Both below- and aboveground shoalgrass structure influence whelk predation on hard clams

Abstract

Seagrass introduces above- and belowground biogenic structure and complexity into otherwise relatively unstructured sand-flat habitat. Observation of burial depths by unmarked and ribbon-tagged knobbed whelks Busycon carica in sand and dense and sparse shoalgrass Halodule wrightii confirmed that shoalgrass structure inhibits whelk burrowing. Whelks showed a strong preference for sand-flat habitat during spring and summer, creating a partial refuge from whelk predation for hard clams Mercenaria mercenaria inside shoalgrass. By early autumn, shoalgrass blades sloughed off, reducing aboveground biomass by over 81%, leading to a massive invasion of seagrass habitat by whelks now exhibiting no habitat preference. The enhanced permeability of the seagrass bed coincided with the large decline in aboveground shoalgrass biomass, a measure of strength of the physical boundary contrast (BC). As shown by tethering whelks in sand, intact shoalgrass, and shoalgrass with aboveground structure removed by clipping, belowground vegetation played a stronger role than aboveground vegetation in limiting the efficiency of predation on hard clams. From summer into autumn, average daily production of empty shells of whelk-consumed hard clams increased, with some evidence of greater increases within seagrass habitat, implying a seasonal breakdown of the refuge function of seagrass. The mechanism by which a strong BC inhibits whelk entry into and use of seagrass habitat is unresolved, perhaps acting as a physical barrier to passage into seagrass or alternatively acting as a behavioral cue indicative of expectation of lower predation efficiency associated with intact but not with clipped seagrass. © Inter-Research 2012.