesting that NPY and NPYR might be induced before and after the two peak periods of food intake in abalones, respectively. These findings demonstrate the feeding rhythm and ingestion rate of abalones under different light/dark cycles, as well as the relationships among digestive enzyme activities, gene expression levels of NPY and NPYR, and feeding activity, which in turn provides a useful reference for the development of optimal feeding strategies and improved food utilization in abalone aquaculture production.