Female fiddler crabs (Uca mjoebergi) are polyandrous and usually mate with at least 2 males during a breeding season. They mate either on mudflats or in a burrow. In a recent paper (R.A. Slatyer et al., 2012 LINK HERE) the authors investigated the possible benefits from the multiple male mating. They found that females mate initially with their male neighbors early in the season seemingly just to get it over and out-of-the-way. There is no romance in the way of courtship and they do it right on the surface for anyone to see. These neighbors with benefits guys then provide help in defending the female’s burrows. Finally, later in the breeding season she chooses a male based on his claw size and how fast he can wave it. This pair then enters his burrow to mate like respectable crabs.
Polyandry occurs because it offers benefits to the female and in this case the two types of mating situations (surface, burrow) offer different benefits. Mating on the surface provides protection by her neighbor-sex partner and in the burrow the superfine male of her choice will sire fitter offspring.
For more information check out the link to the paper above and if you want to learn more about the fiddle crab click here.