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.
Unnam Nasir, Zen Faulkes, Color Polymorphism of Sand Crabs, Lepidopa Benedicti (Decapoda: Anomura: Albuneidae), Journal of Crustacean Biology, Volume 31, Issue 2, 1 April 2011, Pages 240–245, https://doi.org/10.1651/10-3356.1
Journal of Crustacean Biology