Retinal topography in reef teleosts. II. Some species with prominent horizontal streaks and high-density areae.

The retinal ganglion cell layer of five species of reef teleosts was studied from Nissl-stained whole-mounts and the distribution of neural elements determined quantitatively. Iso-density contour maps of neurons in the ganglion cell layer revealed a temporal area centralis (ranging from 3.5 to 8.3 x 10(4) cells/mm2) which often extended into a horizontal streak (ranging from 1.4 to 5.0 x 10(4) cells/mm2) across the retinal meridian. Species possessing a marked horizontal streak were found to inhabit open water and perceive their environment with an uninterrupted view of sand-water horizon. The behavioural significance of these horizontal areas of acute vision is also discussed.