New research published in Royal Society Open Science presents a novel species of sea anemone belonging to the genus Paracalliactis, discovered in the deep waters off the Pacific coast of Japan. Professor Akihiro Yoshikawa, from Kumamoto University, tells us more about the recent discovery of Paracalliactis tsukisome.
About the study
The newly identified sea anemone Paracalliactis tsukisome, was described based on specimens collected at depths of 200–500 meters off the coasts of the Mie and Shizuoka prefectures, Japan from 2017 to 2024. This species differs from other Paracalliactis species in terms of number of tentacles and colouration, number of mesentery cycles, tubercle distribution, and cnidae (venom) distribution in the columns and mesenterial filaments. Its phylogenetic position was also supported by the data obtained for its mitochondrial genes 12S, 16S, and COIII and nuclear genes 18S and 28S.The species was identified as a new member of Paracalliactis genus, previously unrecorded in Japan. Moreover, P. tsukisome sp. nov. was found exclusively on shells inhabited by the hermit crab Oncopagurus monstrosus.
The species name “tsukisome” is from the ancient Japanese colour “Tsukisome (桃花褐)”, meaning pale pink, which appears in volume 12 of the Manyōshū, the oldest surviving anthology of Japanese poetry. In this poem, a kimono dyed in ‘tsukisome’ symbolises half-hearted affection, with the poet contrasting it with sincere and faithful love: “I won’t meet you with a half-hearted affection like the pale pink hue of a kimono”. This name aptly reflects the pale pink colouration (columns and tentacles) of the new species and its species-specific symbiosis with the hermit crab, evoking a deep, faithful bond akin to that described in classical poetry.
Observation of the mouth, tentacles, and oral discs of living Paracalliactis species is particularly difficult because live specimens fully retract their mouths soon after collection and rarely relax again in captive conditions. Therefore, the records of living specimens are extremely limited. Using marine laboratory facilities, the researchers succeeded in recording the species in motion for the first time. These videos will help further morphological and behavioural studies on the diversity and ecology of the Paracalliactis species in the future.
In addition, stable isotope analyses (δ¹³C and δ¹⁵N) suggests that P. tsukisome sp. nov. consumes the host faeces and suspended organic particles from the surrounding environment. Three-dimensional imaging revealed a unidirectional attachment pattern that was consistently positioned near the shell aperture or carcinoecium edge, a likely adaptation associated with feeding behaviour and carcinoecium formation, indicating possible recognition of direction.
Furthermore, body size data from newly obtained specimens, museum-deposited specimens, and literature-related taxonomic information were used to elucidate the benefits rendered to hermit crabs. These findings demonstrate that host hermit crabs benefit from this association, as O. monstrosus has a significantly larger body size than other Oncopagurus species, suggesting a strong, mutually beneficial partnership between P. tsukisome and O. monstrosus.
Although sea anemones are radially symmetrical, P. tsukisome can form asymmetric, snail-shaped structures, a phenomenon that is extremely rare in evolutionary terms. This finding offers valuable insights into how ‘simple’ animals perceive spatial orientation, and theoretically providing a model for studying early forms of body asymmetry.
As this species is sometimes collected as bycatch in deep-sea trawl fisheries, it may be accessible for future experimental studies and aquarium displays. The researchers hope that museum and aquarium visitors who see this “pale pink sea anemone” will be reminded of the Manyoshu’s expression of love and the enduring “bond” between the anemone and its hermit crab companions.
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We are deeply grateful to our Editor, Associate Editor, Editorial Coordinator, and reviewers for their valuable contributions to the progress and improvement of this paper. Their constructive and thoughtful feedback significantly strengthened the study. We especially appreciated the editorial team, which remained highly engaged throughout the process and provided prompt, supportive responses to our inquiries. Their thoughtful assistance greatly enhanced our publication experience.
About the author
Akihiro Yoshikawa is an Associate Professor at Aitsu Marine Station, Center for Water Cycle, Marine Environment and Disaster Management, Marine Science Laboratory, Kumamoto University. He conceptualized and conducted this study as the corresponding author. He is interested in taxonomy, phylogeny, ecology, and evolution of marine invertebrates on the deep-sea floor.
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