New Species Discovered!-A “Pale Pink” Sea Anemone That Builds a Home for Hermit Crabs
— Named After an Expression of “Love” in the Manyoshu, Japan’s oldest poetry anthology —
Summary- A new species of sea anemone of the genus Paracalliactis was discovered from the deep waters off the Pacific coast of Japan. This remarkable anemone constructs a shell-like “home” for hermit crabs using its own secretions. This species has been named Paracalliactis tsukisome after a word expressing “love” in the Manyoshu1, Japan’s oldest collection of poetry.
- Examination of museums specimens revealed that this shell-forming ability may have evolved through co-evolution with hermit crabs.
- This unique species provides a valuable model for studying how simple animals with radially symmetrical bodies perceive direction and orientation in three-dimensional space.
A research team led by Associate Professor Akihiro Yoshikawa (Kumamoto University, Center for Water Cycle, Marine Environment and Disaster Management), Lecturer Takato Izumi (Fukuyama University), and Senior Researcher Kensuke Yanagi (Natural History Museum and Institute, Chiba) collected sea anemones from depths of 200–500 meters off off the Pacific coast of Japan—specifically the Kumano-Nada (Mie Prefecture) and Suruga Bay (Shizuoka Prefecture). Unlike typical anemones, this species secretes and constructs a unique shell-like structure —called a carcinoecium— that expands the hermit crab’s living space.
Morphological examinations and DNA analysis revealed that the species belongs to the genus Paracalliactis. but its distinct characteristics did not match any previously described species. The team therefore described it as a new species and named it Paracalliactis tsukisome, publishing their findings in Royal Society Open Science (October 2025) with a detailed taxonomic description and high-quality video recordings of live specimens.
Collaborative research with museums and aquariums uncovered further insights:
- P. tsukisome may feed on fecal matter or debris produced by its hermit crab host.
- The anemone may create its coiled “shell” by moving its body in a single direction.
- The host hermit crabs tend to grow larger than other non-associated species.
These results suggest a mutualistic relationship that benefits both species. The anemone’s ability to form asymmetric, shell-like structures—a rare case among radially symmetrical animals—may have evolved through long-term co-evolution with hermit crabs.
This research was supported by JSPS KAKENHI (Grant Nos. JP20J00120, JP21K20591, JP23K14002, JP24KJ2210), the Invertebrate Research Foundation (KO2020-04), and the Sasakawa Scientific Research Grant from the Japan Science Society (2020-4012, 2023-5020).
Sea anemones, close relatives of corals, generally do not have the ability to form hard skeletons. However, some deep-sea anemones are known to secrete materials to construct a shell-like structure for hermit crabs. Because sea anemones are radially symmetrical 2-lacking front-back or left-right distinction- It is unclear how they can produce such asymmetric snail-like structure.
How can a sea anemone recognize direction? Direct observations and experiments are difficult in the deep sea, so this question has remained a mystery. The research team shed new light on how P. tsukisome and its hermit crab partner coexist, and how the anemone’s remarkable shell-forming ability may have evolved, utilizing museum specimens, taxonomic literature, and newly collected specimens.
The new species, Paracalliactis tsukisome differs from other Paracalliactis species in color, tentacle structure, and internal anatomy. Genetic analyses of mitochondrial and nuclear DNA confirmed its classification as a new member of the Paracalliactis genus—recorded for the first time in Japan.
The species name “tsukisome” (桃花褐) refers to a pale pink mentioned in volume 12 of the Man'yōshū (万葉集), the oldest surviving anthology of Japanese poetry. In ancient poetry, a “tsukisome-dyed kimono” symbolized gentle yet sincere affection-a fitting tribute to the anemone’s delicate color and its faithful partnership with its hermit crab host.
Video observations of living individuals—rarely possible for deep-sea species—captured the anemone’s movement and feeding behavior for the first time. Stable isotope analyses suggested that the anemone feeds partly on its host’s waste and surrounding organic particles. 3D CT imaging revealed a consistent, unidirectional attachment pattern near the shell’s opening, suggesting a basic sense of orientation.
Measurements of hundreds of museum specimens showed that hermit crabs living with P. tsukisome grow larger than related species, confirming a mutually beneficial relationship.
Image Title: Paracalliactis tsukisome
Image Caption: Paracalliactis tsukisome, a newly discovered sea anemone living in symbiosis with hermit crabs on the deep-sea floor off Japan. Its name is derived from the ancient Japanese poetry anthology Man'yōshū.
Although sea anemones are radially symmetrical, P. tsukisome can form asymmetric, snail-shaped structures—a phenomenon extremely rare in evolutionary terms. This finding offers valuable insight into how simple animals perceive spatial orientation and could provide a model for studying early forms of body asymmetry.
Because this species is frequently caught as bycatch in deep-sea trawl fisheries, it could serve as an easily accessible model for future 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 companion.
- Akihiro Yoshikawa, Associate Professor, Center for Water Cycle, Marine Environment and Disaster Management, Marine Science Laboratory (Aitsu Marine Station), Kumamoto University / Faculty of Science / Graduate School of Science and Technology
- Takato Izumi, Lecturer, Department of Marine Biosciences, Faculty of Life Science and Biotechnology, Fukuyama University
- Takayuki Kanki, Researcher, Faculty of Engineering, Kyushu University
- Takeya Moritaki, Curator, Toba Aquarium
- Madoka Kitajima, Enoshima Aquarium
- Naoya Ohtsuchi, Assistant Professor, Atmosphere and Ocean Research Institute (AORI), The University of Tokyo
- Taeko Kimura, Professor, Graduate School of Bioresources, Mie University
- Yuxiao Gou, Doctoral Student, AORI, The University of Tokyo
- Ryuji Hattori, Doctoral Student, AORI, The University of Tokyo
- Mahiro Yumiba, Doctoral Student, AORI, The University of Tokyo
- Kotaro Shirai, Associate Professor, AORI, The University of Tokyo
- Michela L. Mitchell, Senior Medical Scientist, Women’s and Children’s Health Network / Adjunct Senior Lecturer, The University of Adelaide
- Toshihiko Fujita, Director, Department of Zoology, National Museum of Nature and Science / Professor, Graduate School of Science, The University of Tokyo
- Kensuke Yanagi, Senior Curator, Coastal Branch of Natural History Museum and Institute, Chiba
1. Manyoshu: Japan’s oldest poetry anthology (8th century), comprising 20 volumes and about 4,500 poems, also the source of Japan’s current era name “Reiwa.”
2. Radial symmetry: A body plan in which body parts are arranged around a central axis, as in jellyfish and most sea anemones.
Title: Mutualism on the deep-sea floor: a novel shell-forming sea anemone in symbiosis with a hermit crab
Authors: Akihiro Yoshikawa*, Takato Izumi, Takayuki Kanki, Takeya Moritaki, Madoka Kitajima, Naoya Ohtsuchi, Taeko Kimura, Yuxiao Gou, Ryuji Hattori, Mahiro Yumiba, Kotaro Shirai, Michela L. Mitchell, Toshihiko Fujita, and Kensuke Yanagi
Journal: Royal Society Open Science
DOI: 10.1098/rsos.250789
Akihiro Yoshikawa Ph. D
Aitsu Marine Station, Kumamoto University,
6061 Aitsu, Matsushima, Kami-Amakusa City, Kumamoto, 861-6102, Japan
e-mail1: akiyoshikawa@kumamoto-u.ac.jp
e-mail2: akj.kt.ex80@gmail.com
