What anatomical feature assists fish in sensing dissolved substances in water?

The olfactory sac is the anatomical feature that plays a crucial role in helping fish sense dissolved substances in water. Fish rely heavily on their sense of smell, which is facilitated by the olfactory sac. This organ is equipped with sensory cells that detect chemical cues in the water, allowing fish to navigate their environment, find food, and communicate with other fish.

The olfactory sac is located in the nasal cavities of fish and serves as a highly sensitive detector of various chemical gradients in their aquatic surroundings. When water flows over the olfactory receptors in the sac, it triggers a response that allows the fish to 'taste' the chemicals present, informing them of the presence of prey, predators, or potential mates.

In contrast, gills are primarily responsible for gas exchange, the swim bladder is involved in buoyancy control, and the spinal cord is part of the central nervous system that transmits signals but does not play a direct role in the detection of dissolved substances. Understanding these functions highlights the specialized adaptation of the olfactory sac for chemical sensing, distinguishing it from the other structures listed.