Detrital Subsidy to the Supratidal Zone Provides Feeding Habitat for Intertidal Crabs

Beach-east wl"ack of tam'hie origin is considered a spafiM subsidy to the maa-hle4en-emn-ial trallsifion zone. We fotmd lhat lhe x~'ack line on sm~d aald ga'aveI beaches of Vancouver Island was fi-eq~mnted by h~terlidaI purple shore crabs~ Hernig~tpsus nudus (Daua 1851 ) and densely colonized by de~h ivorous talitsrid ampbipods . Amphipods spend the day bm'ied in sm~d and forage on beach w*aek ditring the night. H. nudus were found in supralidal ~'rack patches hnmediate ly after nightly high tides in field censuses, but spent mos t o f the day and ebb tides either submerged subtidally or tfidden unde rnea th h~tertidal rocks and boulders . In feeding trials, intertidal shore crabs were capable o f pre)~ng on talitfid amphipods . We consider H. nudus an orrmivore feeding on both fresh and d e c a j n g macroalgae as well as a~mnM prey. Although riving s-upratidally, amphipods were significantly pre fe r red over intertidal fitlorine snails by foraging shore mabs. Handl ing t ime of amphipods was significantly shor ter than for littorine snails. While amphipods had a reduced lisk of pl~edation by H. nut/us when bur ied in the sand, foraging t m d e m e a t h wrack patches did not reduce predat ion pressm'e on maaphipods by shore crabs. Rates of amphipod consumpt ion by shore crabs were higher at darkness than da)light. In addit ion to an apparent day-night rhylhrn, tidal he ight and l ime elapsed since previous high tide had a significan~ influence on shore crab density within the vcrack. We conclude thai beach-cast vcrack a ~ as a spatial subsidy by virtue o f providing a valuable food source to talitrid amphipods , which are in turn consumed by shore crabs thai ride the nightly high tide into supratidal wrack patches to reduce the risk o f pass ing bare sand on their way to a feeding habitat rich in valuable prey.