Compared to other parts of anatomy, the anus is underappreciated—but without a way to expel waste from the body, all the work that takes places at other stages of digestion wouldn't be good for much. Now scientists may have a better understanding of how the anus evolved after identifying the first "transient" anus in jellyfish, New Scientist reports.
As biologist Sidney L. Tamm reports in the journal Invertebrate Biology, the warty comb jelly (which is really a type of ctenophore, not a cnidarian like true jellyfish) doesn't have a visible anus most of the time. While studying the comb jelly, he discovered it forms one only when it needs to defecate. When waste builds up, the creature's gut or gastrodermis expands until it touches the outer layer known as the epidermis. At this point, the gastrodermis and the epidermis fuse together and form an orifice where there wasn't one before. Waste is disposed of through the newly opened anus, and once the jelly has finished its business, the hole closes up again and the gut and the epidermis go back to being two separate layers.
These two components each consist of a single cell layer, so the anal opening forms quickly. Mature warty comb jellies have to grow a new anus every hour or so, while their larvae do it about once every 10 minutes.
Jellyfish and comb jellies are simple organisms, with one inner tract instead of a more complex system of organs. Some jellies have one opening in their gastrodermis they use for eating, expelling waste, and exchanging reproductive materials. The discovery of the warty comb jelly's transient anus shows how this one-orifice system may have evolved into a tract with a permanent anus millions of years ago.
Comb jellies are some of the oldest animals on Earth, with ancestors appearing as far back as 700 million years ago. Scientists believe they helped set the evolutionary groundwork for systems that are essential to complex life today, such as the nervous system and digestive tract.
[h/t New Scientist]