New deep‑sea partnership: a pink anemone that builds a hard home for hermit crabs
You don't need to travel to outer space to find alien life — Earth's deep ocean is full of strange, beautiful organisms. A team from Kumamoto University has described a softly colored deep‑sea anemone that forms an intimate, mutually beneficial relationship with a hermit crab species.
The study, published in the journal Royal Society Open Science, identifies a new species, Paracalliactis tsukisome, found attached to the shells used by the hermit crab Oncopagurus monstrosus. Researchers observed the pair at depths of about 200–500 meters (656–1,640 ft) off the coasts of Mie and Shizuoka prefectures in southern Japan.
Unlike most sea anemones, which are made of soft tissues and lack hard skeletons, P. tsukisome secretes a rigid, shell‑like material called a carcinoecium. That structure grows to envelop and reinforce the gastropod shell that the hermit crab occupies, effectively building a fortified dwelling around the crab.
The anemone feeds mainly on suspended organic particles and on the crab's feces, a surprising but efficient recycling of nutrients on the deep seafloor. Using 3D imaging and micro‑CT scans, the team found that the anemone attaches to the shell in a consistent, one‑directional pattern — a behavior likely tied to both its feeding strategy and how it constructs the carcinoecium.
The relationship appears mutualistic: hermit crabs hosting the anemone reach larger body sizes than related crabs without it, suggesting the partnership offers a clear advantage to the crab while the anemone gains a stable substrate and food supply. A similar partnership is known in the Atlantic, where the hermit crab Pagurus arrosor often carries a single Calliactis anemone on its shell.
The species name, tsukisome, refers to a pale pink dye term recorded in the ancient Japanese anthology the Man'yōshū (the Collection of Ten Thousand Leaves). In classical poetry a tsukisome‑dyed kimono evoked gentle, sincere affection; the researchers say the name honors both the anemone's delicate color and its close bond with its hermit‑crab partner.
Akihiro Yoshikawa, co‑author and marine biologist at Kumamoto University: "This discovery shows how even simple animals like sea anemones can evolve surprisingly sophisticated behaviors. Their ability to build a shell‑like structure offers a fascinating clue to how animals perceive space and direction."
Beyond the curiosity of a pink anemone building a home, the finding highlights how deep‑sea species recycle resources and form tight ecological partnerships. The researchers say further study of the carcinoecium formation, attachment behavior, and physiological benefits to both partners will deepen our understanding of deep‑sea biology and evolutionary innovation.
