What Can We Learn from Sea Anemones?

If you’ve ever been stung by a jellyfish, then you’ve inadvertently encountered a representative from a group of animals called the Cnidaria. (If you’re wondering how to pronounce it, you skip the ‘c’ and only pronounce the ‘n’ so it rhymes with ‘I Dare-y Ya’.) Don’t worry if you’ve never heard this word before: although there are over 10,000 different species belonging to the Cnidaria, jellyfish are probably the most famous representatives.

Cnidarians – including the jellyfish – are the so-called ‘sister group’ to a much larger group of animals that scientists call the Bilateria. The ancestors of the bilaterians and cnidarians split into separate branches on the tree of life around 700 million years ago. As the bilaterians set off down their own evolutionary path, they underwent an enormous diversification. In fact, bilaterians encompass the vast majority of the animals that we know and recognize, including vertebrates, insects, mollusks and worms, to name a few. The cnidarians, including the jellyfish, are neither as numerous nor as diverse. They also have a much simpler body organization compared to the bilaterians: cnidarians don’t have a defined left and right, front and back, or head and tail. Instead, they have a single opening that allows them to both eat and expel their waste.

At first glance the cnidarians might not make for an obvious research target. If the bilaterians represent such a remarkable array of animal life, why wouldn’t scientists focus their attention on them? Indeed, mapping what the first ever bilaterian looked like, what genes it had, and how it developed, are big questions in evolutionary biology. But because bilaterians are so diverse, it can be difficult to work backwards and define what their starting point looked like. If we look sideways, to the much simpler cnidarians, we can begin to understand which genes and cell types might have already been around all those hundreds of millions of years ago. By uncovering the genetic blueprints and materials needed to make these simple cnidarian critters, we can begin to understand how the same tools are deployed to make more complicated animals.

For evolutionary biologists, the starlet sea anemone Nematostella vectensis is a popular cnidarian that is easy to keep in the lab. Nematostella looks quite different from the animals we see day-to-day: it has just a single opening at one end surrounded by up to 16 tentacles. Inside Nemoatostella’s transparent body, you can see eight thread-like structures, called the mesenteries, that are involved in reproduction and digestion. The mesenteries are also home to something called the cnidocytes. In Latin, cnido- means “stinging,” and cyto- means “cell.” That jellyfish sting? Well, you’ve got the cnidocytes to thank for that. The cnido part of this word also gives its name to the group Cnidaria. Otherwise, Nematostella lacks many of the features we’re used to seeing in an animal, including a proper gut or a central nervous system.

But despite the relatively simple structure of Nematostella, scientists made a surprising finding when they sequenced its genome. Because cnidarians have fewer types of cells than bilaterians, it had previously been thought that the number of genes in their genomes might reflect this. Scientists hypothesized that fewer genes would be necessary to provide the instructions for a simpler animal like a sea anemone. Instead they found that Nematostella had genes that are related to almost all of those found in the genome of bilaterians. Even more intriguingly, they found that blocks of these genes were kept in more-or-less the same order between vertebrates and sea anemones. This means that a relatively complex genomic structure was already in place prior to the evolution of the more complex animal body plans we see in the Bilateria. In other words, there is no simple link between how complicated a genome is, and what that animal might look like.

Knowing that Nematostella and Bilaterians share the same type of genes, we can begin to figure out what they do. This is by no means an easy feat, but we can investigate it through many different approaches using Nematostella in the lab. One fun experimental technique is to label a gene with a fluorescent marker. When a fluorescently-labeled gene turns on in a cell, it makes a protein that glows, which allows scientists to visualize where and when that gene is expressed in Nematostella. Animals that have modifications made to their genomes are called transgenics – and scientists have made some beautiful transgenic Nematostella that let us visualize genes in the nervous system and in the muscle, amongst other tissue types, and map how their expression changes as the animal grows.

Alternatively, we can discover the function of a gene by turning it off or reducing its expression early on in the animal’s development and observing the effects. Experiments like these have shown, for example, that many of the genes necessary to make the heads and tails and nervous systems of bilaterians already exist in the Cnidaria. The fact that Cnidaria don’t have proper heads, tails, or a complex nervous system shows us that genes are a bit like Legos: the same core starter bricks needed to make a simple structure like a house can be expanded on and deployed in different ways to make much more elaborate designs like a castle or a spaceship. Nematostella already has those starter pack genes, so by comparing how they are used in different combinations in different animals, we can understand how different genes contribute to more complicated animal body plans.

There’s still a lot left to learn about Nematostella, but as we continue to discover how the gene toolkit of the sea anemone is used, we’re building a more complete picture of the history of animal evolution. And this will help us in many ways, from unpacking developmental questions like how an arm or leg is made, to getting a comprehensive understanding of the dazzling array of animal life we see on Earth.  So, next time you see a jellyfish, you’ll know that it represents a pretty informative group of animals. Just be sure to watch out for those cnidocytes!