Retina - Vertebrate and Cephalopod Retina Differences

Vertebrate and Cephalopod Retina Differences

The vertebrate retina is inverted in the sense that the light sensing cells sit at the back side of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones. By contrast, the cephalopod retina has the photoreceptors at the front side of the retina, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot.

The cephalopod retina does not originate as an outgrowth of the brain, as the vertebrate one does. It is arguable that this difference shows that vertebrate and cephalopod eyes are not homologous but have evolved separately.

In 2009 Kröger anatomically showed in Zebrafish that though the inverted arrangement is nonadaptive in that it creates avoidable scattering of light (and thus loss of light and image blur), it has space-saving advantages for small-eyed animals in which there is a minimal vitreous body, as the space between the lens and the photoreceptors’ light-sensitive outer segments is completely filled with retinal cells.

The difference between vertebrate and cephalopod retinas presents an interesting puzzle of evolutionary path which is not yet fully settled. From an evolutionary perspective, a convoluted structure such as the inverted retina can generally come about as a consequence of two alternative processes; (a) an advantageous "good" compromise between competing functional limitations, or (b) as a historical maladaptive relic of the convoluted path of organ evolution and transformation. Vision is an important adaptation in higher vertebrates. Therefore, if the retina is indeed "wired wrongly" or "badly designed" (from an optical engineering point of view) then it is sensible to look for it to possibly have some very significant physiological advantage. One such suggestion is based on the argument that the mammalian photoreceptor amplification process requires vast quantities of metabolic energy, and consequently, it requires massive and homogeneous supply of blood. Indeed, a unique network of blood vessels is well adapted to provide the photoreceptor layer with copious quantities of blood. This shows that the inverted retina is an adaptation to deliver abundant quantities of oxygen to the retina commensurate with its high energy demands and with good maintenance by the retinal pigment epithelial (RPE) cells against photo-oxidative damage, which, while on the face of it is exacerbated by the oxygen-rich blood in the choroid, is none-the-less eliminated by the process of opsin disc recycling it enables. This latter effect allows the photoreceptor cells to have a long (i.e. decades) useful life. The cephalopods have a non-inverted retina which is comparable in resolving power to the eyes of many vertebrates, however, the photoreceptors are not maintained, and this forces all invertebrates to either have a short life (of a few years) in a photopic environment or spend most of their lives in darkness. A third possibility, of having easily replaced stalk-eyes (some lobsters) or retinae (some spiders, such as Deinopis ) is rare.