Sea anemones and hermit crabs kind a mutualistic relationship in Japan

Japanese researchers have described a new species of sea anemone that seems to share a mutually useful relationship with hermit crabs.

The pale pink sea anemones, now named Paracalliactis tsukisome, have been discovered connected to the shells of hermit crabs (Oncopagurus monstrosus).

The researchers described the anemone based mostly on 36 specimens that fishing trawlers collected between 2017 and 2024 from numerous places off the coast of Japan at depths between 192 and 470 meters (630 and 1,542 toes).

The anemones, the group noticed, have been all connected to the tops of hermit crab shells and spatially oriented in the identical path. “3D CT imaging revealed a consistent, unidirectional attachment pattern near the shell’s opening, suggesting a basic sense of orientation,” Akihiro Yoshikawa, the research’s lead creator from Japan’s National Museum of Nature and Science, advised Mongabay by e-mail.

The anemone’s asymmetry and sense of orientation have been uncommon as a result of sea anemones, like starfish, normally show radial symmetry, which means a line drawn via their center can divide the animals into halves many various methods, like slicing a pie. Radial symmetry permits anemones to work together with their atmosphere from all instructions equally.

P. tsukisome “can form asymmetric, snail-shaped structures—a phenomenon extremely rare in evolutionary terms,” Yoshikawa mentioned. “This finding offers valuable insight into how simple animals perceive spatial orientation and could provide a model for studying early forms of body asymmetry.”

The group’s evaluation discovered that anemones feed partly on the waste of their host, hermit crabs, and on surrounding natural particles. So, the hermit crabs present anemones with a gradual provide of meals and a construction to connect to. In return, the anemones supply the hermit crabs free residence enhancements. The anemones secrete a shell-like construction, known as carcinoecium, onto the present gastropod shell the hermit crab is inhabiting, which reinforces and enlarges the crab’s shell.

The researchers discovered that hermit crabs with P. tsukisome connected to them “were significantly larger” than different crabs. With bigger, stronger shells, the hermit crabs with anemone companions don’t want to maneuver to new shells as typically, saving vitality and decreasing publicity to predators, the research notes.

The researchers say that is an instance of true mutualism on the ocean ground.

“So far, this new sea anemone has always been found with this specific hermit crab,” Yoshikawa mentioned.

As for the anemone’s species title, tsukisome is a Japanese phrase that refers back to the pale pink coloration of the Japanese crested ibis (Nipponia nippon). It additionally seems in an historic Japanese poem about love and devotion.

Yoshikawa mentioned the title, reflecting the anemone’s delicate pink coloration and bond with its crab host, evokes “a deep, faithful bond akin to that described in classical poetry.”

Banner picture: Paracalliactis tsukisome, residing symbiotically with hermit crabs on the seafloor off the coast of Japan. Image courtesy of Yoshikawa et al. 2025. (CC BY-NC)