Trichoplax Adhaerens – The World’s Biggest – and Only – Placozoan

The Trichoplax

Imagine a flat blob of pink-white jelly stuck to the wall. Now imagine it oozes its way along, continually changing shape as it goes. It reaches something edible, creeps over it, enfolds it and slowly digests its prey. Finally, imagine this jelly-like creature does all this on the wall of your fish tank (not your room), and instead of being a horrible monster, is only slightly larger than the head of a pin. You’ve just imagined the amazing Trichoplax Adhaerens. A picture of a Trichoplax – seen through a microscope – is shown to the right. There’s a larger version of this picture (originally taken by Oliver Voigt) at the bottom of the page.

Now what’s a tiny animal like the Trichoplax doing on a website of the world’s largest animals? Well, Trichoplax Adhaerens is a Placozoan, in fact, it’s the only Placozoan. It’s easiest to be the biggest when you are one-of-a-kind! The Trichoplax was discovered in 1883, but it was only in the 1970s that scientists realized that it was an animal so different from other types of animal that it deserved its own “phylum”. They called this phylum Placozoa. Phylums are the largest groupings of animals. You can see a list of all the different phylums on the page about animals.

The name Trichoplax comes from Greek words meaning “hair” and “plate”, and Adhaerens from the fact that it adheres – sticks to – the surface it is moving along. It was first discovered on the glass of a sea-water aquarium in Austria, making it perhaps the only creature ever discovered in captivity. In fact, it was only in 1989 that a population of Trichoplax was discovered in the wild, in the seas off Japan.

Is there really only one Trichoplax?

Well, in 1896, another Trichoplax was announced, and named Trichoplax Reptans. However, nobody has ever found another Trichoplax Reptans, so quite probably it never actually existed. On the other hand, in recent years scientists have studied the DNA of Trichoplax Adhaerens, and found that there is a lot more variation than they would expect in a single species. This has led some to wonder if Trichoplax Adhaerens is actually many many species that just happen to all look and act the same.

So far, however, the official word is that Trichoplax Adhaerens is the only type of Placozoan. Therefore, at 2.5 mm (0.1 inches), 1 mm (0.04 inches) wide, and 0.025 mm (0.01 inches) thick, the Trichoplax Adhaerens weighs in as the biggest!

What is Trichoplax Like?

Trichoplax Adhaerens has only 4 different types of cells (people have over 200). It therefore presents a number of puzzles for zoologists.

• How does it tell its body where to go, when it has no nerve cells?
• When it has digested its food, how is it shared fairly between all the cells?
• How does its cells work together when its body has no fixed shape?
• If the body of a Trichoplax is broken up into its individual cells, they find their way back together again and rebuild the original Trichoplax. How do they do this amazing trick?
• If a Trichoplax is cut in half, the two halves can grow into two new Trichoplaxes. How does this happen?

Scientists know that Trichoplax lives in most tropical and subtropical seas around the world. They suspect it eats algae. It can also eat bacteria and other small food particles that touch and get stuck on its back!

In Summary

The Trichoplax is an absolutely amazing creature.

• It can eat through any part of its body surface,
• It can change shape and squish around,
• If you chop it in half, you get two smaller Trichoplaxes,
• Even if it gets all mashed up, it re-forms,
• It’s very hard to see – light shines straight through it,
• It’s unlike any other animal in the animal kingdom.

If it were, say, 2000 times bigger, it would be a perfect monster for a scary movie. As it is, it’s the world’s biggest and only Placozoan, and an amazing member of the animal kingdom that you might just find in your salt-water aquarium.