A notch in time

Native to the Indo-Pacific coral reefs, the alluring Korean angelfish (Pomacanthus semicirculatus) captivates us with its colourful designs and surprises us with its natural history. As the juvenile angelfish (seen on the cover) matures, its coloration will change to that of an adult so different in coloration that it was considered a different species before its natural history was understood. Angelfish prey on benthic invertebrates and perhaps some sponges, but both hunting its prey and avoidance of its own predators requires good vision. As a result, these rather intelligent reef fish have very interesting visual systems.

These generally predatorial fish require binocularity especially in their sometimes monotonous, three dimensional blue world. Most fish, including the angelfish, have their eyes positioned on the sides of their heads, making stereopsis difficult. Through subtle adaptations and countless generations, these successful fish have solved this problem in a most peculiar manner.

The fish eye probably originated approximately 450 million years ago just before the Devonian period, but truly came into its own during the Devonian. The vertebrate eye probably had its humble beginnings in the Cambrian …