Vision

Fishes will often move their eyes to place images on such ‘areas’, the shape and location of which vary and sample visual space most effectively for a given species. The adnexa comprises eyelids, conjunctiva, extraocular muscles and the lacrimal system. Eyes with high sensitivity have enlarged/plentiful rods connected to few ganglion cells, large pupils, reflective tapeta and short focal-length lenses. Closer to the coast, water is often green, and much freshwater appears red, as longer wavelengths are preferentially transmitted but over much shorter distances. The photoreceptor inner and outer segments of some turbid freshwater and deep-sea teleosts are grouped into bundles, usually surrounded by reflective crystals forming a ‘tapetal cup’. Colour vision in fishes was first shown behaviourally over a century ago in minnows; it has more recently been demonstrated in a variety of other marine and freshwater species with multiple cone pigments.