Relative eye size and optical ratios of nocturnal and diurnal teleost reef fish. Panel (a) shows that nocturnal reef teleosts (black circles) have larger eye diameter for given body mass than diurnal species (yellow circles). Open circles are elopomorphs, which have very elongated bodies that make a comparison with body mass as independent variable impossible. The plot of the square of lens diameter against the product of eye diameter and axial length (OPTmorph1) reveals that nocturnal reef teleosts have a relatively large lens (b). Nocturnal fish have more circular pupils than diurnal fish (c), and if one combines the pupil ratio with OPTmorph1, cancelling out the ratio of lens diameter and axial length, a fairly clear delineation between groups of diel activity pattern emerges (d).