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Lecture 2:

Listen to part of a lecture in a marine biology class.

So we’ll continue today with the topic of fish movement and we’ll focus on the role of fins. You can think of fins pretty much like the arms and legs of humans.It’s what allows movement,basically.But as we’ll see,they also do a lot more.Before we go on with the function of specific fins,I’ll just say a few general remarks so you get an overall picture. Fins are mainly used for moving the fish forward,for propulsion,as it’s called.But they are also needed for stability,steering and braking.

That’s right.Fish have to have a way to put on the brakes too. Now,some fish are so fast they are not good at stopping even with their fins.Requiem sharks for example.

Well,you know how sharks are built for speed,but these requiem sharks have difficulty stopping.They are unable to brake themselves efficiently so they must take sharp turns in order to slow down or stop.Because of this brake problem,requiem sharks avoid swimming into coral reefs.Coral reefs are usually crowded,don’t allow a lot of space for large fish to take sharp turns.So these sharks roam around the outside of the coral reefs. Uh,fish lives in a three-dimensional world,you know,with forces pushing and pulling it in all directions.

The forces I’m referring to are gravity,which pulls the fish downward,buoyancy, which holds it up in the water,and something called drag. Now,drag means to slow down,to impede movement.It’s a term that applies to the effect of the force acting on any object moving through a fluid,or through air.So for instance the force of air on an airplane.Drag acts to pull the airplane in the opposite direction to the direction of motion.Well,all of these forces,gravity,buoyancy,and drag could be potentially dangerous for fish if it weren’t for the fact that they have…yup,you guess it!Fins! So fins allow fish to keep from rolling or flipping over.They stabilize the fish’s head.Fins are crucial for determining the speed at which fish travel,to change direction.In a word,fins allow fish to maneuver efficiently through the water.

Okay.Remember most fish have two general types of fins.The first type is median fins. Median fins are located along the midline of the fish,such as the dorsals on the back.And the caudal fins,the tail fin.The other type of fin is the paired fins at each side of the fish. For today,the caudal fin is the one we’ll focus on.The caudal fin,that is,the tail fin,is the main propulsive agent for most fish.It keeps them moving forward.Some fish have a broad tail,that gives the fish a quick burst of speed from a standing position,and um,this is useful for going after a meal,or swimming away from a predator.

You’ll find that fast long-distance swimmers have a very long and narrow caudal fin,which is very efficient. Now,do all fish have caudal fins?No,not at all. Some seahorses and eels get along without them just fine.Well,a seahorse swims in upright, vertical position.It looks like it’s standing upright,so it uses the fin on its back,the dorsal fin. as if it were a caudal fin. And eels,well,don’t really need to swim very rapidly.And like I said,most are missing the caudal fin. But most fish do have caudal fins.I’ll give you two examples of caudal fins and we’ll see how they work.Okay? Let’s see.I’ll focus on the tuna and the blackfish.

The tuna’s body is built for speed.But the caudal fin,the caudal fin is key here because its moon-shaped tail creates hardly any drag in the water and it could reach really high speeds of even 60 to 80 kilometers per hour.That’s pretty amazing. Then there’s the really interesting case of the black fish,which has a rounded caudal fin,as you can see.And in sudden bursts,it almost acts like a propeller on a plane that can even produce the sound that resembles an explosion.I bet you never thought a fish fin could do that.And you can see how this is useful to a fish when it goes to catch something or tries to avoid a predator. It needs to accelerate in an instant.

going to take a broad effort.