A newly identified species of hermit crab-associated sea anemone, Paracalliactis, has been described by a group of marine biologists from Japan and Australia. This crab creates and maintains a distinctive shell-like structure known as a carcinodecium, which enhances the living space for its hermit crab host.
Paracalliactis. Image credit: Yoshikawa et al., doi: 10.1098/rsos.250789.
Paracalliactis is the ninth recognized species within its genus, Paracalliactis.
The genus previously included eight species, one of which was unverified; Involves Paracalliactis and one yet-to-be-described species inhabit shells occupied by hermit crabs.
Most Paracalliactis species thrive on the deep-sea floor (at depths of approximately 200 to 4,700 meters), while Paracalliactis rosea and Paracalliactis sinica are noted at varying depths from shallow to deep (50–3,000 m and shallow depths of 39–40 m, respectively).
“Our findings indicate that even simple organisms like sea anemones can exhibit unexpectedly complex behaviors,” stated the first author, Dr. Akihiro Yoshikawa, a researcher at Kumamoto University.
“The capacity of animals to construct shell-like structures offers intriguing insights into how they navigate and perceive their environment.”
The Paracalliactis hermit crabs have been found inhabiting the shells of Oncopagurus monstrosus, which resides at depths ranging from 200 to 500 meters along the Pacific coast of Mie and Shizuoka prefectures.
Unlike typical sea anemones, which lack a hard skeleton, this species secretes a carcinogen that fortifies and enlarges the crab’s shell.
The research team’s stable isotope analysis indicated that the sea anemones derive a portion of their nutrition from organic particles and crab feces, demonstrating an unusual yet effective recycling method on the deep sea floor.
Additionally, 3D imaging using micro-CT scans revealed that sea anemones attach to their shells in a consistent, unidirectional manner, potentially linked to both feeding and shell formation.
Consequently, hermit crabs gain the advantage of increased body size through this association, underscoring a true symbiotic relationship between the two species.
“Our study provides the first quantitative evidence of mutualism within the carcinogen-forming context, illustrating a remarkable case of deep-sea symbiosis and hypothesizing how mutual benefits evolve over time, fostering the development of carcinogen-forming abilities and species-specific mutualisms,” the authors concluded.
Their paper was published on October 22nd in Royal Society Open Science.
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Akihiro Yoshikawa et al.. 2025. Deep seafloor symbiosis: A new shell-forming sea anemone coexists with hermit crabs. R. Soc. Open Science 12(10):250789; doi: 10.1098/rsos.250789
Source: www.sci.news
