Clownfish Fatten Up Their Best Buddies To Upgrade Their Homes

The relationship between clownfish and sea anemones is one of the oldest and most famous examples of a form of symbiosis called mutualism, in which each organism benefits the other. Theirs is the ideal partnership, a prime example of couples privilege. By day, the clownfish chases away predators that would eat the sea anemone in exchange for a shelter that stings anyone else that dares encroach. And at night, the anemone swaddles the fish in its stinging tentacles.

But scientists are still unraveling the full extent of this particular evolutionary collaboration, which offers the fish and anemone more than just shelter and food. The clownfish spawn in the anemone, and their feces fertilize their host with ammonia. The fish also aerate the anemone at night, offering their hosts fresh oxygen. They remove waste and other unwanted materials. And in certain situations, they deliver tasty meals right to the anemone's door.

Larger anemones are more luxurious homes, offering clownfish more room to grow and lay their eggs, so the fish have an incentive to fatten their hosts. In captive experiments, some dating back to the 1880s, scientists have observed clownfish carrying food to their host anemone. But scientists were unsure what, exactly, this meant. Were the clownfish using the anemone as a larder, saving snacks for later? Were they offering unreasonably big dishes to the anemone in the hopes that the invertebrate spewed some smaller, bite-sized shreds more suitable for the mouth of a clownfish? Or was this behavior just a beneficent quirk of captivity?

Now, a group of researchers in Japan have documented wild clownfish actively feeding their anemone hosts, and they've also confirmed that this feeding strategy helps the anemone grow faster. They recently published their results in the journal Scientific Reports.

The researchers first made the discovery on a dive where they offered a piece of clam to a species of clownfish called the Clark's anemonefish. The clownfish sucked the clam into their mouth and swam down to deposit the snack in the anemone's tentacles.

To test the behavior more broadly, the researchers conducted a series of experiments over the course of 100 dives. They offered clams, squid, and shreds of fish to clownfish living alongside 26 different bubble anemones. If the morsel was tiny—smaller than three millimeters—the fish gobbled it up. But the larger the snack, the most likely the clownfish were to bring it to their host. This could indicate clownfish give anemones food that is too big for them to eat, or that seems meaningfully large enough for the host. In fact, when the researchers offered the clownfish large and small sea urchins, the clownfish abandoned the smaller ones and brought the more substantial ones to the anemone.

The clownfish were not selfless; if they were hungry, they were more likely to eat their fill before ferrying food to their home anemone. But even while full, the clownfish did not abandon any food offered by the divers. This commitment suggests the clownfish understand that the benefits of feeding their anemone trickle back to them, the researchers explained.

Amid the planned experiments, the divers also noticed the coral reef was littered with the bodies of dead sea cucumbers, which appeared to have died as a result of unusually warm seas. On two occasions, they watched as a clownfish carried a slice of dead sea cucumber back to their anemone host, suggesting this behavior occurs even without a human offering a snack.

Clownfish are small, but live a long time. One paper from 1986 recorded a Clark's anemonefish living to 13 years old before disappearing from the reef. Another paper from 2007 estimated that the lifespan of the clown anemonefish Amphiprion percula—the species that comes to mind when you say "clownfish"—is 30 years. This is twice as long as any other clownfish or damselfish living on coral reefs. "For anemonefish, which cannot leave their sea anemone, feeding their hosts is extremely important and will ultimately benefit themselves," Yuya Kobayashi, a PhD student at Osaka Metropolitan University and an author on the paper, said in a press release.

So it makes sense that clownfish are in their anemone partnership for the long haul. If you'll be spending 30 years with your anemone buddy, you may as well help them bulk. I eagerly anticipate Pixar's threequel to Finding Nemo, a heartwarming flick about the first anemone to start protein-maxxing, in which Marlin feeds the family's anemone the Musashi bar with 45 grams of protein, and the anemone begins to thirst for human blood.