plankton

Let’s explore!

The ocean's floating zoo of tiny animals

A Kid's Guide to the little animals of the sea

Zoo

Tiny crabs float with the zooplankton

Most phytoplankton are smaller than the period at the end of this sentence, so small that they can only be seen through a microscope, but many can be seen with a plain magnifying glass.

Copecods live everywhere and are tiny as dust. It is possible that there are more copepods than any animal on earth.

Because zooplankton live in the ocean, they have found ways to float easily. Some have long spines or feathery hairs that let them spread their body weight over a bigger area.

The medusa grows into a jellyfish. By utilizing air-filled floats and oil that makes them more bouyant, they join the water column. They are one of the world’s most efficient swimmers.

Shrimp use their legs and tail to move up and down in the ocean water column.  At night, they swim up to the surface to feed on phytoplankton and other tiny drifters. During the day, they sink into deeper water where they are safer from predators.

The arrow worm injects venom into its victim before swallowing it whole.

An arrow worm is transparent except for its two black eyes and whatever it had for lunch inside its stomach. Its mouth is ringed with hooked teeth that can latch onto passing copepods or larval fish. 

grasping spines

For four to six weeks, they float and eat phytoplankton before they change to another stage and molt. While they are floating zooplankton, they are called phyllosoma.

Even lobsters spend part of their early life in the swimming column of zooplankton. When they are young, they have thin, flat, transparent bodies with really long legs. 

Nudibracts sea slugs have a free-swimming larval stage. 

Nudibracts or sea slugs are the most beautiful zooplankton of all. 

Although they are bottom dwellers, they float in the plankton column after they are born, eating other zooplankton before reaching adulthood and returning to the bottom.

The tunicate is named for the tougher jelly-like cover— sort of like a “tunic“—that protects it. 

The way they eat is pretty simple. They have a inhalant opening that pulls floating food in. Inside, they filter out food particles using a basket-like shape called a bronchial basket before expelling the water out through a second opening in a squirt.

Jellyfish hatchlings are carried by the currents into the zooplankton world after they are born near shore. 

For their first two months of life, they feed on plankton.

See what you know about you’ve learned about jellyfish stage.

notochord

dorsal nerve tube

Tunicates have a tadpole stage where they join the floating zoo before they attach themselves to a solid surface. 

Tunicates are classified by scientists as chordates because they have these three structures: notochord, dorsal nerve tube, and muscle tissue behind the digestive tract (postanal tail).

muscle behind mouth

Even as tiny, tiny tadpoles, tunicates have little suckers so they can attach themselves to a rock or reef.

Suckers 

An octopus belongs to the family called cephalopod. All their members have a distinct head with a beak and muscular tentacles near the mouth, like octopus, squid and cuttlefish.

Scene

 The life cycle of an octopus has four stages—egg, larva, juvenile, and adult. After the sun sets, hundreds of small octopuses emerge from daylight hideouts in rock crevices to feed on crabs and clams.

Yolk

For three months, they will use the yoke sac for nutrition. Once that’s gone, they will be fry, and swim up to the surface to feed. But the column is dangerous because they can be eaten by zooplankton bigger than them during the 3 months of floating.

The little fish eggs carry their own food inside a sack of egg yolk attached to their bellies.

A nudibranch, even though they are shell-less snails when they are mature, have a shell to protect their guts when they are very young. 

velum

After the egg stage, they transform into a larval form, called Veliger larva. In their free-swimming shelled larval stage, nudibranchs have furry cilia called Velum they use for locomotion and feeding.

shell

foot

Polychaetes are tiny tiny worms with a head, a tail, and body segments. Plychaetes are small but tough. There are 10,000 different kinds of these little worms. Some can withstand very high temperatures.

As they grow, they develop into a stack of baby jellies, and are released one new jelly after another.

Jellyfish begin life in a stalked phase attached to something solid. 

Baby crabs are calle zoea. They have a spine on the back that makes them too painful for other zooplankton to swallow. To grow, they have to shed their shell and grow a new one, a process called molting. They'll go through several tough shells as they grow larger.

Crab zoea are tiny, but their head spine may make them a painful mouthful for other zooplankton to swallow.

Zoea larvae swim by moving their maxillipeds (the legs that work like arms for moving food to the mouth) and pereopods (their other legs).

The growing larva find a hard surface to call home, and excrete adhesives to hold on. As they grow, they become affixed to that location, and are no longer floating in the zooplankton column.

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Lobsters have compound eyes that move about on stalks. They have exception eyesight that helps them spot intruders.

Drifting among the plankton are tiny lobsters. The tiny lobster larvae are very transparent, making it difficult for fish to see them.

Fish eggs are in the water column along with zooplankton. They aren't really zooplankton because they don't need to eat phytoplankton.

Instead of finding food in the water column, they often are the food for many other floating animals.

Alevin is the name given to baby fish, right after they emerge from their egg sack. They are still carrying part of their yolk food. Once the alevin looses the egg sack, it is called a fry (recently hatched fish), and swim up to the surface to feed.