Why are shark's eyes on the sides of their heads?
Posted by Odd_Blueberry_8570@reddit | marinebiology | View on Reddit | 15 comments
If I'm remembering correctly, predators usually have eyes on the front of their head/face(like falcons), and prey have eyes on the side(like deer). So considering that they're predators, why do some sharks have eyes on the side of their face?
EquivalentSpot8292@reddit
All fish do other than a few benthic species… they need a more 360 degree view of their environment. Similar to birds.
stargatedalek2@reddit
Tuna have forward facing eyes, as do most macropredatory birds. The reason sharks don't have forward facing eyes is that they aren't relying on vision to hunt with.
IndividualText4931@reddit
I have filleted thousands of tuna and never once saw one with forward facing eyes.
stargatedalek2@reddit
An animals eyes don't need to be on a flattened forward surface to be forward facing. They can still be on the sides of the skull but point forwards.
You can see what I mean if you look at tuna from the front.
https://www.freedive.net/yellowfin/yellowfin4.htm
EquivalentSpot8292@reddit
No, they do not amigo.
stargatedalek2@reddit
An animals eyes don't need to be on a flattened forward surface to be forward facing. They can still be on the sides of the skull but point forwards.
You can see it if you look at tuna from the front.
https://www.freedive.net/yellowfin/yellowfin4.htm
It's even more obvious with birds of prey.
https://images.squarespace-cdn.com/content/v1/55525830e4b0ac0977512301/1593708342640-94W3C1UVSCYYT7KUNR2H/Lussky-IMG_9773.jpg?format=750w
https://www.shetzers.com/bald-eagle-cry-head-on/
https://www.pexels.com/photo/head-of-eagle-19131342/
Even vultures have forward facing eyes.
https://africageographic.com/wp-content/uploads/2019/09/vulture-Dewald-Kleynhans-Vulpro.jpg
wyrd_werks@reddit
Because they are fish
Fanged_Monster@reddit
It’s largely because of how light moves in the water and how hydrodynamics works. For many aquatic organisms their primary senses used for hunting are hearing, smell, and pressure sensitivity. Having eyes on either side of the head creates a slightly bigger blind spot but at the gain of being very very efficient at swimming and reducing drag. You also see this in cetaceans. There isn’t a lot of good in being able to see great if you can’t catch your food because you’re slower than they are
Sharks are no exception to this. The first sense they use to detect prey is their hearing (1km+ away), then they rely on smell and pressure with their lateral lines (~100m), followed by vision (~90m), and electroreception is the last sense and is good to ~1m depending on the species and concentration of ampullae. The distances these senses are good to vary depending on species, but generally speaking they don’t rely on their vision until they’re close enough to their prey to make a chase attempt
Predatory fishes that do have binocular or binocular-ish vision are cryptic ambush predators like toadfish, flatfish, and stonefish. They need to minimize blind spots and don’t really care about swimming quickly because their food comes to them. They need great vision and fast reaction times instead
pacondition@reddit
Side eye
marinebiology-ModTeam@reddit
Your post was removed as it violated rule #8: Responses to identification requests or questions must be an honest attempt at answering. This includes blatant misidentifications and overly-general/unhelpful identifications or answers.
stargatedalek2@reddit
This only applies to predators that primarily track prey by eyesight. Cats, hawks, frogs, Tyrannosaurus, etc. Sharks have very good electroreption and can sense prey that is directly in front of them with that, so they don't loose anything by having a blind spot directly in front of them.
atomfullerene@reddit
Because the "eyes in front = predators" is not really the general rule of biology that people make it out to be. You see something similar in a lot of grade school biology textbooks, which show an image like this comparing binocular vision in a predator and wide field of view in prey.
And that is something you can see in a decent fraction of ground dwelling mammals. The idea was further popularized (on reddit at least) by the webnovel "The Nature of Predators" where it was treated as an ironclad law of nature.
But it's not, really. More forward-facing eyes provide a greater range of binocular vision, which allows for better depth perception over a greater fraction of the field of view. Eyes placed further on the side of the head tend to allow for a greater field of view at the expense of a smaller area with good depth perception (though most animals do have some binocular overlap).
What this means is that animals which have a greater need for depth perception tend to have eyes more in the front, otherwise animals tend to have eyes positioned to provide a greater field of view. This means that the rule holds true for ground-dwelling animals and some birds, because most ground-dwelling herbivores don't need a large area of binocular vision to successfully graze on leaves, and because many carnivores that pounce (or swoop, in the case of birds of prey) onto moving targets do need more depth perception to judge the distance for their attacks.
But it's not a general rule for herbivore and carnivore, it's all about judging distance. Tree dwelling herbivores often have forward facing eyes and good depth perception because they need to judge distances between branches. And predators which don't need to judge distance don't need forward facing eyes.
Which finally brings us back to sharks. Sharks don't have forward facing eyes because they don't need to use binocular vision to judge the distance to their prey. Sharks can use pressure, scent, and electrical currents to sense prey right in front of them, so they don't need to actually see the prey to detect it in front of them. In fact, great whites famously roll their eyes back during an attack to protect them, and greenland shark adults are mostly blind due to eye parasites.
Cha0tic117@reddit
The whole side of the head vs forward facing mostly applies to terrestrial mammals. And even so, there are tons of exceptions, so it's not the best way of judging how an animal feeds.
Sharks have their eyes on the sides of their heads primarily for hydrodynamic reasons. Having a flat surface with forward-facing eyes would cause a lot of drag and make swimming difficult. This is true of most fish species, especially those with the standard fusiform shape.
This doesn't mean that sharks can't see in front of them, quite the contrary. Sharks have excellent binocular vision like most predators, and are able to see quite well. They do possess a "blind spot" directly in front of their nose, which only really affects their vision for things directly in front of them. However, as others have pointed out, sharks have excellent hearing, a great sense of smell, and can detect electricity with their ampullae, so having a small blind spot doesn't affect their ability to detect prey.
KeeblerElvis@reddit
They mostly track prey by smell.
jaydon-420@reddit
sharks are not the top of the food chain. hope this helps