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A Blue View: Colonial Animals

At first glance, you’d probably guess that a bryozoan (or moss animal) is a single organism. But, in fact, this "animal" is comprised of a colony of smaller organisms that work as a team to create a larger, functional invertebrate.

Published March 29, 2016


Photo via Wikicommons.

Bryozoans can be beautiful, existing in the form of lacey or fan-like colonies sometimes mistaken for coral or seaweed. Based on their appearance, it’s no surprise they were thought to be plants until the mid-18th century.

Each bryozoan can be comprised of just a few to many millions of individual organisms called zooids. Though each zooid is rarely greater than a millimeter in size, the colonies they form can span meters. To fuse together, these individual colony members secrete a calcium carbonate shell, which attaches to that of its surrounding zooids.

Zooids have a very simple structure: a U-shaped gut and a crown of ciliated tentacles called the lophophore. When hungry, they extend the lophophore into the water and beat the cilia on their tentacles, sometimes even wagging back and forth, as if dancing, to increase the odds of success. With an extended lophophore, the bryozoan resembles moss, which is how it got its nickname: moss animal.

Each zooid’s role varies based on its colony. In some bryozoans, each colony member has a specialty, such as feeding, defense or reproduction. While in others, each zooid serves the same functional purpose.

These colonial animals exist all over the world, with nearly 5,000 living species. They typically inhabit kelp forests and rocky shores, feeding on phytoplankton, diatoms and bacteria. However, some have earned a bad reputation for their tendency to grow on the bottom of ships, causing drag and reducing a vessel’s efficiency and maneuverability.

Episode Transcript

With cascading tendrils of blue-green tentacles and a translucent, neon bell, the Portuguese man-of-war has an otherworldly appearance. The man-of-war may resemble a jelly and even appear to be a single organism, but don’t be fooled: It’s actually a siphonophore, a colony of four separate, yet genetically identical, animals.

Siphonophores and other colonial animals—with strange names like salps, bryozoans and cnidarians—are a collective of individuals, each specialized to perform a different task. One way to think of colonial animals is like tissues or organs in a human body; each fulfills a critical function that allows the entire body to operate as one.

The delicate but deadly man-of-war’s uppermost polyp is an iridescent, fanned pocket that gently grazes the water’s surface. It is this familiar “sail” that inspired its common name; sailors thought the man-of-war resembled a tiny warship.

They were spot-on with their description, for this man-of-war also comes fully rigged. Its second polyp forms a set of stinging tentacles that can pack a mean punch. As it drifts through the water, its tentacles stun small fish, crabs and shrimp from up to 160 feet away! The man-of-war’s sting is infamous—even washed up on shore, its nematocysts, or stinging cells, can remain active for months afterward.

While the tentacles are its food gatherers, the man-of-war’s third polyp acts as a kind of mouth-stomach. The final polyp is responsible for reproduction.

Weird science? In a word, yes. And there are many others in the animal kingdom—in fact, many organisms that we think of as singular animals are actually collectives.

Take the stony corals. They’re composed of thousands of individual polyps that over time, by tiny increments, form massive reefs that can sink ships.

Or consider the sea squirt, one species of salp. Many boaters are all too familiar with these small, gelatinous critters as “fouling organisms,” as they accumulate on wet surfaces like hulls. Transparent and tube-like, they too are colonial. Stranger still, sea squirts have a primitive backbone, making them more closely related to humans than their gelatinous ocean counterparts.

One of the largest colonial animals is called a pyrosome. This deep-sea tunicate has been called the borg of the sea for its otherworldly appearance. Imagine one of those inflatable air dancers, the long tubes that dip up and down and sideways in front of car dealerships. Place one in the deep in the sea, add bioluminescence and you’ve got a pretty good rendition of a pyrosome.

One diver recently described a 60-foot pyrosome as feeling like “an exquisitely soft feather boa."

Bryozoans—sometimes called moss animals—are colonial too. Like sea squirts, they are considered fouling organisms. But unlike the gooey salps, delicate and lace-like bryozoans can be beautiful. Certain species even secrete microscopic boxes of calcium carbonate that are perfectly rigid and geometric as honeycomb.

Colonial animals challenge us to think critically about individuality. Is a single polyp an individual? What about the group as a whole? Is that ethereal Portuguese man-of-war really an “it” or a “they”?

It's a head-scratcher—for those of us with heads anyway.

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