This time on Jonathan Bird’s Blue World, investigating shark electrosensory systems!
Hi I’m Jonathan Bird, and welcome to my world!
I’ve been swimming with sharks for over twenty years, and usually with a camera.
One thing that has always been true about sharks is that they seem to love my camera.
Sharks get right up close to the lens.
This makes great, dramatic footage.
And lots of TV shows--not this one--try to use this behavior to make sharks look aggressive.
Well, it’s not aggression.
They just want to be TV stars!
No it’s not that either.
Sharks have a snout full of pores called Ampullae of Lorenzini, whose sole purpose is to detect
tiny electrical currents in the water.
All living things make tiny electrical currents, and sharks can sense them.
They use this skill to help them hunt in complete darkness.
My video camera produces a tiny electrical field in the water, and it makes the sharks
curious.
They’re bumping the camera to investigate.
During the day, this shark can see me with its eyes…but in pitch darkness could it
still detect my electrical signature?
And from how away?
To learn more about how the shark’s electrosensory system works, I have come to one of the “sharkiest”
places in the world—the Bahamas.
And I just happen to have brought with me an expert in shark electrosensory systems!
Dr. Stephen Kajiura is a shark biologist at Florida Atlantic University in Boca Raton.
He is going to show me how sharks can sense electricity with a simple experiment.
Basically what we are trying to do here is simulate the electric field produced by a
natural prey item—a little fish or a crab or something.
All sharks have the ability to detect electrical fields, and you can see those little pores
all over the surface of the head—those are all electroreceptors.
So what we are doing is, by creating an electric field in the environment here, what we will
be able to demonstrate is the sharks orienting to just the electric stimulus.
We will have two identical targets, one will be active, one will be a control treatment,
and then you will see that they are going to swim right over the control, ignore it
completely, and bite only at the active electric target.
Dr. Kajiura puts the equipment together.
We will take it underwater to do some tests with the sharks.
Dr. Kajiura, Julia and I gear up for a dive to set up the experiment.
Julia has brought down the plexiglass base.
It will be partially buried in the sand so it won’t draw attention to itself.
Dr. Kajiura connects the electrodes to a cable, so he can back up far enough to keep from
affecting the sharks’ behavior.
Next he buries the cables in the sand.
Working underwater is slow going.
Once everything is set up, he turns on the power.
Now a small electric field is emanating from the experiment.
Think of it like an invisible campfire on the sandy bottom.
It’s only warm if you are close.
The sharks swim by, but they don’t detect it any more than we do.
They are way too high above it in the water column to realize that anything is happening.
After lunch, Connor, our shark handler, shows up with some bait.
He will use the bait to get the sharks closer to our electrical campfire.
The sharks are hungry, so they will focus their attention on Connor, close to the sandy
bottom.
The Lemon sharks are swimming close to the bottom now, looking for food.
Dr. Kajiura turns on the power and we watch.
The very first Lemon shark to pass over immediately takes an interest in the plexiglass.
It can clearly feel the tiny electrical field.
A Tiger shark passes half a meter above the experiment and doesn’t react.
But another Lemon shark cruising just above the sand homes right in on it.
The apparatus is designed to produce an extremely small electrical field, similar to the one
generated by a living thing.
We are talking about a super tiny current.
Shark after shark stops to check it out--even while Connor is feeding the sharks only a
few meters away--but only the sharks that are swimming close enough to the sand to feel
it.
This demonstrates how the electrosensory system of sharks is only good for close range because
the electrical field doesn’t carry far in the water.
They need to use their other senses like hearing, eyesight, and scent to get close enough to
prey for the electrosensory system to work.
Sharks are amazing animals.
They have highly-evolved senses to find prey.
Ultra-sensitive eyes that can see in low light.
A powerful sense of smell.
They can detect the slightest vibrations in the water that might be quarry.
And when they get close to the prey--even in pitch darkness--they have the ability to
detect electrical fields.
Some people think that because sharks have been around for hundreds of millions of years
that they are in some way primitive.
But that’s completely wrong.
Because they have been around for so long, they have developed these incredible senses
to give them the upper hand in the struggle for life in the Blue World.