Sand crabs (Lepidopa benedicti) are highly specialized digging animals that spend the vast majority of their adult life submerged in fine sand. The carapace color of L. benedicti varies from gray to white. Given that the environment in which they live is uniform and dark, most of the functional scenarios to explain color variation seem unlikely to apply. Carapace color has a bimodal distribution, indicating that it is better characterized as a polymorphism than simply as normal variation. Gray crabs are larger and more common than white ones. Crabs mostly swim ventral-side up, which suggests that countershading does not explain the apparent advantage of having a gray carapace.
Nasir, Unnam and Faulkes, Zen, "Color Polymorphism of Sand Crabs, Lepidopa Benedicti (Decapoda: Anomura: Albuneidae)" (2011). Biology Faculty Publications and Presentations. 44.
Journal of Crustacean Biology