Researchers from Kumamoto University and partner institutions have identified a new species of deep-sea sea anemone that creates shell-like structures for hermit crabs. This discovery highlights an extraordinary case of mutualism and co-evolution in the ocean’s depths. The findings were published on November 4, 2025, in the journal Royal Society Open Science.
The newly classified species, named Paracalliactis tsukisome sp. nov., was located at depths ranging from 200 to 500 meters off the coast of Mie and Shizuoka Prefectures in Japan. Unlike typical sea anemones that lack hard skeletons, this species secretes a unique structure called a carcinoecium. This shell-like formation enhances and stabilizes the shells occupied by the hermit crab Oncopagurus monstrosus.
In-depth analyses revealed that the anemone derives its nutrients partly from organic particles and the feces of the hermit crab, showcasing a remarkable example of recycling on the deep-sea floor. Advanced imaging techniques, including micro-CT scanning, illustrated that the anemone consistently attaches to the crab’s shell in a one-directional manner, which may relate to both its feeding and shell-building behaviors.
In return, the hermit crab benefits significantly from this relationship, achieving a larger size than its relatives. This mutualistic interaction suggests a complex ecological relationship between the two species.
The soft pink hue of the anemone led to its name, tsukisome (桃花褐), which translates to “pale pink color” in English. This name is derived from an ancient Japanese term found in the Man’yōshū (万葉集), Japan’s oldest anthology of poetry. In this context, a “tsukisome-dyed kimono” symbolizes gentle yet sincere affection, reflecting both the anemone’s delicate color and its symbiotic relationship with the hermit crab.
“This discovery shows how even simple animals like sea anemones can evolve surprisingly sophisticated behaviors,” said Associate Professor Akihiro Yoshikawa, who headed the study at Kumamoto University’s Aitsu Marine Station. “Their ability to build a shell-like structure is a fascinating clue to understanding how animals perceive space and direction.”
The research underscores the importance of studying deep-sea ecosystems and the intricate relationships that can exist within them. The findings contribute valuable insights into how species adapt and thrive in challenging environments.
For more information, refer to the study by Akihiro Yoshikawa et al, titled “Mutualism on the deep-sea floor: a novel shell-forming sea anemone in symbiosis with a hermit crab,” published in Royal Society Open Science.
