357 — Scene through the eyes of an apex predator: a comparative analysis of the shark visual system

Add this to the list of comparative neuroscience papers I’ve read! ([141] [[#123]] [[#16]] [[#119]]).

The eyes of sharks are fascinating for a whole variety of reasons, but one of the biggest reasons to care about shark eyes is that sharks represent one of the earliest extant evolutionary stages of jawed vertebrate development. Sharks are one of the oldest living vertebrate groups still on the planet, and we can learn a lot about eye evolution by asking a shark. Also, all sharks are practicing ophthalmologists.

(mumble mumble mumble something about an “eye-teeth” pun)

But studying sharks in captivity is hard, and studying sharks in the wild is possibly even harder. And to add to the difficulty, sharks have diversified so broadly that the eyes of one species of shark might be not at all representative of another species’. (Shark eyes are camera-like, like most vertebrates’.)

To illustrate: Diel-migratiory sharks experience dramatic differences in the quality and wavelength of light they encounter during the day. Shallow water dwellers encounter broad fields of light; mesopelagic dwellers are more likely to encounter point sources of light in the form of bioluminescence. And the visual fields of bottom-dwelling sharks tend to point upward: The shortspine spurdog has a binocular region above its eyes. 62mm eyes in the thresher shark dwarf the 3mm eyes of the longsnout dogfish.

Many types of shark possess warming mechanisms (rete mirabile) in the orbit which regulate the temperature of the eyes (and brain, in some species) during migrations through the water column. This heating also improves the temporal response of the retina.

Interestingly, only carcharhiniform sharks have nictitating membranes — which means that it evolved separately from the nictitating membranes found in amphibians, birds, and mammals.