Living Mysteries: Meet Earth’s simplest animal | Science News for Students

Living Mysteries: Meet Earth’s simplest animal

Tiny Trichoplax, discovered in a fish tank, provides a window into the origin of animals
Apr 5, 2018 — 6:45 am EST

Dickinsonia was one of the first animals on Earth. It lived on the ocean floor 550 million to 560 million years ago. It looked like a giant version of the modern-day Trichoplax, the simplest animal now alive.

John Sibbick

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.

A hungry suction cup

At first glance, Trichoplax does not even look like an animal. Its flat body constantly changes shape as it moves. As such, it resembles a blob called an amoeba (Uh-MEE-buh). Amoebas are a type of protist, single-celled organisms that are neither plants nor animals. But when Schulze looked through his microscope in 1883, he could see several clues that Trichoplax truly was an animal.

Trichoplax dividing
Trichoplax can reproduce by splitting in two. Each piece then becomes its own new animal. 
Emina Begovic

Some amoebas are larger than this animal. But an amoeba has just one cell. In contrast, the body of a Trichoplax has at least 50,000 cells. And though this animal lacks a stomach or heart, its body is organized into different kinds of cells that perform different tasks.

This “division of labor between cell types” is a hallmark of animals, explains Bernd Schierwater. He works at the Institute for Animal Ecology and Cell Biology in Hannover, Germany. He’s a zoologist who has been studying Trichoplax for 25 years.

Cells on the underside of Trichoplax have tiny hairs called cilia (SILL-ee-uh). The animal moves by twirling these cilia like propellers. When the animal finds a patch of algae, it stops. Its flat body settles down atop the algae like a suction cup. Some special cells on the underside of this “suction cup” squirt out chemicals that break down the algae. Other cells absorb the sugars and other nutrients released from this meal.

So the animal's entire underside works as a stomach. And since its stomach is on the outside of its body, it doesn’t need a mouth. When it finds algae, a Trichoplax just plops itself onto the food and begins to digest it.

Clues about the first animals

Schierwater believes that the first animals on Earth must have looked a lot like Trichoplax.

When those animals appeared, the oceans were already full of single-celled protists. Much as Trichoplax do, those protists swam by twirling their cilia. Some protists even formed colonies. They gathered into balls, chains or sheets made of thousands of cells. Many protists alive today also form colonies. But these colonies are not animals. They are just clumps of identical, single-celled organisms that happen to be living in harmony.

Then, 600 million to 700 million years ago, something happened. One group of ancient protists formed a new type of colony. Each member’s cell started out the same. But over time, those cells began to change. Once identical, they eventually morphed into two different types. All of the cells still contained the same DNA. They had exactly the same genes. But now the cells began chatting with each another. To do that, they released chemicals that served as messages. These told cells in different parts of the colony to do different things. Says Schierwater, this would have been the first animal.

He suspects that this first animal must have been a flat sheet, much like Trichoplax. It would have been just two cells thick. Those on the bottom let it crawl and digest food. Cells on the top did something else. Maybe they protected the animal from protists out to eat it.

It makes sense that the first animal would be flat. Just consider what the ocean looked like back then. Shallow areas of the seafloor were covered with a gooey carpet of single-celled microbes and algae. The first animal would have crept atop this “microbial mat,” Schierwater says. It would have digested the microbes and algae berneath it — just as Trichoplax does.

That first animal was probably no larger than Trichoplax. It left no fossils. But larger, similar animals evolved over time. Scientists have found fossils that look like giant versions of Trichoplax.

One, known as Dickinsonia, lived some 550 million to 560 million years ago. It was up to 1.2 meters (four feet) across. No one knows whether it would have been related to Trichoplax. It moved and ate the way Trichoplax does, crawling around and then plopping down on a meal. Like Trichoplax, it had no organs — tissues like a brain or eyes that work together to perform a particular task. But its body was a bit complex in other ways. It had front and back ends and left and right sides. Its flat body also was divided into segments, like a quilted blanket.

Mouth and butt — an animal starter kit?

For Schierwater, it is easy to imagine how such a simple animal could evolve a more complex body. Start with a plate of cells, like Trichoplax, whose stomach is its entire underside. The edges of that plate might gradually lengthen until it looked like a bowl sitting upside down. The opening of the bowl might narrow until it looked like an upside-down vase.

Story continues below image.

stomach evolution
This series of drawings shows how early animal shapes may have evolved 500 million to 700 million years ago. The red part shows cells that can digest food. As the body shape evolved from a flat “plate” to a bowl to a vase, those cells formed a stomach inside the animal’s body.
Schierwater lab

“Now you have a mouth,” says Schierwater. It’s the opening of the vase. Inside that vase is now the stomach.

When this primitive animal has digested its food, it just spits back out any unneeded remains. Some modern animals do this. Among them are jellyfish and sea anemones (Uh-NEMM-oh-nees).

Over millions of years, Schierwater suggests, this vase-shaped body stretched. As it got longer, it made a hole at each end. One hole became the mouth. The other, an anus, was where it pooped out wastes. This is the type of digestive system seen in bilaterian (By-lah-TEER-ee-an) animals. Bilaterians are a step past anemones and jellyfish on the evolutionary tree of life. They include all animals with right and left sides and front and back ends: worms, snails, insects, crabs, mice, monkeys — and, of course, us.

Deceptively simple

Schierwater’s idea that the first animal looked like Trichoplax gained some support in 2008. That year, he and 20 other scientists published its genome (JEE-noam). That’s its full string of DNA, containing all of its genes. Trichoplax might look simple on the outside. But its genes pointed to a somewhat complex inner life.

Trichoplax diagram
A cross section showing structures inside of the body of a Trichoplax, the simplest known animal. It has only six different types of cells. Sponges, another simple type of animal, have 12 to 20 cell types. Fruit flies have around 50 cell types and humans have several hundred.
Smith et al/Current Biology 2014

This animal has only six types of cells. For comparison, a fruit fly has 50 types. But Trichoplax boasts 11,500 genes — 78 percent as many as a fruit fly.

In fact, Trichoplax has many of the same genes that more complex animals use to shape their bodies. One gene is called brachyury (Brack-ee-YUUR-ee). It helps form the vase shape of an animal, with its stomach on the inside. Another gene helps divide the body — from front to back — into different segments. It’s known as a Hox-like gene. And as this name implies, the gene is similar to Hox genes, which shape insects into front, middle and back parts. In people, Hox genes divide the spine into 33 separate bones.

“It was a surprise” to see so many of these genes in Trichoplax, says Schierwater. This suggests that a flat, primitive animal already had many of the genetic instructions that animals would need to evolve a more complicated body. It was just using those genes for different purposes.

First nerves

Trichoplax turned out to have 10 or 20 of the genes that in more complex animals help create nerve cells. And this really grabbed the interest of biologists.

In 2014, scientists reported that Trichoplax has a few cells that act surprisingly like nerve cells. These so-called gland cells are scattered across its underside. They contain a special set of proteins known as SNARE. These proteins also show up in the nerve cells of many more complex animals. In those animals, they sit at synapses (SIN-apse-uhs). These are places where one nerve cell connects to another. The proteins’ job is to release chemical messages that move from one nerve cell to the next.

A gland cell in Trichoplax looks much like a nerve cell at a synapse. It, too, is packed with little bubbles. And just as in nerve cells, those bubbles store a kind of messenger chemical. It’s known as a neuropeptide (Nuur-oh-PEP-tyde).

Last September, scientists reported that gland cells actually control the behavior of Trichoplax. When this animal creeps over a patch of algae, these cells “taste” the algae. That informs the animal that it’s time to stop creeping.

A single gland cell can do this by releasing its neuropeptides. Those neuropeptides tell nearby cells to stop twirling their cilia. This puts putting on the brakes.

The chemicals also communicate with other nearby gland cells. They tell their neighbors to dump out their own neuropeptides. So this “stop and eat” message now spreads from cell to cell across the entire animal.

Carolyn Smith looks at Trichoplax and sees a nervous system that is just starting to evolve. In a sense, it is a nervous system without nerve cells. Trichoplax uses some of the same nerve proteins that more complex animals use. But those aren’t yet organized into specialized nerve cells. “We’re thinking of it as like a proto-nervous system,” says Smith. As early animals continued to evolve, she explains, “those cells essentially became neurons.”

Smith is a neurobiologist at the National Institutes of Health in Bethesda, Md. She and her husband, Thomas Reese, discovered the nerve-like properties of gland cells. Three months ago, they described another part of Trichoplax’s proto-nervous system. They found cells containing a kind of mineral crystal. That crystal always sinks to the bottom of the cell, whether Trichoplax is level, tilted or upside down. In this way, the animal uses these cells to “feel” which direction is up and which is down.

Creature carries snake-like venom

Trichoplax isn’t just teaching biologists about evolution, however. Scientists still are learning surprisingly basic things about how this animal lives. For one thing, it can fly! (Sort of.) Also it is deadly poisonous. And it may spend part of its life sneaking around in an entirely different shape —  a disguise that scientists still haven’t recognized.

For a century after Trichoplax’s discovery, people had thought the animal could only crawl. In fact, they are skilled swimmers. And that may be how they spend much of their time, Vicki Pearse discovered. She’s a biologist, recently retired from the University of California, Santa Cruz. Back in 1989, she was traveling from one island to another in the Pacific Ocean.

She collected Trichoplax wherever she went. Afterward, she spent hours watching them under a microscope. One day, she spotted one swimming through the water “like a little flying saucer.” Once she learned to look for it, she often saw the animals swimming this way.

This wasn’t the only weird discovery that she made that year. Another time at her microscope, she watched Trichoplax being chased by a snail. She was sure she was going to see the little fellow get eaten. But as soon as the snail caught hold of Trichoplax, it pulled back as if it had touched a hot stove.

“They look completely defenseless,” she says of Trichoplax. “They’re just a little blob of tissue. They should be delicious.” But not once did she see a hungry predator actually eat one. Instead, the hunter always seemed to change its mind at the last second. “There must be something nasty about them,” Pearse thought.

The mystery was solved years later, in 2009. That’s when another scientist discovered that Trichoplax can sting an animal that tries to eat it. That sting can actually paralyze its would-be predator. It uses tiny dark balls, found on its upper side, to do this.

People had always thought those balls were just globs of fat. But instead, they hold some kind of venom that Trichoplax releases when attacked. In fact, the animal has genes that look a lot like the venom genes of certain poisonous snakes, such as the American copperhead and the West African carpet viper. A little blip of that venom means nothing to a big human. But if you’re a tiny snail, it can ruin your day.

Secret life

Pearse believes that scientists are still missing something big about Trichoplax. These animals usually reproduce by splitting in half. That gives rise to two animals. At least that’s what scientists see when they grow them in the laboratory. Once in a while, Pearse has seen one of these animals break into a dozen or more tiny pieces. Each would go on to become a new little animal.

Trichoplax triple split
Trichoplax does not always just divide into two new animals. Sometimes it divides into three, as this one is doing. The animal has even been seen breaking up into 10 or more pieces that each develop into complete new animals.
Schierwater lab

But Trichoplax also reproduces sexually, as most other animals do. Here, a sperm — a male reproductive cell — seems to fertilize an egg cell from another individual. Scientists know this because they can find Trichoplax whose genes are a mix of two others. This suggests that the animal had a mother and father. Trichoplax also has genes that are involved in making sperm. Despite this genetic evidence of sex, says Pearse, “no one’s ever caught them at it.”

She also wonders if these animals have another life stage that no one knows about. Many sea animals, such as sponges and coral, start out as a tiny, baby larvae. Each larva swims around like a little tadpole. Only later does it land on a rock and grow into a sponge or a coral — one that will stay put for the rest of its life.

Trichoplax could also have a swimming larval stage. That larva’s body could look very different from the “sticky hairy plate” into which it later morphs. It also could help explain why such a simple-seeming animal has so many genes. Shaping and building that larval body would require many genetic instructions.

Pearse hopes that scientists can one day answer all of these questions. “These are mystery animals,” she says. “They have all kinds of puzzles waiting to be solved.”

A Trichoplax feeds on algae. A dye emits red light as the algal cells break open, spilling their contents into the water. The Trichoplax eat chemicals spilled out from the dying algae.
PLOS Media/YouTube

Power Words

(more about Power Words)

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

amoeba     A single-celled microbe that catches food and moves about by extending fingerlike projections of a colorless material called protoplasm. Amoebas are either free-living in damp environments or they are parasites.

animal ecology     A branch of biology that deals with the relations of animals to one another and to their physical surroundings. A scientist who works in this field is called an animal ecologist.

anus     The opening at the end of an animal's digestive system through which solid waste leaves the body.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

bilaterian         Any of the animals whose bodies generally exhibit two-sided (or bilateral) symmetry. That means they would have equal numbers of appendages (such as legs, fins or antennae) on the right and left sides, for instance. The best known animal phyla are bilaterians, including fish, birds, mammals (including humans), amphibians and reptiles. Not included in this group are sponges, placozoans (such as Trichoplax), jellyfish and other jellies.

biology     The study of living things. The scientists who study them are known as biologists.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

cilia     (singular cilium) Small hairlike features that occur on the surface of some cells and larger tissue structures. They can move and their wavelike motion can propel liquids to move in a particular direction. Cilia play an important role in many biological functions throughout the body.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

digest     (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

egg     The unfertilized reproductive cell made by females.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. 

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

exotic     An adjective to describe something that is highly unusual, strange or foreign (such as exotic plants).

fat     A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.

fertilize     (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

fruit     A seed-containing reproductive organ in a plant.

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genome     The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

larva     (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microscope     An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

morph     Short for metamorphosis, it means to change from one form to another (such as from a caterpillar to a butterfly). Or it can mean to evolve or mutate, where one or more parts of the genome undergo some sort of change in their chemistry — and potentially in their function.

muscle     A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.

National Institutes of Health (or NIH)      This is the largest biomedical research organization in the world. A part of the U.S. government, it consists of 21 separate institutes — such as the National Cancer Institute and the National Human Genome Research Institute — and six additional centers. Most are located on a 300 acre facility in Bethesda, Md., a campus containing 75 buildings. The institutes employ nearly 6,000 scientists and provide research funding to more than 300,000 additional researchers working at more than 2,500 other institutions around the world.

nerve     A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.

nervous system     The network of nerve cells and fibers that transmits signals between parts of the body.

neurobiologist     Scientist who studies cells and functions of the brain and other parts of the nervous system.

neuron     An impulse-conducting cell. Such cells are found in the brain, spinal column and nervous system.

neuropeptides      This is the most diverse group of signaling molecules in the brain. Each member of this group is a peptide — a molecule made of two or more amino acids (the building blocks of proteins). Neuropeptides ferry messages between nerve cells in the brain.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

organ     (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

Pacific     The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

protist     A broad group of mostly single-celled organisms that are neither plants nor animals. Some, like algae, may appear plant-like. Those known as protozoans may appear animal-like. And still others appear fungi-like.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sea anemone     An animal that usually lives on the seafloor and reefs. Although the young larvae disperse through the water, they eventually settle and permanently anchor themselves to a solid structure. They have a tube like structure and resemble a soft, flower. But what appear to be petals are actually stinging tentacles that ring their mouths.

sex     (in biology) An animal’s biological status, typically male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal reproductive organs, and external genitals. It can also be a term for some system of mating between male and female animals such that each parent organism contributes genes to the potential offspring, usually through the fertilization of an egg cell by a sperm cell.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

sperm     The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

sponge     A primitive aquatic animal with a soft, porous body.

synapse     The junction between neurons that transmits chemical and electrical signals.

taste     One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

tree of life     A diagram that uses a branched, treelike structure to show how organisms relate to one another. Outer, twiglike, branches represent species alive today. Ancestors of today’s species will lie on thicker limbs, ones closer to the trunk.

vein     Part of the body’s circulation system, these tubes usually carrying blood toward the heart.

venom     A poisonous secretion of an animal, such as a snake, spider or scorpion, usually transmitted by a bite or sting.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.


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