Visualization of the chemical defense molecule formoside binding to sensory structures in a model fish predator.

Sensory perception of chemical threats coming from an organism's environment relies on the coordination of numerous receptors and cell types. In many cases, the physiological processes responsible for driving behavioral responses to chemical cues are poorly understood. Here, we investigated the physiological response of fish to an unpalatable compound, formoside, which is employed as a chemical defense by marine sponges. Construction of fluorescent probe derivatives of formoside allowed visualization of this chemical defense molecule in vivo, interacting with the cells and tissues of the early larvae of a model predator, the zebrafish (Danio rerio). This revealed the precise chemosensory structures targeted by formoside to be in the taste buds and olfactory epithelium of developing zebrafish. Mechanosensory neuromasts were also targeted. This study supports the involvement of a previously identified co-receptor in detection of the chemical defense and provides a springboard for the long-term goal of identification of the cellular receptor of formoside. Extension of this approach to other predators and chemical defenses may provide insight into common mechanisms of chemoreception by predators as well as common strategies of chemical defense employed by prey.