Japanese researchers have described a new species of sea anemone that appears to share a mutually beneficial relationship with hermit crabs.
The pale pink sea anemones, now named Paracalliactis tsukisome, were found attached to the shells of hermit crabs (Oncopagurus monstrosus).
The researchers described the anemone based on 36 specimens that fishing trawlers collected between 2017 and 2024 from various locations off the coast of Japan at depths between 192 and 470 meters (630 and 1,542 feet).
The anemones, the team observed, were all attached to the tops of hermit crab shells and spatially oriented in the same direction. “3D CT imaging revealed a consistent, unidirectional attachment pattern near the shell’s opening, suggesting a basic sense of orientation,” Akihiro Yoshikawa, the study’s lead author from Japan’s National Museum of Nature and Science, told Mongabay by email.
The anemone’s asymmetry and sense of orientation were unusual because sea anemones, like starfish, usually display radial symmetry, meaning a line drawn through their middle can divide the animals into halves many different ways, like slicing a pie. Radial symmetry allows anemones to interact with their environment from all directions equally.
P. tsukisome “can form asymmetric, snail-shaped structures—a phenomenon extremely rare in evolutionary terms,” Yoshikawa said. “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 team’s analysis found that anemones feed partly on the waste of their host, hermit crabs, and on surrounding organic particles. So, the hermit crabs provide anemones with a steady supply of food and a structure to attach to. In return, the anemones offer the hermit crabs free home improvements. The anemones secrete a shell-like structure, called carcinoecium, onto the existing gastropod shell the hermit crab is inhabiting, which reinforces and enlarges the crab’s shell.
The researchers found that hermit crabs with P. tsukisome attached to them “were significantly larger” than other crabs. With larger, stronger shells, the hermit crabs with anemone partners don’t need to move to new shells as often, saving energy and reducing exposure to predators, the study notes.
The researchers say this is an example of true mutualism on the sea floor.
“So far, this new sea anemone has always been found with this specific hermit crab,” Yoshikawa said.
As for the anemone’s species name, tsukisome is a Japanese word that refers to the pale pink color of the Japanese crested ibis (Nipponia nippon). It also appears in an ancient Japanese poem about love and devotion.
Yoshikawa said the name, reflecting the anemone’s delicate pink color and bond with its crab host, evokes “a deep, faithful bond akin to that described in classical poetry.”
Banner image: Paracalliactis tsukisome, living symbiotically with hermit crabs on the seafloor off the coast of Japan. Image courtesy of Yoshikawa et al. 2025. (CC BY-NC)
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