A new pink sea anemone that “builds a home” for hermit crabs discovered off Japan’s deep-sea coast

Researchers from Kumamoto University and collaborating institutions have discovered a new species of deep-sea sea anemone that builds shell-like “homes” for hermit crabs — an extraordinary case of mutualism and co-evolution in the ocean depths.

The newly identified species, Paracalliactis tsukisome sp. nov., was found living on the shells inhabited by the hermit crab Oncopagurus monstrosus at depths of 200–500 meters off the Pacific coasts of Mie and Shizuoka Prefectures, Japan. Unlike typical sea anemones, which lack hard skeletons, this species secretes a shell-like structure known as a carcinoecium, which expands and reinforces the crab’s shell.

Stable isotope analyses revealed that the anemone feeds partly on organic particles and the crab’s feces — an unusual but efficient form of recycling on the deep-sea floor. Meanwhile, 3D imaging using micro-CT scanning showed that the anemone attaches to the shell in a consistent, one-directional pattern that may be linked to both feeding and shell-building behavior. The hermit crab, in turn, benefits from this partnership by achieving a larger body size than its relatives, suggesting a true mutualistic relationship between the two species.

The soft pink anemone was named tsukisome （桃花褐）— meaning “pale pink color” — after an ancient Japanese word found in the Man'yōshū (万葉集), Japan’s oldest anthology of poetry. In ancient poetry, a “tsukisome-dyed kimono” symbolized gentle yet sincere affection-a fitting tribute to the anemone’s delicate color and its faithful partnership with its hermit crab host.

“This discovery shows how even simple animals like sea anemones can evolve surprisingly sophisticated behaviors,” said Associate Professor Akihiro Yoshikawa of Kumamoto University’s Aitsu Marine Station, who led the study. “Their ability to build a shell-like structure is a fascinating clue to understanding how animals perceive space and direction.”

The study was published in Royal Society Open Science on October 22, 2025.