Olfaction, the sense of smell, is crucial for animal survival, especially in localizing prey or food. However, little is known about the fish behavioral responses to specific food odors and which olfactory receptor might be activated by the food odors. Here, we examined the evolutionary relationships and classified the subfamilies of olfactory receptors (ORs), and conducted behavioral and gene expression analysis in grass carp (Ctenopharyngodon idellus). We identified 196 intact ORs in the grass carp genome and classified them into 8 subfamilies: β, δ, ε, η, γ, κ, θ, and ζ. 12 representative ORs (4 β, 4 δ, and 4 ε) were highly expressed in the olfactory epithelium (OE) of 24-month-old grass carp. Behavioral exploration found that grass carp showed different behaviors to different food odors at a series of concentrations. Specifically, 0.01 μmol/L amino acid mixtures and 0.005 × Hydrilla verticillata extracts strongly changed the preference index and swimming trajectories in 6-month-old grass carp. In addition, the two food odors stimulation differentially induced ORs expression in three tissues, especially strong in the olfactory bulb and brain. Altogether, our results suggest that olfaction plays an important role in the regulation of fish feeding behavior and provide a basis for further studies on the function and specific recognition of olfactory receptors in fish.
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