New Species of Symbiotic Sea Anemone Discovered Off Coast of Japan

Scientifically named Stylobates calciferthe new sea anemone species lives in a symbiotic relationship with a species of hermit crab called Pagurodofleinia doederleini.

External morphology of Stylobates calcifer with host hermit crab Pagurodofleinia doederleini. The upper (A) and posterior (B) views and the marginal views of the right and left sides (C, D, respectively) of the living specimen. The upper and marginal views (E, F, respectively) of the preserved specimen. Scale bars – 5 mm. Image credit: Yoshikawa et al., doi: 10.1086/719160.

Stylobates is a small genus of sea anemones in the family Actiniidae.

It previously comprised four species, all of which exclusively inhabit deep water: Stylobates aeneus from Guam and Hawaii, Stylobates birtlesi from north australia, Stylobates cancrisocia from East Africa, and Stylobates loisetteae from Australia.

The new Stylobates species was collected at a depth of 100-400 m around the Pacific side — middle of Honshu Island to Kyushu — of Japan.

Named Stylobates calciferit was found on the shells inhabited by the hermit crab Pagurodofleinia doederleini.

The anemone occupies the entire top of the hermit crab’s shell, attaching itself by means of a hard shell-like secretion called a carcinoecium.

This phenomenon is not new to science as approximately 35 species of anemones have mutually beneficial relationships with hermit crabs.

“The symbiotic association between hermit crabs and sea anemones is an example of mutualism,” said University of Tokyo researcher Akihiro Yoshikawa and colleagues.

“At least 35 symbiotic sea anemone species belonging to 14 genera have been found on the chelipeds of hermit crabs or the shells they inhabit.”

“Carcinoecium-forming mutualism is a remarkable relationship between the sea anemones and hermit crabs living on the deep-sea floor.”

“Carcinoecium-forming sea anemones settle on the gastropod shells inhabited by the hermit crabs and produce a pseudo-snail shell (carcinoecium) covering over the shell that is likely beneficial for both parties: the hermit crab may not have to find new, larger shells , and the sea anemone may not have to change the substrate.”

In their study, the authors were able to closely observe and describe Stylobates calcifer‘s behavior when the hermit crab changes shells, thereby extending knowledge of their natural history and how symbiosis is maintained.

“In a series of first ever live recordings of the living state, the crab can be observed attempting to urge the anemone, still attached to the old shell, to detach and move to occupy its new shell,” they said.

“It alternatively taps and pinches the anemone with its walking legs and chelipeds.”

“Although no apparent reaction was initially observed by the sea anemone, its position was gradually moved and peeled off from the shell by the intense efforts of the hermit crab.”

“After about 43 hours from the host’s shell change and 18 hours from detachment from the old shell, the sea anemone began to mount and completely cover the new shell.”

“It was positioned on the shell with tentacles facing upward, to enable feeding on suspended particulate matter from the water column and detritus from the hermit crab’s feeding.”

The researchers suggest that the benefit to the hermit crab includes safety from parasites and predators that could affect its growth or shorten its life.

“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,” they said.

“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 behavior interaction of the rarely studied carcinoecium-forming mutualism in the deep sea.”

The team’s paper was published in The Biological Bulletin.

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Akihiro Yoshikawa et al. 2022. Carcinoecium-Forming Sea Anemone Stylobates calcifer sp. nov. (Cnidaria, Actiniaria, Actiniidae) from the Japanese Deep-Sea Floor: A Taxonomical Description with Its Ecological Observations. The Biological Bulletin 24 (2); doi: 10.1086/719160

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