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Acid waters may wipe out some of the smallest animals in Earth’s oceans.

What’s in a scoop of seawater? You might think that a scoop of water couldn’t hold much. Think again. Loads of living things swim, float, and splash around in a small scoop.

Now, you will not see these creatures bathing in a dipper of water. That’s because many of them are too small to see with the unaided eye. Others are almost clear, or transparent, so you can’t see them either. Combined, all these creatures are called marine microfauna. They are the smallest animals in the sea.

A microscope reveals this normally hidden world. Bug-like creatures dart about. Nearly clear worms float by. Crinkled critters are sprinkled through the water. Big-eyed baby crabs crawl by.

These are a few of the millions of mini mites that abound in Earth’s oceans. Yet these creatures may be heading for trouble. Let’s explore this small world.

Four oceans cover most of Earth’s surface. They are, from largest to smallest, the Pacific Ocean, Atlantic Ocean, Indian Ocean, and Arctic Ocean. The Pacific Ocean alone is 15 times larger than the United States. All four oceans combined are twice the size of Earth’s dry land.

The four oceans are all connected. Water from one ocean flows into the others. So they are often combined and called the ocean. The ocean isn’t the same everywhere, though. Many different habitats make it up.

Life abounds in the habitats located near the surface of the ocean. There, sunlight warms ocean water. Near the Equator, colorful coral reefs rise from the seafloor. In other areas, towering kelp forests sprout.

Little sunlight reaches below 200 meters (656 feet). Without sunlight, the water is very cold. Still, some sea creatures make their homes in the deepest parts of the ocean.

Many creatures live around volcanic vents, for example. A volcanic vent marks a volcano on the ocean floor. These vents warm the ocean floor, creating hot spots in the deepest and darkest parts of the ocean. Fish and other sea creatures live throughout the ocean. Sharks and rays live near the surface. Whales and giant squid live in deeper waters. Anglerfish live in some of the deepest waters.

Lilliputian life-forms also make their homes almost everywhere in the ocean. They even outnumber their larger neighbors. For example, scientists recently found 20,000 kinds of bacteria in one liter (about a quart) of seawater.

Bacteria aren’t the only tiny tots in the vast ocean. Some of these critters wear shells. Others are jelly-like blobs. Some swim. Others float. Tiny sea dwellers include baby squid, octopuses, and fish. There are also wee animals called copepods.

Not all of these teensy critters stay small. Some join forces to work together. Take the blue button. It is related to the jellyfish. While it looks like one animal, it isn’t. Many small animals combine to make the blue button.

Each of the arms that make up the blue button has a different job. For example, some catch prey. Other arms digest the prey.

Other pint-size sea creatures simply grow up. A swordfish starts out small, but it grows into a hungry predator. An adult swordfish can be three meters (ten feet) long. It swims along, looking for smaller prey.

All the critters in the sea are part of the ocean food chain. A food chain is made of the plants and animals that eat one another.

Because of their size, the tiny critters are near the bottom of the food chain. For example, the giant whale shark feasts on these puny animals. It slurps in mouthfuls of water. It then forces the water out its gills. As the water flows out through the gills, the shark swallows the bitty beasties in the water.

The whale shark and other animals that eat this way are called filter feeders. That’s because the sharks have filters in their bodies that trap marine microfauna.

Large predators are just one of the problems marine microfauna face. An invisible gas is changing the water in which they live.

Cars and factories make carbon dioxide gas. Most of this gas rises into Earth’s atmosphere. There it acts like a blanket, trapping the sun’s heat and causing global warming. Global warming is the worldwide rise in Earth’s average temperature.

The gas also causes problems in seawater. The ocean soaks up about a third of the carbon dioxide pumped into the skies. That’s about 25 million tons of carbon dioxide each day.

Carbon dioxide makes an acid when it mixes with water. That acid makes it harder for animals to make shells and skeletons. This is especially true for the smallest sea creatures.

If more of the gas fills the ocean, small snails and other creatures that wear shells may not be able to survive. That will mean less food for the animals that eat shellfish. Many animals could starve and die.

It may seem hard to believe that what people do on land can affect sea creatures. Yet it can. More and more, scientists are learning that all living things are connected. That includes us. Human actions can mean big changes for even the tiny animals in our oceans.

Article by Fran Downey. Top-of-page photo by Ingo Arndt/Minden Pictures. "Water Wonders" appears on page 2 of the Nov.-Dec. 2007 issue.