What is plankton? The Real Monsters of the Ocean

Night on the coral reef

As darkness slowly creeps over the coral reef, the night dwellers begin to appear. Coral relies upon its symbiotic algae to feed themselves during the day, but after dark the coral polyps unfold their sticky tentacles. These tentacles grab food (plankton) floating by in the water.

Black tip reef sharks, which have been snoozing lazily on the sandy bottom during the day, become violent predators. They travel in packs with some sharks flushing out and eating prey from the fish cowering in their reef lairs. Other sharks catch whatever the flushing sharks missed.

A chambered nautilus, the less sexy cousin of octopus and squid, slowly ascends from 2,000 feet deep to 300 feet deep, its shell’s chambers acting like a submarine’s ballast.

Plankton!

Then the real stars of the night show up, barely visible to the naked eye. The longest vertical migration takes place every night. Plankton rise from depths of 1,500 feet to feed near the surface.

I was lucky to see some of these other-worldly creatures on a Blackwater dive called “Pelagic Magic,” through Jack’s Diving Locker in Kailua-Kona on the Big Island of Hawaii. Divers hold onto ropes under the boat and like a seat back entertainment center on an airplane seat, each diver gets their own individual show. The highlight of my dive was seeing a squid flush red, catch a meal and dart to the surface, all in a blink of any eye.

Types of plankton

Plankton are the real monsters of the ocean, as the myriad of shapes and colors is outstanding. The larval form of many animals, like a crab or sea urchin, look nothing like the adults.

Much of the plankton is translucent because in the depths of the ocean they virtually disappear. Other plankton are bright red, which at the surface are obvious, but at depth they are barely visible. This is because red is the first color to be lost under the water. The further down you descend in the ocean, the less colors you see.

Phytoplankton vs. Zooplankton

There are two main types of plankton, the phytoplankton vs. zooplankton. “Phyto” means plant in Greek, and “Zoo” means animal (think of what lives at a zoo!). So the phytoplankton are the plants of the ocean, while the zooplankton are the animals of the ocean. Phytoplankton are very important because they make up the base of the food web, and they are responsible for one out of every two breaths you take! Just like how animals on land eat plants (do you eat your veggies?), the zooplankton depend on the phytoplankton for food. And plenty of larger animals-fish, whale sharks, manta rays, baleen whales-depend on plankton for as their food.

Did you know krill (part of the plankton) are the most abundant animal on Earth?

By-the wind sailors, also known by their scientific name Velella velella, have been washing up by the thousands along the West Coast of the United States. They have been found from Monterey, California all the way up the coast to Oregon.

What are Velella? They are distantly related to jellies as both are cnidarians. Velella are closely related to the Portuguese Man of War. They have a blue elliptical base and a transparent triangular “sail.” An individual is actually a hydroid polyp. A polyp is like the less recognizable stage in a jelly’s life when it is anchored to the sea floor, though Velella polyps are free-floating. In this polyp form, they spend their whole lives at the surface. Velella are at the mercy of the wind to get anywhere. They are found in warm and temperate oceans.


Velella eat by using their tentacles which hang beneath the surface. Like jellies, the tentacles have nematocysts, or stinging cells, to catch their food. These stinging cells are not dangerous to humans, though each person’s tolerance to their venom varies.

There are two forms of Velella. One has a left-to-right orientation of its sail, and the other form has a right-to-left orientation of its sail.

The Velella life cycle (like many “jelly-like” creatures) can be summarized as polyp-medusa-egg-planula-polyp. The polyp stage is the one written about in this post. The medusa is the free-floating stage, like any jelly you can think of. The eggs are microscopic and part of the plankton. Planula are the “free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species,” i.e. the fertilized egg.

Next time you are at the beach, look for these By-the-Wind Sailors!

Sources used: Univ. of Michigan animal diversity page

San Francisco Chronicle article “Beached blue wonders”

Wikipedia page on planula

Or specifically, one of the creatures with the most biomass on Earth (we don’t know for sure though as so much is being harvested by humans or eaten by animals). That would be krill, the small shrimp-like creatures that large whales love to gobble down.

My name is Karl, and I am a krill. I live in Antarctic waters. What is it like to be part of a collective or large group? Well, not only am I small at 2.4 inches or 6cm long, but I live in a swarm of krill. A swarm may be as dense as 10,000 individuals per cubic meter and may stretch for many kilometers. There are 82 kinds of species.

The total biomass, or the total weight of krill is estimated at 379 million tons. Half of that is eaten by animals such as whales, seals, penguins, squid and fish each year! Blue whales eat 4 tons of us a day! So my life expectancy is less than a year unless I’m lucky and can live up to 5 years. That is if I don’t get caught by a krill fishery either. Krill is used in supplements, aquaculture, aquarium trade, as bait and as food for humans in places such as Japan and Russia.

As a wee lad, I ate microscopic food stuck to the ice. I also depended on the ice for shelter from predators. Climate change is causing the sea ice to melt earlier in the season, and this is a problem for young krill. Some populations have declined up to 80% due to climate change.

Let’s hope climate change doesn’t wipe out my species, as it would be detrimental to animals higher on the food chain like whales and seals.

Also see Krill guide: what they are, and why they are so important

and
Prochlorococcus (Blue-Green Bacteria) are the Earth’s Most Abundant Photosynthesizing Biomass