Remarkable symbiotic relationship found in the deep sea

Hermit crabs are found in oceans all over the world, occupying habitats that range from shallow coastal waters to the deepest of seas. They have soft, fragile abdomens that need to be protected from damage and are adapted to occupy the empty shells of mollusks, carrying these shells around like mobile homes while they forage on detritus on the ocean floor. 

A study, led by Dr. Yoshikawa from the University of Tokyo, has now identified a species of hermit crab off the coast of Japan that carries a specific and previously unknown species of anemone on it back. The anemone, described for the first time in this study, has been named Stylobates calcifer and it is only found on the shells of a single host species of hermit crab, Pagurodofleinia doederlein. In fact, the anemone is so important to the hermit crab that, when the crab moves into a new shell home, it will go to great lengths to take the anemone along. 

At least 35 symbiotic sea anemone species have been found living on the legs of hermit crabs or on the shells they inhabit. Symbiosis implies a close physical relationship between organisms of different species, and in the case of hermit crabs and their anemones, this is a mutualistic relationship from which both species benefit. Having an anemone covering one’s back may provide camouflage and protection, while riding through the water on the shell of a crab may ensure access to food, particularly the scraps of food left over after the crab eats dinner.

Dr. Yoshikawa and his team collected 16 specimens of the new, deep-sea anemone from depths of 100 – 400 meters between 2017 and 2020. All the specimens came from the ocean on the Pacific side of Japan, and were used for detailed studies of the anatomy, morphology, histology and genetics of the species. Based on the results of these analyses, the researchers have described and named the new species of anemone, Stylobates calcifer, in a publication in The Biological Bulletin, a journal of the University of Chicago Press. 

In addition to describing the new species of anemone, the researchers were able to record live video footage of the process whereby a hermit crab that chose to move house succeeded in coaxing its anemone to detach from the old shell and settle on the new one. This is not an easy task as the anemone occupies the entire top surface of the hermit crab’s shell, and is attached by means of a hard, chitinous secretion called a carcinoecium. This structure helps the anemone cling to the uneven surface of the mollusk shell, but also strengthens and enlarges the shell, which gives the crab extended occupation of its shelter. 

Despite this, a hermit crab will eventually outgrow its shell and will need to find new, more spacious accommodation. The researchers were able to observe the behavior involved when a captive hermit crab changed shells, thereby extending the understanding of how this symbiotic relationship between hermit crab and anemone is maintained. 

Initially, the hermit crab left its old shell and tested out its new one. Once convinced that the new shell was a better option, it tapped and pinched the anemone (still attached to the old shell) with its walking legs and chelipeds (legs that bear claws). It also climbed on top of the anemone and tapped it with the new shell, continuously trying to encourage the anemone to detach. By this physical stimulation and provocation, the crab was able to peel the base of the anemone off the surface of the old shell, but it took approximately 12 hours of intense activity on the part of the crab.

The crab then tried all means of tactics to get the anemone to attach itself to the new shell, which was now on the crab’s abdomen. The crab dragged the anemone around, manipulating it, presenting it with the new shell and continuously stimulating it with legs and chelipeds. Initially, the anemone did not respond in any way and the crab’s efforts appeared fruitless. However, after the anemone was turned upside-down and placed on its tentacles, the crab maneuvered itself, along with its new shell, into a suitable hollow in the anemone’s foot. 

Eventually, 18 hours after the anemone’s detachment from the old shell, it began to show ‘climbing’ behavior and mounted itself onto the new shell. It soon covered the surface of the shell and was positioned vertically, with its tentacles facing upwards, ready to access food particles suspended in the water column. The entire process took 43 hours from the time that the hermit crab initially changed shells. 

The study authors acknowledge that all their behavioral observations involved only a single crab-anemone pair because of the difficulties involved with retrieving and maintaining deep-sea species in captivity. Nevertheless, they suggest that their observations reveal details of a remarkable symbiotic relationship between deep-sea organisms. Future research would need to focus on how the anemones recognize the host hermit crabs on the ocean floor and then build and retain their highly specific association with them. 

The live video recordings are available at https://www.journals.uchicago.edu/doi/10.1086/719160#_i32

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By Alison Bosman, Earth.com Staff Writer