Sharks, as a group, have a diversity of eyes. Some sharks have tiny eyes; some sharks have notably large ones. Their pupils can be round like primates’, slit like cat eyes, or even crescent-shaped, depending on the species. Some species have additional anatomical features; some are more powerful than others; you get the idea.
The chondrichthyian eye has a light-sensitive retina populated by rod cells for dim lighting and cone cells for bright lighting packaged in a rigid, round eyeball. From there, features diverged between species for adaptations to different environments. For example: the pupil.
This is the most common form of pupil in extant sharks and vertebrates. Found in chimaeras, Gulper sharks and Kitefin sharks, to name a few, this dilated ocular feature is ideal for consistently dark conditions, like the deep sea. These pupils can still constrict to a near-pinhole aperture, much like our pupils shrink down in the brightest daylight. Other sharks with round pupils include Blacknose sharks, whose eyes can take up to 20 minutes to dilate.
Horizontal Slit Pupils
Bonnethead shark pupils are slit in a vertical orientation and can dilate in an impressive three minutes, indicating advanced rapid ocular response to variance in lighting. Research reports the pupil of the Scalloped Hammerhead shark (not pictured) to be nearly round with a slight horizontal slant, but not as pronounced as the pupil of the bonnethead shark.
Vertical Slit Pupils
Vertically slit pupils found in Whitetip reef sharks can constrict to a nearly closed position, but much more slowly than horizontal slit pupils. While research concludes that the vertical slit pupil in the Lemon shark is adapted for both brightly and dimly lighted regions, maximum dilation in Lemon sharks was tested to take up to 25 minutes, indicating “adaptation to a less dynamic habitat or nocturnal activity.”
Oblique Slit Pupils
This pupil style is similar to the vertical and horizontal slit pupils but with a slight slant to it. You’ll find this feature on many bottom-dwelling species such as Leopard sharks and Smallspotted catsharks, as well as some coral reef creatures like Nurse sharks, Brownbanded bamboo sharks and the Epaulette carpetshark. Even a few flat-bodied ambush predators like the Japanese angelshark and Pacific angelshark have these eyes.
In skates and rays, crescent-shaped pupils with several dimples called pupillary apertures provide advanced focus, contrast and resolution in bright conditions.