Developmental shifts in functional morphology of the retina in Atlantic tarpon, Megalops atlanticus (Elopomorpha: Teleostei) between four ecologically distinct life-history stages

The Atlantic tarpon, Megalops atlanticus, is a large piscivorous fish that supports economically important recreational fisheries in the Gulf of Mexico, Caribbean, and Florida Atlantic coast. Megalops atlanticus undergoes ontogenetic shifts in morphology, hatching in the open ocean as larvae (less than 1 cm in length), moving into hypoxic turbid mangrove marshes as juveniles (around 10 cm in length), and then moving into coastal oceanic waters as adults (over 100 cm in length). In this study, photoreceptor distributions, opsin distributions, and photoreceptor absorbance characteristics were studied with light microscopy, transmission electron microscopy, antiopsin immunofluorescence, and microspectrophotometry, respectively, at four ecologically distinct life-history stages-premetamorphic larva, settlement stage, juvenile, and adult. The purposes of this study were 1) to determine the extent to which the retina of M. atlanticus changes over the course of development and 2) to relate these retinal changes with ecological shifts between developmental stages. The new data presented here indicate that the M. atlanticus retina changes substantially in rod and cone distributions and absorbance characteristics over the course of development and that these changes correlate closely with those in habitat and behavior. We show that M. atlanticus has a rod-dominated retina at the larval stage (which is unusual for teleost larvae) and that the scotopic visual system becomes far better developed with maturity, adding a substantial tapetum and high densities of small, bundled, and stacked rod cells. We also show that there are shifts in cone and rod spectral sensitivities and an increase in the diversity of spectrally distinct cone classes, including the addition of ultraviolet cones as fish mature into adults.