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

Abstract

Samples of the New Zealand cockle Austrovenus 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.

Distribution of parasites in their second ­intermediate host, the cockle Cerastoderma edule: community heterogeneity and spatial scale

“…Conversely, for 4 other trematode species, lower infections occurred at the lower level stations. The latter case has previously been observed, although reasons for its occurrence remain unclear (Poulin et al 2000, Mouritsen et al 2003; a possible reason is the distribution of the infected upstream first intermediate host (Curtis 2007).…”

“…Another major difficulty is that parasite distribution displays a species-dependent aggregative pattern (Mouritsen et al 2003, Thieltges & Reise 2007, Faltynkova et al 2008. Consequently, the significance of a given number of samples in order to describe a parasite community varies according to trematode species (Thieltges & Reise 2006).…”

“…Little is known about spatial heterogeneity in infection levels of second intermediate hosts and about the contributing factors at different spatial scales. Concerning bivalves as second intermediate hosts, a positive relationship between size or tissue weight and infection levels has been described and is usually ascribed to a higher filtration rate of larger individuals resulting in a higher exposure to infective stages (de Montaudouin et al 1998, Mouritsen et al 2003, Thieltges & Reise 2006. The distribution and abundance of the first intermediate host (the upstream host) seems to be important in determining infection levels in the second intermediate hosts (Thieltges & Reise 2006, 2007, but other factors also have an influence, including the density of the target host (Thieltges & Reise 2007), host condition (Mouritsen et al 2003) and the ambient benthic community (Thieltges et al 2008, Mouritsen & Poulin 2010.…”

Climate change, parasitism and the structure of intertidal ecosystems

“…The fact that the general activity of these ectothermic invertebrate hosts will also increase as a function of temperature, in turn increasing the likelihood of contact between host and parasite (see e.g. Mouritsen & Jensen, 1997;Mouritsen et al, 2003), only serves to reinforce the process. Given the ubiquitous presence and ecological importance of trematodes in intertidal systems (see above; Sousa, 1991;Mouritsen & Poulin, 2002b), global warming is therefore likely to induce community-wide changes mediated by increased trematode parasitism.…”

Climate influences parasite-mediated competitive release

“…As a further requirement, these circumstances have to coincide in spring prior to the amphipod's burst of summer reproduction, as a high density of second intermediate hosts will effectively dilute the number of parasites reaching individual hosts (e.g. Mouritsen et al 2003;Thieltges and Reise, 2007). These conditions were all met in 1990 and, interestingly, for the most parts correlated with the NAO index.…”