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

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.