We’re dedicating each Monday to a different thing that makes sharks just that much more awesome… without the dramatically infused headlines.
Sharks- and rays!- have an array of senses to help them go about their daily lives. They have to be at the top of their game… otherwise they wouldn’t have survived five mass extinction events. Their sensory organs can be placed into the five categories we know: sight, hearing, taste, touch, smell, and another two: electroreception and pressure changes.
Shark smell is something everybody talks about. “Oh, a shark can smell a drop of blood a gazillion miles away!” So… yeah… that may be a bit exaggerated.
Don’t get me wrong, they ARE super-sensitive to smells (i.e., potential prey dying from blood loss or potential mate giving off pheromones). They’re a heck of a lot more sensitive to smells than us mere humans and can locate prey just by smell from over hundreds of ft/m away. I mean, their brain’s olfactory lobes make up 2/3 of the total weight of the shark brain. Clearly, smell is a big deal.
How do sharks smell? They lack a nose like we do, but instead have nares (which are like nostrils) that can be found under their snout. Water goes in through the openings, and passes over sensory cells (olfactory lamellae) and then exit through the olfactory sac. These sensory cells are wrinkled and packed together tightly, so the most surface area is used, allowing for better interpretation of smells. Basically, dissolved particles in the water bind to the receptor cells and send a chemosensory signal.
Sharks smell is directional, meaning that they use their regular S-shaped pattern of swimming to an advantage; if the smell comes from the right, the right nasal opening will receive the smell first, and the shark’s brain will connect the dots and come to the conclusion, “Oh! The smell is coming from the right. Go right.” And so on and so forth.
If the smell is lost, they will swim in an exaggerated S-shape to find it again. They continue this swimming- and moving the head from side to side - to hone in on precise locations, as well.
Shark eyes are more than just “bottomless pits of black,” like many horror movies claim. They’re more complex than that!
Many sharks rely heavily on their eye sight (i.e., great white shark chasing seal)… and many don’t use their eyes at all (i.e., hammerhead shark digging for stingray under the sand). It just depends on the lifestyle of the shark!
Shark eyes can’t focus, but some sharks can dilate or contract their pupils (kind of like us)… or have that already done and look like cats (which is jawsome). Some sharks have no cones for sensing color, but others do possess those rods and cones! The ratio of rod to cone cells in humans is 4:1, but this ratio varies in sharks. For example, the Spiny Dogfish has a ratio of 50:1, the Sandbar Shark 13:1 and the White Shark 4:1 (like us). However, a shark's ability to tell one color from another is still undecided.
Dr. Eugenie Clark once tested lemon sharks (trained to push a white panel lowered into their pen where they were rewarded with food) where the panel color was changed to yellow and the shark pretty much flipped out.
But, this may not be due to color but a change in brightness instead.
Still, almost all sharks possess excellent vision in low light conditions. Shout out to the tapetum lucidum; this is a layer of mirrored crystals behind the retina that reflect light back onto the retina and strengthen the image clarity.
Speaking of cool eye things, some shark species have a toughened layer known as the nictitating membrane that covers the eye in its entirety during feeding, basically rendering the shark blind. And still yet, some sharks, like the great white shark, actually stick their head out of the water. How good their eyes adapt to being above water is still unclear. (I can totally imagine hearing the shark mini screaming, “My eyes! My EYES!” though)
Touch can be divided into a few things: both physical and distant.
Basically, the shark brushing up against something and its multiple nerve endings interpreting that touch OR the lateral line sensing pressure, temperature, water currents, etc.
Touch can also include the barbells around the mouths of some sharks, used to look for prey under the sand. Sharks also touch through their teeth- the teeth are flexible, have a rotating axis and multiple nerve endings that are pressure sensitive. These test bites can sometimes be fatal to prey that is… well, unintended and not designed to be “checked out” by shark teeth… such as humans.
Lateral line: The lateral line system runs from head to caudal fin on a shark (seen below); it’s made up of sensory cells in jelly filled pores. This jelly conducts shock waves to the sensory cells. Vibrations (e.g., swimming) indicate whether there are animals in the area and if the animals in the area are sick or injured… aka potential food.
The pit organ is made up of two bigger-than-normal denticles that cover a small hole in the skin. At the bottom of said hole are sensory hair cells. Many sharks have more of these organs along their back, sides and lower jaw. We don’t really know what pit organs do… but they may sense things like temperature changes, water currents, etc.
Ampullae of Lorenzini
So, this is one of my favorite things to teach people. They can detect electrical fields. This is anything from muscle movement of a heart beating to the vibrations from your $20,000 underwater camera.
Sharks respond to erratic electrical signals (such as those from a wounded animal) more than those sedentary ones from a camera strobe light. This may be why those who are bitten by a shark may be repeatedly bitten.
These pores are separate from the lateral line, in that these pores are clustered around the snout of the shark. They are long canals filled with jelly that transports these signals to the brain. Some sharks have as many as 1500 around their snout while others may just have a few hundred.
Speaking of teeth and munching, this sense is also a chemosensory function. The shark has gustatory cells lining the mouth and throat that, once the chemical molecules from the object attaches itself to these cells, allows the brain to assess if the meal is a “yes” or “no” item. This sense isn’t as highly adapted as the others a shark possesses.
Shark ears are internal. No, really. Those holes behind the eyes of sharks and rays aren’t their ears, but spiracles, that help with breathing. Anyway, the shark ear is composed of three cartilage tubes, filled with fluid and hair cells that, when sound hits them, vibrate and is registered by the brain. The three tubes allow for sharks to sense the X, Y and Z axis of their environment, allowing them to balance (and, you know, not be upside down or something silly like that). The otoliths are actually in charge of that whole gravity and orientation component; the otolith is a small area in the ear that is made up of calcium carbonate.
By the way, sharks like low frequencies (like AC/DC songs apparently?) around 100 Hz. These frequencies are best produced by sick or wounded animals.
Can you guess what we'll cover next week?