Range: Cosmopolitan Habitat: Calm intertidal zones Size: Microscopic; About the size of one grain of sand Diet: Algae and detritus Threats: Snails and ribbon worms Life span: Unknown

Transcript:

Today on Animal Fact Files we’re talking about placozoa. Rewind to 1883 when these animals were discovered by a scientist who wasn’t even looking for them. They just showed up stuck to the side of aquarium tanks, and the scientist probably announced something along the lines of “by golly that little booger thing is an animal!” Thus, the little booger thing was given a name which roughly translates to “the sticky, hairy plate.”

Placozoa are the simplest animals living on our planet. That’s not meant to be an insult, they’re seriously simple in design. Placozoa are made up of just a handful of different cell types. When compared to the hundreds of cell types found in mammals, a handful truly does indicate a simple design. Most known placozoa are about the size of a grain of sand, they’re sort of circular in shape, they appear relatively flat, and they have tiny hair-like structures which they use for movement.

Placozoa don’t have legs or feet, noses or eyes, or even a heart or a butthole. They don’t have any kind of symmetry, either - though they do have a definite top and bottom. They’re unlike any other animals discovered thus far, so they’ve been classified in their own phylum - which, for those of you unaware, is basically one step away from even being classified as an animal. Here’s a diagram to give you a better idea.

For nearly 100 years after their discovery, placozoa were thought to be just an abnormal larvae of some kind of jellyfish. It wasn’t until the 1960s that they were more appropriately positioned in the placozoa phylum. And even then, there was only one species classified in this phylum, which gave this animal a unique position among animals as the only animal with a single species classified in its phylum, but recently more species have been classified. It’s widely believed in the scientific community that more placozoa species exist. Even some of the species that are currently classified look identical from the outside; only their genetics reveal these species’ differences, so the likelihood that more species will be discovered is high.

In the wild, placozoa are found in intertidal zones with calm water and lots of hard, textured rocks and surfaces. They’re able to tolerate some differing levels of salinity and they’ve been found in the Atlantic, Pacific, and Indian oceans as well as the Mediterranean Sea. They seem to have a preference for tropical and subtropical temperatures, as well. Placozoa eat algae and detritus by crawling over top of their food, enveloping it, releasing digestive fluids, and then sucking up the remains. They might seem like harmless sesame seed-sized specks, but they’ve been observed to pack quite the punch. Snails have been seen recoiling from placozoa after a brief touch. To a huge human, there’s no threat, but for a small snail, it’s a day ruiner. Some animals are able to eat placozoa, however, and those are some species of snails and ribbon worms.

Placozoa reproduce both asexually, or without a partner, via fission and budding and sexually. Asexual reproduction is the more common method of reproduction in placozoa, though sexual reproduction happens in warmer water that is densely populated with placozoa individuals. Scientists have never witnessed placozoa in the act of sexual reproduction but it’s known that they do it because evidence exists showing individuals with two separate parental genes. And, although they have been kept in lab settings for more than a year, scientists aren’t sure how long placozoa can live.

For more facts on placozoans, check out the links in the description. Give a thumbs up if you learned something new today, and thank you for watching Animal Fact Files.