Spatial heterogeneity in parasite loads in the New Zealand cockle: the importance of host condition and density

Samples of the New Zealand cockleAustrovenus stutchburyi (Veneridae) collected at two di¡erent intertidal levels were found to be heavily infected by the echinostome trematode Curtuteria australis which use the mud £at whelk Cominella glandiformis and shorebirds as ¢rst and de¢nitive host, respectively. Parasite loads were on average more than 2-fold higher at the high site than at the low site, whereas the density of Curtuteria australis metacercariae per square metre showed the opposite trend.This pattern appeared unrelated to the abundance of ¢rst intermediate hosts and C. australis prevalence within this host that did not di¡er between sites. At both sites there was a positive relationship between parasite load and cockle condition, a likely byproduct of the relationship between condition-based ¢ltration capacity and the rate at which individual cockles encounter the free-swimming trematode larvae. The results suggest that the between-site variation in parasitism is a consequence of the much higher density of cockles at the low site. There, the dense cockle population is able to clear the water column of parasites and consequently individual cockles virtually ‘compete’ for parasite larvae. This will depress mean infection intensity per cockle but elevate mean parasite density per square metre.