Coming up next on Jonathan Bird’s Blue World, Jonathan explores the fascinating biology
of slates and rays!
Hi I’m Jonathan Bird and welcome to my world!
You know me…I love sharks!
It doesn’t matter if it’s a big shark like the Great White or whale shark, or if
it’s a small one like the white tip reef shark.
Sharks are cool!
Skates and rays are the evolutionary cousins of sharks—they are contained within the
class Chondrichthyes along with the sharks, but they are in their own sub-division known
as the Batoids.
The Batoids share many characteristics with sharks, such as the cartilaginous skeleton,
the lack of a swim bladder and multiple gill slits.
In fact, they are so similar that you almost could think of them as flat sharks.
But there are some flat sharks.
The Wobbegong shark, for example, actually looks more like a ray than a shark, because
it’s flattened and tends to lie on the bottom.
So how do you tell a ray from a flat shark?
First of all, skates and rays are always flat.
Biologists call them dorso-ventrally flattened.
But unlike a dorso-ventrally flattened shark, a skate or ray has wing-like pectoral fins
that are just an extension of its body, making it look a bit like a flying carpet.
Or maybe if you squint a little, a bat, hence the name “Batoids.”
Also, skates and rays have their gills on the underside of their bodies, while a shark—even
a flattened shark--has them on the side.
One of the most well-known rays is the stingray.
This is a Southern Stingray, common in the Caribbean.
It gets its name from its stinger—a sharp venomous spine at the base of the tail.
This powerful weapon is used to defend the animal from its most feared predator—sharks!
Wait a minute!
Sharks eating their own evolutionary cousins?
That is so uncivilized!
The Southern Stingray feeds on animals that live in the sand, like mollusks and worms.
The stingray’s mouth is on its underside, just like its gills.
It blows water into the sand to excavate around prey, then slurps it up with powerful suction.
Because it feeds in the sand, the Southern Stingray tends to swim close to the bottom,
though it certainly doesn’t have to.
But this poses a problem—with breathing!
A shark breathes by taking water in through its mouth and then sending it back out through
the gills to extract oxygen from the water.
However, since the stingray has its mouth on its underside and swims so close to the
bottom, you would think that the stingray would get a lot of sand into its gills.
And you would be right, except for one thing.
On top of the stingray’s head are a pair of openings called spiracles.
They are kind of like big nostrils.
Instead of breathing in through its mouth, the stingray breathes in through the spiracles
on top of its head and away from the sand.
Sharks have spiracles too, but they are really small on most sharks, and they can’t breathe
through them.
The only sharks that breathe through their spiracles are—you guessed it!
Flattened bottom dwellers like the Wobbegong!
Notice how this ray swims, by flapping its wings and gliding along just over the sand.
By comparison, meet the Little Skate, common in the waters of New England.
It looks just like a miniature stingray, but it’s a skate because it has no stinger.
Although it can swim by flapping its wings, it often scoots along the bottom by repeatedly
pushing off the bottom with its pelvic fins.
Kind of like riding a skateboard.
This type of locomotion has been termed “punting.”
Most Batoids have pelvic fins like this, but only a few use them to move in this way.
And if you think that’s weird, check out this strange looking ray!
The Torpedo Ray doesn’t look much different from any other ray when it’s lying on the
bottom.
But when it gets up to swim…it has a vertical tail fin like a shark!
Instead of flapping its wing-like pectoral fins, it swishes its tail back and forth to
swim!
And that’s not even the weirdest thing about this unique ray.
The Torpedo Ray is also known as the Electric Ray because it can produce 50 volts of electricity
to shock its prey!
Most rays can’t electrocute their dinner into submission.
The Spotted Eagle Ray spends a lot of time soaring through the water like the Eagle for
which it’s named.
But look closely.
It has a shovel-shaped face for digging around in the sea floor.
It’s a bottom-feeder like the stingray.
And look here!
Not just one but three stingers!
Another ray that likes to swim up off the bottom: the manta ray.
If ever there was an animal that looked like a flying carpet, this is it.
Unlike most other rays, this animal eats plankton.
It swims through the water with its mouth wide open, allowing the tiny plankton to go
into its mouth and get stuck on comb-like filters on its gills.
Then it swallows them down.
Mantas don’t have spines for protection.
They rely partly on their large size to deter predators.
But they can jump pretty far out of the water to escape predators too!
Sometimes that doesn’t help.
This manta has a bite cleanly taken out of it by a shark!
Ouch, that’s gotta hurt.
It probably happened when this ray was much smaller.
Manta rays are ovoviviparous, meaning that they produce eggs that hatch internally.
Baby manta rays emerge from their mother alive and swimming.
All rays use this reproductive strategy.
However skates, like the Little Skate, are oviparous—they lay eggs.
This is the egg of a skate.
The mother produced 5 to 15 of them.
They are laid on the sand and given no parental protection.
Inside the egg, a baby skate develops, for up to a year, dependent on water temperature.
Finally, the day comes when the baby skate slowly forces its way out of the egg case
and swims away.
Even a newborn knows to camouflage itself quickly to avoid predators.
Skates and rays are a diverse group.
Not all Batoids look like the typical ray.
This animal is called a Sawshark or sawfish.
It looks more like a shark, but it is a ray.
Don't believe me?
Where are its gill slits?
A shark's gills would be here, but they're underneath.
And look, its breathing through large spiracles.
The Sawfish uses its saw to hunt by swinging it back and forth in the sand, impaling soft-bodied
creatures on the teeth.
So skates and rays are a diverse group of animals with many adaptations to survive.
And while they are not as well-known as their cousins the sharks, they’re every bit as
fascinating.