Perception and Evolution

The freshwater eel is notable not only for great sushi, but also for chromophore substitution (Beatty, 1984; Lythgoe, 1991). As young adults these eels spend most of their time in fresh water and, like many other freshwater creatures, have a longwavelength visual pigment known as porphyropsin, with a maximal sensitivity at 522 nanometers. As the time draws near for the first leg of their breeding migration, which takes them into coastal waters, the porphyropsin changes to a rhodopsin with maximal sensitivity at 500 nanometers. And as the time draws near for the last leg of their breeding migration, which takes the eels into the deep sea, the rhodopsin changes its maximal sensitivity to 487 nanometers, a value typical for many deepsea creatures. This is but one of many engaging examples of the adaptation, both phylogenetic and ontogenetic, of perceptual systems to environments. The variety of adaptations in vision alone is remarkable. The optical systems used include: pigmented pits without lenses or mirrors, found in some platyhelminthes, protochordates, coelenterates, annelids, and molluscs (Salvini-Plawen & Maxx, 1977); multiple pigmented tubes, found in some tube-worms; spherical lens eyes, found in fishes, some molluscs, alciopid annelids, and copepod crustaceans (Pumphrey, 1961); corneal refraction, found in many terrestrial vertebrates and arachnids (Land, 1985); apposition compound eyes, found in many diurnal insects and crustacea; refracting superposition compound eyes, found in nocturnal insects and some crustacea (Exner, 1891; Kunze, 1979; Nilsson, 1989); simple mirror eyes,