Ecology

Habitat

The habitat usually occupied by C. polypus is the shell of several species of hermit crab, Dardanus sp. There may also be one or more anemones attached to the shell. The hermit crab initiates shell transfer, as the anemone shows no preference to attach itself to the shell that does or does not contain a hermit crab.

Symbioses and Commensalism

C. polypus has been found to have associations with 5 species of hermit crab from the genus Dardanus; D. arrosor, D. impressus, D. haani, D. gemmatus, D. deformis (Ross, 1974). The symbiosis that C. polypus has with Dardanus sp. is a mutualistic relationship in which the sea anemone provides camouflage and defends the crab against predators by using its nematocysts in return for more proficient dispersal and a substrate in which to attach. (Ross, 1971)

Dardanus sp. initiate contact then actively collect and move C. polypus from surrounding substrate, and also from one shell to another (Ross, 1970). From first contact to when the anemone is sitting upright on the shell takes roughly 15 minutes. However, in some cases it can be as brief as 5 minutes or as long as 3 hours. (Muscatine, 1974)

Crabs with more anemones attached are more likely to survive an encounter with a predator than crabs without. In studies investigating the impact of the anemones of the survivability of the hermit crab after having encountered Octopus vulgaris, it was found that those with anemones attached caused the octopus to withdraw. All Dardanus without Calliactis were ingested, while all Dardanus with at least one symbiotic Calliactis survived.(Ross, 1971)

C. polypus also has an endosymbiotic relationship with zooxanthellae, a photosynthetic algae. Zooxanthellae are a diatom that occur mainly within the gastrodermal tissue of the tentacles and oral disc, as they are exposed to sunlight for the longest period of time. The photosynthate produced by the algae can sometimes account for up to 90% of the anemone’s nutrition, depending on the amount of prey available. (Ruppert, 2004) In return for this huge influx of excess organic carbon for the anemone, the algae receive nutrients, carbon dioxide and a place to live that is positioned in direct sunlight for a majority of the time. (Ruppert, 2004)