pinnipeds. However, it prefers benthic bivalve mollusks, especially clams, for which it forages by grazing along the sea bottom, searching and identifying prey with its sensitive vibrissae and clearing the murky bottoms with jets of water and active flipper movements. The walrus sucks the meat out by sealing its powerful lips to the organism and withdrawing its piston-like tongue rapidly into its mouth, creating a vacuum. The walrus palate is uniquely vaulted, enabling effective suction. The diet of the Pacific walrus consist almost exclusively of benthic invertebrates (97 percent). Aside from the large numbers of organisms actually consumed by the walrus, its foraging has a large peripheral impact on benthic communities. It disturbs (bioturbates) the sea floor, releasing nutrients into the water column, encouraging mixing and movement of many organisms and increasing the patchiness of the benthos. Seal tissue has been observed in a fairly significant proportion of walrus stomachs in the Pacific, but the importance of seals in the walrus diet is under debate. There have been isolated observations of walruses preying on seals up to the size of a bearded seal. Rarely, incidents of walruses preying on seabirds, particularly the Brünnich's guillemot (Uria lomvia), have been documented. Walruses may occasionally prey on ice-entrapped narwhals and scavenge on whale carcasses but there is little evidence to prove this. Predators Due to its great size and tusks, the walrus has only two natural predators: the orca and the polar bear. The walrus does not, however, comprise a significant component of either of these predators' diets. Both the orca and the polar bear are also most likely to prey on walrus calves. The polar bear often hunts the walrus by rushing at beached aggregations and consuming the individuals crushed or wounded in the sudden exodus, typically younger or infirm animals. The bears also isolate walruses when they overwinter and are unable to escape a charging bear due to inaccessible diving holes in the ice. However, even an injured walrus is a formidable opponent for a polar bear, and direct attacks are rare. Armed with its ivory tusks, walruses have been known to fatally injure polar bears in battles if the latter follows the other into the water, where the bear is at a disadvantage. Polar bear–walrus battles are often extremely protracted and exhausting, and bears have been known to break away from the attack after injuring a walrus. Orcas regularly attack