David Kyle Johnson

And that incorporates their tail fin, right?

But it's their whole body.

And so, you know, if you undulate in one direction, it moves you forward.

If you reverse the undulation, you swim backwards.

So, yeah, eels can swim backwards into a crevice to hide there to get their prey, for example.

Undulating fish swimming backwards, no big deal, right?

There's lots of examples of that.

All right, but what about non-undulating fish?

There are some fish that can swim backwards using their pectoral fins.

Sometimes they have a specific fin on their belly that they could use.

But it's very rare for a fish to actually use its tail fin for propulsion.

That's why I specifically said that.

What some fish do is they swim backwards with their dorsal fins and they use their tail fin as a rudder to sort of control where they're going.

But they're not propelling themselves with their tail fin.

And the reason why most fish cannot do this is just because of the anatomy of the tail fin.

The tail fin evolved for propulsion in one direction forward.

However, there is at least one fish, and that is the bluegill.

That evolved new muscles, right?

It has a separate tail fin muscle anatomy.

And those muscles allow it to, if you can imagine this, so the tail fin is not just going back and forth.