Land crabs are the largest terrestrial invertebrates on Aldabra. The 12 species occupy most trophic levels, and some crabs are responsible for the transfer of energy from land to sea and vice versa. Some species may be responsible for preventing, or severely reducing colonization of Aldabra by exotic plant species. Other species are important in hastening leaf litter breakdown by digestion, while scavenging forms reduce the number of carrion-breeding flies. Burrowing forms may aerate the soil, and the littoral crab, Grapsus tenuicrustatus, by feeding on surface encrusting alga may speed erosion and aid soil formation. The hermit crabs form an important part of the diet of the flightless rail, and by depositing gastropod shells on land may provide drinking and breeding places for other animals.