The Search for the World’s Simplest Animal
On a sunny pre-pandemic afternoon at the beach near Santa Cruz, California, children shriek as the waves demolish their sand castles, and seagulls squawk over a discarded bag of salt-and-vinegar potato chips. Pelicans, sea lions, and fishermen flock to the end of an old wooden pier, attracted by schools of fish that shelter in the wreckage of a half-submerged tanker, the SS Palo Alto. The pier bristles with fishing poles, their long lines trailing into the water.
At the end of one of these lines, I hope, dangles an elusive, mysterious creature—not a fish, but the world’s simplest animal, Trichoplax adhaerens. Named after the Greek words for “hairy, sticky plate,” Trichoplax belongs to one of the most ancient animal lineages on Earth, a phylum known as Placozoa that is more than 650 million years old.
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.
The way to trap a wild Trichoplax, according to experts, is to place glass microscope slides in a plastic rack that will hold them securely, but spaced far enough apart to allow seawater to flow through. Tether the case to a piece of fishing line, dangle it over the side of a pier or dock to a depth of at least a meter, and let it hang there for a week or two. If you are lucky, a Trichoplax will float into the case, stick to the glass, and start to clone itself.
Manu Prakash, the biophysicist I’m meeting on the pier, has not been very lucky lately. Although he captured one Trichoplax in Puerto Rico in 2018, it died before he could get it home to the lab, and he hasn’t caught one off the California coast in a year.
Prakash is 40, with dark brown curls and the concave, avid posture of someone who has been peering through microscopes since he was in elementary school—when he built his first scope, from a pair of when he decided to build a much bigger, more powerful microscope that could image every cell inside a freely moving animal.
