This deep-sea anemone seems to be an amazing roommate. Scientists recently discovered this creature, which was named Paracalliactis tsukisome, living in harmony with hermit crabs off Japan’s coast in the Pacific Ocean. P. tsukisome dwells on the shells of the hermit crab Oncopagurus monstrosus, which live around 650 to 1,600 feet below the ocean’s surface.
According to a recent study published in Royal Society Open Science, the spiky sea anemone builds a shell-like structure called a carcinoecium, which adds more square footage to the home of its hermit crab companion. The name tsukisome is fitting—it comes from a classical Japanese word meaning “pale pink color,” which is also associated with a “deep, faithful bond” described in ancient Japanese poetry.
Shell building is an unusual tactic for sea anemones, which don’t often possess rigid skeletons. In fact, it’s “surprisingly sophisticated behavior” for a simple animal like a sea anemone, said study author Akihiro Yoshikawa, an associate professor at Kumamoto University in Japan, in a statement.
Beyond finding a place to settle down, P. tsukisome also benefits from its crabby partner by snacking on its feces—an “unusual but efficient form of recycling on the deep-sea floor,” according to the statement.
This is a rare look at an unexpectedly mutualistic relationship forged by co-evolution in the ocean depths. 
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Lead image: Yoshigawa, A., et al. Royal Society Open Science (2025).
