The Development of Form and Function in Fishes and the Question of Larval Adaptation

Variation in the Development of the Fish Retina

Barbara I. Evans and Howard I. Browman

doi: https://doi.org/10.47886/9781888569582.ch7

Abstract.—Although the basic structure of the vertebrate retina is similar across taxa, high variability in specific features of the fish retina reflects the differences in visual microhabitat of these species. The vertebrate retina is the first step in the neural integration of visual information. A great deal of retinal function can be inferred from structure, and as these relationships continue to be revealed, we are gaining new insights into how vision is integrated by the nervous system. Among fishes, the developmental rate and acquisition of retinal structures is highly variable. While some species develop all structures early in embryogenesis, others delay acquisition of the full adult retinal complement of cells until months after hatching. Given the tight relationship between structure and function, differences in the timing of retinogenesis have implications for the visionbased survival skills of the early life history stages and for the overall ecology and fitness of the species. Although much of the observed variation may be related to altricial versus precocial life history strategies, we suggest that protracted retinal development also reflects and separates the constraints imposed by the requirements of foraging and predator avoidance. As evidenced by a typically monochromatic all-cone retina, the eye of early fish larvae is adapted for efficient foraging in bright light. At later stages, an improved ability to identify the presence of predators is acquired via addition of rod photoreceptors for low light vision, as well as multiple cone spectral channels (and regularly geometric cone mosaics) for increased contrast and motion sensitivity. The larval retina of some species exhibits further specializations, such as the pure rod retina of the eel leptocephalus and the pure green cone retina of many marine teleosts. Overall, variation in the development of the teleost retina can be viewed as a continuum from very rapid to greatly delayed. The developmental trajectory of the visual system in any given species represents a product of evolutionary history, developmental constraints, and foraging and predation pressures.