A new species of sea anemone associated with the hermit crab. Paracalliactis tsukisomeAccording to a team of marine biologists from Japan and Australia, it secretes and creates a unique crustacean-like structure known as a carcinecium, which expands the living space of the hermit crab host.
Paracalliactis tsukisome. Image credit: Yoshikawa etc.., two: 10.1098/rsos.250789.
Paracalliactis tsukisome is the ninth active sea anemone species of its genus, Parakalliactis.
Previously, this genus included eight species, one dubious species, featuring Paracalliactisand one undescribed species, all found exclusively on hermit crab shells.
Majority Parakalliactis species live on the deep-sea bottom (depth about 200–4700 m), and two species, Paracalliactis pink And Chinese paracalliactiswere recorded at shallow and deep depths of 50–3000 m and at shallow depths of 39–40 m, respectively.
“Our discovery shows how even simple animals such as sea anemones can develop surprisingly complex behaviors,” said the lead author. Dr. Akihiro Yoshikawaresearcher from Kumamoto University.
“Their ability to build a shell-like structure is a fascinating clue to how animals perceive space and direction.”
Paracalliactis tsukisome was found alive on hermit crab shells. Monstrous Oncopagurus at depths of 200-500 m off the Pacific coast of Mie and Shizuoka prefectures, Japan.
Unlike typical sea anemones, which do not have a hard skeleton, this species secretes a carcinecium, which expands and strengthens the crab's shell.
The team's stable isotope analysis showed that the anemone fed in part on organic particles and crab feces—an unusual but effective form of recycling on the deep sea floor.
Meanwhile, 3D imaging using microCT scanning revealed that the anemone attaches to the shell in a sequential, unidirectional pattern, which may be related to both feeding and shell-building behavior.
The hermit crab, in turn, benefits from this partnership by achieving a larger body size than its relatives, suggesting a true mutualistic relationship between the two species.
“Our study provides the first quantitative evidence of mutualism in carcinecium-forming associations, highlighting a remarkable example of deep-sea symbiosis and hypothesizing how mutual benefits are refined over time, facilitating the evolution of carcinecium-forming abilities and species-specific mutualistic relationships,” the authors concluded.
Their paper was published on October 22 in the magazine Royal Society Open Science.
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Akihiro Yoshikawa etc.. 2025. Mutualism on the deep sea floor: a new shell-forming anemone in symbiosis with a hermit crab. R. Juice. Open Science 12 (10): 250789; two: 10.1098/rsos.250789
