Intertidal facilitation and indirect effects: causes and consequences of crawling in the New Zealand cockle

Bioturbation by the ghost shrimp Callianassa filholi and the lugworms Abarenicola affinis as well as coverage by macroalgae cause the New Zealand cockle Austrovenus stutchburyi (Veneridae) to relocate by crawling longer distances on the sediment surface. On the surface, the cockles become targets for sublethal predation by benthic-feeding fishes, which crop off their feet. This renders the cockles unable to bury for up to 8 wk, thus exposing them to a 5-fold higher predation pressure from shorebirds and the whelk Cominella glandiformis than buried conspecifics. Shell dimensions, foot size, general condition, gender, age, and infections by gymnophallid trematodes do not influence the crawling activity of cockles. However, heavy infection by the digenean trematode Curtuteria australis and the shell-boring spionid polychaete Boccardia acus reduces the distance travelled, and these organisms may hence reinforce the impact of the biogenic disturbers by forcing repeated crawling of the cockles to reach an undisturbed site. The presence of biogenic disturbers and some parasites therefore benefits predating fishes by providing them with an otherwise inaccessible food source, and these in turn benefit birds and whelks through foot-cropping, thus increasing the availability of prey. The gymnophallids also receive an advantage through transmission success to their definitive shorebird hosts. The question as to whether Curtuteria australis benefits, depends on the balance between its loss to fishes (unsuitable hosts) and its increased trophic transmission to its bird hosts. The spionid B, acus on the other hand is negatively affected, since its fate is linked to that of its substrate, which is the shell of the cockle. If the cockle is predated, the attached B. acus will perish also.