NARRATOR：

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

FEMALE PROFESSOR：

And the sea is teeming with tiny organisms, but they don't get as much popular attention as say, whales.

Microscopic algae just aren't as exciting I suppose.

And yet those organisms are the foundation of the bulk of the marine food chain.

Without plankton, which is the global term for these tiny organisms, there will be no whales.

Plankton is found both in fresh water and marine environments.

Again, it's a term we use for any small organisms that float along with the current, either because they are too small or weak to swim against it, or because they don't have any capacity at all to move by themselves.

Plants and plant-like plankton are called phytoplankton while animals and animal-like plankton are called zooplankton.

For over a century now, researchers have been trying to solve the mystery about zooplankton.

You see, some species of zooplankton migrate are...um...not the way birds do when the seasons change. But daily, in the phenomenon we call Diel Vertical Migration or DVM.

In the Diel Vertical Migration, sole plankton swim up near the surface of the water during the night and swim down to deeper water during the day.

Depending on the species and region, this can be a round trip of between 100 and 400 meters.

For a tiny microscopic organism, that's a huge distance.

Remember now, zooplankton can't swim very well and DVM requires a lot of energy.

So, there must be an important benefit to these daily up-and-down commuting.

We're not exactly sure what this benefit is.

Though there are several compelling theories.

I'll talk about them in a moment, but first I want to talk about what we do know or rather what we are pretty sure we know.

So, researchers generally agree that the stimulus for zooplankton DVM is light.

Zooplankton tend to swim away from sunlight into deeper water where the sun's rays barely penetrate.

At night, when the sun no longer illuminates shallower water, zooplankton head back toward the surface.

Now, why would light cause zooplankton to expend all that energy in migrate?

One popular theory is that zooplankton are hiding during the day from visual predators, uh...those animals that hunt by sight.

The darkness provides safety during the day.

Then at night after migrating upward, they have an opportunity to feed on phytoplankton that float at the surface.

Make sense, doesn't it?

But what do we do with the data showing that many kinds of zooplankton don't dive deep enough during the day to become invisible to predators or that others dive deeper than it's necessary to escape hunters' eyes.

And that some zooplankton are bioluminescent, which means they have special organs that light up and make them visible even at great depth.

Well, despite all these, we believe predator avoidance is a possible explanation because of studies done in fresh water lakes.

It turns out there is a correlation between the presence or absence of vertical migration, and the presence or absence of fish that find their prey by sight.

But what are some other possible explanations?

Some researchers suggest that zooplankton migrate to avoid the sun's ultra-violet light.

That would explain why some zooplankton are found at such great depth.

Visible light may not penetrate very far down, but ultra-violet light can.

And we know that some zooplankton have special pigments that protect them from the damage ultra-violet light can cause.

That could be why some zooplankton are able to stay closer to the surface during daylight hours.

And there is a third theory.

Although it takes a lot of energy for the zooplankton to migrate, they conserve energy while floating in deeper colder water.

So, while they're not feeding, they are quietly digesting in cooler water.

But remember, zooplankton consist of any number of different organisms from microscopic worms to crab larvae to tiny fish.

And they are found in a large range of marine habitats: cold water, warm water, shallow water, deep water.

So, there may be different reasons for different species.