Evolution with Hermit Crabs

Members of the genus Calliactis are crucial to understanding the evolutionary history of the symbiosis between hermit crabs and anemones, as it is the most diverse and widespread genus of symbiotic sea anemones with 19 species (Gusmão and Daly, 2010). It is also the most commonly found commensal anemone. (Ross, 1974) Within the genus, behaviour towards the symbiotic hermit crabs varies from relying heavily on the activity of the crab (like C. polypus), responding to chemical signals from the gastropod shells, or both. (Gusmão and Daly, 2010)

Studies conducted on anemone-crab associations have found that these relationships have evolved on a number of occasions within the order Actinaria. (Ross, 1974) It is stated by Ross that the symbiotic relationships between the three genera (Calliactis, Adamsia, Paracalliactis) that associated with hermit crabs evolved independently of each other. However, model-based analyses of the three genera found that Adamsia is nestled within Calliactis, indicating a common origin of symbioses for the members of these genera, ruling out that the three groups evolved independently. (Gusmão and Daly, 2010) 

Therefore it is concluded that there is not a single origin for a hermit crab and gastropod symbiosis within Actinaria, but it has evolved on at least two separate occasions. (Gusmão and Daly, 2010)

Figure 1: Dardanus pendunculatus with Calliactis polypus attached to its shell. (Hobgood, 2006)