Trichoplax lacks nearly all the usual animal characteristics: It has no muscles, no stomach, and no neurons. Its minute, translucent body consists of just two layers of cells, surrounding a gooey, fibrous middle, and under a microscope it looks like a deflated beach ball covered in hair. Yet this shapeless, brainless animal can do remarkable things, including hunt for algae and defend itself with venom. Its human fans think the species is a budding scientific superstar, carrying clues to the origins of multicellular animals, brains, and cancer.
Emily Underwood, The Search for the World’s Simplest Animal, The Atlantic, June 8, 2020
Trichoplax is the simplest animal on Earth. It has no mouth, stomach or brain. Yet it can teach how these and other organs evolved.
Living Mysteries launches as an occasional series on organisms that represent evolutionary curiosities.
Franz Eilhard Schulze had a laboratory full of beautiful sea creatures. In the 1880s, he was one of the world’s top experts on ocean sponges. He found many new species and filled saltwater aquariums at the University of Graz in Austria with these simple sea animals. They were striking — brightly colored with exotic shapes. Some looked like flower vases. Others resembled miniature castles with pointy towers.
But today, Schulze is best remembered for something very different — a drab little animal no larger than a sesame seed.
He discovered it one day by pure accident. It was hiding in one of his fish tanks. Creeping along the inside of the glass, it was dining on the green algae that grew there. Schulze named it Trichoplax adhaerens (TRY-koh-plaks Ad-HEER-ens). That’s Latin for “hairy sticky plate” — which is about what it looks like.
To this day, Trichoplax remains the simplest animal known. It has no mouth, no stomach, no muscles, no blood and no veins. It has no front or back. It is nothing but a flat sheet of cells, thinner than paper. It is only three cells thick.
This little blob might look boring. But scientists are interested in Trichoplax precisely because it is so simple. It shows what the very first animals on Earth might have looked like, 600 million to 700 million years ago. Trichoplax is even providing hints about how simple animals later evolved more complicated bodies — with mouths, stomachs and nerves.
The Ancestor’s Tale, Rendezvous 30: Placozoans (Trichoplax)
There really isn’t much to them. They’re basically like amoeba but multi-cellular. They have two layers. It is believed that the top side is for protection while the bottom side is for consumption. There seems to be no head. It just grows arbitrarily. It apparently looks like a jellyfish larva so it was mistakenly classified as that until modern molecular techniques told a different story. They are the only species in their own phylum.
...the sole member of the monotypic phlyum Placozoa. T. adhaerens is an unique species of very flat organism, that lacks any organs or internal structures. Trichoplax are very flat in appearance, as they only consist of two cellular layers. The first layer consists of ciliated “cover cells”, and the second layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these is the “fibre syncytium”, a liquid-filled cavity strutted open by star-like fibers. Trichoplax feed by absorbing food particles (in a similar fashion to amoebae), with their underside. Reproduction is done asexually by budding, but Trichoplax is also capable of reproducing sexually. The phylogenetic relation between Trichoplax and other animals is still under debate, several hypotheses have been suggested. One of the more recent mitochondrial genome analyses suggests that Trichoplax is a basal eumetazoan, which indicates it is more closely related to the Cnidaria, which contrasts with one of the older hypotheses which place Trichoplax as one of the most basal metazoans, with a relation to sponges.
Trichoplax was first discovered on the walls of marine aquarium, and is rarely encountered in the wild. Field specimens are often found in the coastal tidal zones of tropical and subtropical seas, on substrates like the trunks and roots of mangroves, mollusc shells, coral fragments, or simply just on rock. Places where they have been collected include, the Red Sea, the Mediterranean, the Caribbean, Hawaii and other Pacific Islands, New Guinea, and on the Great Barrier Reef.
