Which structure in fish detects chemical signals in the water?

The structure in fish that detects chemical signals in the water is primarily the olfactory bulb, which is part of the olfactory system. Fish utilize their sense of smell to detect a wide range of chemical cues in their environment, such as food particles, predators, and potential mates. The olfactory bulbs receive signals from the olfactory receptors located in the nasal cavity, and these receptors are highly sensitive to dissolved chemicals in the water.

In contrast, the lateral line system is responsible for detecting vibrations and changes in water pressure, which helps fish sense their surroundings and navigate. The swim bladder serves primarily a buoyancy function and is not involved in chemical detection. Gills, while they are essential for the respiration of oxygen, do not play a role in sensing chemical signals in the water. Thus, the olfactory bulb is the key structure that enables fish to detect and respond to chemical stimuli in their aquatic environment.