Oceanic Crabs Switch to Monogamous Lifestyles When Living in Smaller Spaces

Researchers have found that oceanic crabs (Planes minutus) form monogamous relationships when they live in small refuges, such as the backs of sea turtles. The findings are in a paper that was just published in the journal Biology Letters.

Oceanic crabs (Planes minutus) lead an odd lifestyle. They live in floating homes throughout the open ocean. These refuges may be made of driftwood, weeds, or even plastic trash. Some oceanic crabs pick especially unusual refuges—the backs of loggerhead sea turtles (Caretta caretta). The crabs sometimes live in the space between the turtle’s tail and shell in a type of symbiotic relationship. Researchers wondered if living in cramped quarters would affect the crabs’ mating strategies.

The research team conducted a survey of oceanic crabs in the North Pacific Subtropical Gyre, collecting specimens for later analysis. The team compared refuge size and type to the number of crabs present. The team also looked into the crabs’ mating strategies to see if they changed based on the type of home. They found that in large refuges, the crabs grouped together in inclusive colonies. In smaller spaces, however, the crabs became more territorial and lived in monogamous male-female pairs. This was especially true for crabs living on sea turtles; the available space can only hold a couple of crabs to begin with, leading them to pair up in exclusive relationships. This was the first recorded case of symbiosis (in this case, a symbiotic relationship with loggerhead sea turtles) changing crabs’ mating tactics.

The findings suggest that oceanic crabs switch to monogamous lifestyles when space is limited. This allows same-sex pairs to live peacefully in cramped conditions, including the backs of sea turtles or small barnacle clusters. The authors note that this is a unique case of symbiotic relationships affecting mating strategies, an understudied phenomenon.


Joseph B. Pfaller et al. Sea turtle symbiosis facilitates social monogamy in oceanic crabs via refuge size. Biology Letters (2016).

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