# Mutualism on the deep-sea floor: a novel shell-forming sea anemone in symbiosis with a hermit crab

## TL;DR

A new sea anemone species forms a mutualistic relationship with a hermit crab in the deep sea, providing a shell-like structure that benefits both organisms.

## Contribution

The study identifies a novel species-specific mutualism between a sea anemone and a hermit crab in the deep sea.

## Key findings

- The sea anemone Paracalliactis tsukisome sp. nov. secretes a shell-like carcinoecium that enhances the hermit crab's living space.
- Stable isotope analysis suggests the anemone gains nutrition from host faeces and organic particles.
- 3D imaging revealed a unidirectional attachment pattern linked to feeding and carcinoecium formation.

## Abstract

Interspecific species interactions are fundamental evolutionary forces that shape the traits and adaptive strategies of biological communities. However, their diversity and dynamics in deep-sea ecosystems are poorly understood because of their inaccessibility. Here, we report and describe a newly identified species-specific, hermit crab-associated sea anemone named Paracalliactis tsukisome sp. nov. The sea anemone secretes and constructs a unique shell-like structure known as a carcinoecium, which expands the host hermit crab’s living space. Stable isotope analyses (δ¹³C and δ¹⁵N) suggested that P. tsukisome sp. nov. consumes nutritional benefits by consuming host faeces and suspended organic particles from the surrounding environment. Three-dimensional computed tomography imaging elucidated a unidirectional attachment pattern, which was consistently positioned near the shell aperture or carcinoecium edge—a likely adaptation linked to feeding behaviour and carcinoecium formation. The host, Oncopagurus monstrosus (Alcock, 1894), substantially benefits from this association, attaining larger body sizes than other Oncopagurus species, highlighting the functional role of the carcinoecium as an effective shell enhancement in the deep-sea environment. This study provides the first quantitative evidence of mutualism in carcinoecium-forming associations, highlighting a remarkable example of deep-sea symbiosis and hypothesizing how reciprocal benefits are refined over time, fostering the evolution of carcinoecium-forming abilities and species-specific mutualistic relationships.