How to become the most beautiful crab in the sea? Sea anemone: I will help you with your styling

Except for hermit crabs like coconut crabs that grow their own hard shells, most hermit crabs do not have protective shells, and only have a soft body exposed outside. So they need to find a shell to protect themselves. Generally speaking, hermit crabs will give priority to conch shells, after all, they are solid and durable.

But one group of hermit crabs chose not to eat hard sea snails made of calcium carbonate, but to eat living animals made of muscle and mucus: sea anemones.

Cool "tentacle coat" - anemone

This strange combination was found on the seabed of Japan. The dozens of "tentacles" on the head of a Doederleini hermit crab (Pagurodofleinia doederleini) are sea anemones. This type of sea anemone has long been known to the Japanese and has been named Himekinkara anemone (note that sea anemones look like plants, but are actually predatory animals). However, it has not been recognized as an independent species in academia until recently, when it was published in academic journals together with its companions.

Image credit: Akihiro Yoshikawa

Scientists have given the sea anemone a formal name that sounds quite cartoonish: Stylobates Calcifer. Stylobates means "column base," a term used specifically to describe the platform at the bottom of the column at the top of the stepped foundation in ancient Greek and Roman columns. It vividly describes the shape and function of the sea anemone - its tentacles are similar to columns, and it has a chitinous structure similar to a base that can fix it on hermit crabs.

You may be more familiar with Calcifer, a character in Howl's Moving Castle, a fire demon who signed a contract with Howl. With the contract, Howl and Calcifer have a mutually beneficial symbiotic relationship. Calcifer has Howl's heart, and Howl lets Calcifer move the castle for him.

Howl and Calcifer

Image source: Howl's Moving Castle

In reality, sea anemones have red tentacles that resemble Calcifer's red flames. At the same time, in reality, hermit crabs and sea anemones have a similar relationship, but it is not a magical contract, but pure mutual use.

Let's combine! Hermit crab!

In 2020, scientists brought a pair of hermit crabs and sea anemones back to the laboratory, placed them in an aquarium with flowing seawater, and observed how they lived together.

At first, the scientists did not let the hermit crab wear the "tentacle coat", but gave it an ordinary sea snail (Glossaulax reiniana), and the anemone was also attached to a shell. After a while, they put them together.

The ordinary conch shell that was first given to the hermit crab

Image source: ViaNet Conchology

Sure enough, the hermit crab also loves new things and gets bored with the same things. The moment it saw the anemone, it immediately crawled over and wanted to take off the conch on its body and put on a cool "tentacle coat".

The hermit crab with the conch on its back can't wait to move to a new house after seeing the anemone

Image credit: Akihiro Yoshikawa

But changing shells is not as easy as imagined. On February 20, 2020, the hermit crab saw the Cassifa sea anemone for the first time in the laboratory, and as soon as they met, they wanted the anemone to attach to themselves.

The anemone and hermit crab that have not yet merged

Image credit: Akihiro Yoshikawa

The hermit crab first climbs onto the anemone, taps it quickly with its front pedipalps, and pinches it with its claws, just like giving it a massage. The "massage" is rhythmic, about 6 times a minute.

Trying to "persuade" the anemone to leave its hermit crab

Image credit: Akihiro Yoshikawa

The crab spent 12 hours trying to get the anemone to work, but to no avail. The anemone was a living thing after all, and you can't just change it if you want to. The crab returned to its previous shell in embarrassment after the failure.

But the crab did not give up after failing the first time. The tentacles of the new home were still very tempting to it. On February 28, 2020, the crab tried again, using the same massage technique and the same perseverance. This time, the crab spent 43 hours massaging the anemone and finally got the anemone out of its shell.

After the fusion is completed, the hermit crab looks like a pink rose bud (the kind with tentacles) that is about to bloom, and its tentacles sway with the water flow in the tank.

Of course, this is not the end. This is just the molting, not the installation. It took about 12 hours for the anemone to climb up, and another 4 hours for it to fully adapt to its new host, the hermit crab. Only then did they truly "sign the contract" and merge successfully.

Hermit crab after fusion

Image credit: Akihiro Yoshikawa

After the fusion is completed, the hermit crab looks like a pink flower bud, with its tentacles sometimes contracted and about to bloom, and sometimes unfolded into Cthulhu tentacles that sway with the water flow.

Scientists added filters to photos of hermit crabs and anemones

Image credit: Akihiro Yoshikawa

Mutual benefit or forced cooperation?

It took so long to merge successfully, do you think it was worth it? Of course it was worth it, and it was good for both parties. For the hermit crab, not only is the new home stylish and cool and full of Cthulhu atmosphere, it looks very different from other ordinary hermit crabs, and it is the most handsome man in the sea.

The most important thing is that the new home anemone can secrete toxic substances , which are more lethal than ordinary snail shells and can protect it from harm by parasites and predators.

For the Calcifer anemone, the fusion is like having a mount. Originally it could only filter plankton in a fixed place, but now it can sit on the "roof" every day to eat fresh food.

A hermit crab covered in tentacles

Image credit: Akihiro Yoshikawa

However, whether this mutually beneficial symbiotic relationship is true has not been fully confirmed, and it is only a speculation of scientists. Because hermit crabs can survive without the Calcifer anemone, the anemone is not essential for survival. In addition, it is a bit forced for the hermit crab to remove the anemone and put it on its own body, otherwise the hermit crab could have successfully made the anemone voluntarily transfer to its body from the beginning.

Image credit: Akihiro Yoshikawa

By the way, this is not the first time that crabs have bullied anemones. Lybia tessellata, also known as boxing crabs, likes to put anemones on their hands and use them as poisonous boxing gloves. Their relationship is not like mutualism, but more like slavery...

The boxing crab's toxic 'boxing gauntlet'

Image source: AquaInfo

Image source: AquaInfo

In fact, this is not the first time that sea anemones have been discovered attached to hermit crabs. In addition to the Cassifa anemone, there are also the Adamsia, Calliactis, and Paracalliactis genera, all of which can be installed on hermit crabs.

A very important reason why they can attach is that their abdominal mouth produces a chitin substance called carcinoecium that adheres to the back of the hermit crab, thus completing the fusion.

Image source: Species New to Science

Image source: Semantic Scholar

As to whether it is true mutualism, there is actually no absolute answer. Perhaps the sea anemone is just living under the protection of the hermit crab and wants to survive but has no choice but to "use" the hermit crab.

Editor｜Wang Tingting References:

1.https://www.journals.uchicago.edu/doi/10.1086/719160

2.https://www.sciencedirect.com/science/article/abs/pii/S1055790310002083

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo