DATE2025.10.22 #Press Releases

【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 research team led by Dr. Yoshikawa (Associate Professor, Kumamoto University), Dr. Izumi (Lecturer, Fukuyama University), and Dr. Yanagi ( Senior Researcher, Coastal Branch of Natural History Museum and Institute Chiba), has discovered a new species from the deep sea off the Pacific coast of Japan—specifically the Kumano-Nada (Mie Prefecture) and Suruga Bay (Shizuoka Prefecture). The sea anemone secretes and constructs a unique shell-like structure known as a carcinoecium, which expands the host hermit crab’s living space.
Morphological examinations and molecular phylogenetic analyses using DNA sequences confirmed that this species is an undescribed species of the genus Paracalliactis. Then, it has been named Paracalliactis tsukisome.

The species name “tsukisome” is from the ancient Japanese color “Tsukisome (桃花褐),” meaning pale pink, which appears in volume 12 of the Manyoshu, the oldest surviving anthology of Japanese poetry. This name aptly reflects the new species’ pale pink colouration (column and tentacles) and its species-specific association with the hermit crab—evoking a sense of faithful and enduring affection, as expressed in classical Japanese poetry.

Ecological observations conducted in collaboration with museums and aquariums revealed the following:
1.Paracalliactis tsukisome may feed on feces and other waste produced by its hermit crab host.
2.The anemone may create its coiled “shell” by moving its body in a single direction.
3.The host hermit crabs tend to grow larger than other non-associated species.

These findings suggest a mutualistic relationship that benefits both species. Because sea anemones are generally radially symmetrical, the ability of P. tsukisome to form an asymmetric, snail-like structure is an exceptionally rare phenomenon in evolutionary terms. This discovery provides valuable insight into how simple animals perceive spatial orientation and may serve as a model for studying the early evolution of body asymmetry.

Professor Toshihiko Fujita(Director, Department of Zoology, National Museum of Nature and Science / concurrently affiliated with the Department of Biological Sciences, The University of Tokyo) also contributed to this study.

Illustrations: Chihiro Kinoshita

Links

Kumamoto University, Atmosphere and Ocean Research Institute (The University of Tokyo), National Museum of Nature and Science, Fukuyama University, Natural History Museum and Institute (Chiba), Mie University, Toba Aquarium, Enoshima Aquarium, Kyushu University, and the University of Adelaide.

Journals

Journal name	Royal Society Open Science
Title of paper	Mutualism on the deep-sea floor: a novel shell-forming sea anemone in symbiosis with a hermit crab