Melanopsin, a photopigment produced in a special class of light responsive retinal ganglion cells, regulates pupil constriction and circadian activity in mammals. Unexpectedly, zebrafish have five melanopsin genes that are expressed in different cell types throughout the retina, and whose functions are unknown. For example, as shown here, Opn4m2 is transcribed in horizontal cells that modulate photoreceptor signaling. Graduate student Vanessa Matos-Cruz is performing a collaborative project between the Halpern and Hattar (Johns Hopkins) laboratories to understand the diverse roles of melanopsin-related proteins in fish.