Here we describe Stylobates calcifer sp. nov. (Cnidaria, Actiniaria, Actiniidae), a new carcinoecium-forming sea anemone from the deep-sea floor of Japan. Stylobates produces a carcinoecium that thinly covers the snail shells inhabited by host hermit crabs Pagurodofleinia doederleini. The new species is distinct from other species by the shape of the marginal sphincter muscle, the distribution of cnidae, the direction of the oral disk, and host association. The species’ novelty is supported by the data of its mitochondrial genes 12S, 16S, and COIII and nuclear genes 18S and 28S. Also, we conducted behavioral observation of this new species, focusing on the feeding behavior and interaction with the specific host hermit crab. Our observations suggest that this sea anemone potentially feeds on the suspended particulate organic matter from the water column or the food residuals of hermit crabs. When the host’s shell changed, intensive manipulation for transference of S. calcifer sp. nov. was recorded. However, although the hermit crab detached and transferred the sea anemone to the new shell after shell change, the sea anemone did not exhibit active or cooperative participation. Our data suggest that the sea anemone may not produce a carcinoecium synchronously to its host’s growth, contrary to the anecdotal assumption about carcinoecium-forming sea anemones. Conversely, the host hermit crab’s growth may not depend entirely on the carcinoecium produced by the sea anemone. This study is perhaps the first observation of the behavioral interaction of the rarely studied carcinoecium-forming mutualism in the deep sea.