Scientists discover a new pink sea anemone species that 'builds homes' for hermit crabs

In the deep waters off central Japan, scientists have discovered a pale pink sea anemone, named Paracalliactis tsukisome, that builds its own shell-like home – then shares it with a hermit crab partner.

The newly described species forms an unusual alliance in which both animals gain protection and stability on the dark seafloor.

Found between roughly 630 and 1,640 feet (190 to 500 meters) below the surface off Mie and Shizuoka on Japan’s Pacific coast, the pair live in low-light and food-limited conditions.

Their partnership, built on construction and recycling, reveals how even simple organisms can evolve remarkable teamwork to survive the deep.

Surprisingly sophisticated behavior

The two organisms form a mutualism, a partnership in which both species benefit. Here, both parties gain food and shelter.

Paracalliactis tsukisome secretes a carcinoecium, a shell-like covering that extends and strengthens the crab’s housing. This structure grows with the animal rather than requiring a new snail shell.

The work was led by Akihiro Yoshikawa, an associate professor at the Aitsu Marine Station (AMS) of Kumamoto University. His research focuses on deep-sea symbioses and the evolution of shell-forming anemones.

“This discovery shows how even simple animals like sea anemones can evolve surprisingly sophisticated behaviors,” said Yoshikawa.

Testing the crab-anemone alliance

In ecology, stable isotope analysis – a way to track light and heavy atoms through diets – shows who eats what over time. It offers a long view of feeding that is hard to capture with snapshots.

The team measured carbon and nitrogen in muscle tissue from anemones and crabs. The anemones showed a slightly higher nitrogen signal, consistent with mixed meals of feces and drifting particles.

The pattern points to resource sharing rather than one-sided freeloading. It fits a two-way exchange in a food-poor seafloor environment.

Critically, the isotope results stand alongside the growth data that compare crab sizes. Both lines support a fair trade between partners.

CT scanning Paracalliactis tsukisome

The researchers used micro-CT scanning, a 3D X-ray method that reveals internal shapes without cutting. The images showed that Paracalliactis tsukisome anchors near the shell opening.

That consistent position can funnel food and guide shell building. It may help the animal lay down new material in a single direction.

One directional growth is key to spiral shells. Seeing it in a sea anemone challenges assumptions about how soft-bodied animals build.

Crabs find room to grow

Crabs paired with Paracalliactis tsukisome reached larger body sizes than several close relatives. Bigger bodies align with safer housing that saves energy for growth.

The genus Oncopagurus includes small deep-water hermit crabs that often live between 160 and 7,575 feet, the monograph details. That background makes a useful yardstick for judging any size advantage.

If a crab gets a durable upgrade to its shell, it can avoid risky moves. Fewer shell swaps can also reduce time spent exposed.

Only a few anemone lineages build rigid coverings for hermit crabs. A related species was described in 2022, showing that shell-making has evolved in separate groups.

This new case adds a rare example from mid-depth Japan. The research helps link behavior, growth, and anatomy to show how a partnership takes shape.

Future work can test if similar anemones evolve the same shell rules. It can also probe how crab behavior selects for better builders.

What makes this anemone unique

The team compared tentacle counts, internal muscles, and stinging capsule types. They also sequenced five genes and built evolutionary trees for classification.

The DNA data formed a tight group that separated from close relatives. The same pattern held across both mitochondrial and nuclear markers.

The analysis strengthens the case for a distinct species rather than a variant. It also helps future surveys identify the animal more quickly.

Researchers deposited specimens and data in public collections so others can verify the work. That record-keeping boosts confidence and supports repeatable science.

Paracalliactis tsukisome has a strategy

The species name tsukisome is derived from a classical Japanese term for a pale pink dye. It reflects both the creature’s soft color and its enduring partnership.

The researchers took inspiration from the Man’yoshū, Japan’s oldest collection of poetry, where the word often symbolized quiet affection.

Unlike most anemones, this one grows a hard extension that reinforces the crab’s shell. The structure forms purposefully in one direction, anchoring near the shell’s opening instead of spreading randomly. This positioning likely improves feeding while keeping the entrance clear.

The inner surface remains smooth and free of sand or debris, suggesting the anemone carefully secretes the material rather than simply letting particles stick.

Hermit crabs and snail shells

A larger crab may reflect many forces – not just help from its anemone partner. Food supply, predation, and shell quality all play a role in shaping body size.

The team’s comparisons narrow those uncertainties but cannot remove them entirely, leaving open questions that future sampling across regions and seasons may answer.

Understanding why crabs build or modify shells helps put these patterns in context. Hermit crabs depend on borrowed snail shells, and shortages can limit survival.

A durable, well-fitted cover reduces risky searches and supports growth without frequent moves, while protecting the crab’s soft abdomen tucked safely inside.

Lessons from Paracalliactis tsukisome

Directional attachment raises questions about sensing and control in a simple body plan, suggesting that even sea anemones may gauge position and respond consistently.

Although Paracalliactis tsukisome sits far from bilaterally symmetric animals on the tree of life, this anemone still organizes space with striking precision.

The finding points to sensory rules worth testing in real time through controlled lab setups that can track how placement responds to water flow and food.

Earlier work on Stylobates hinted at flexible shell coatings and occasional crab transfers, revealing varied strategies among shell builders.

The new species reinforces those findings with broader samples and a clear correlation between body size and shell formation, establishing a valuable baseline for monitoring ecological changes across different locations and over time.

The study is published in the journal Royal Society Open Science.

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