Effects of cockle density and environmental background in the structure of free-living and parasitic communities

Biotic and abiotic factors can play an important role in the parasite community. The likelihood of successful parasite transmission rises with host density. At the same time, high host density can provide a dilution effect that lessens the host's individual burden. In this study, the effects of cockle (Cerastoderma edule) density and environmental factors in the structure of free-living and parasitic communities in two sites of the Ria de Aveiro (Portugal) were investigated, performing every other month sampling for one year. Thirty-five different taxa (2832 individuals) were identified, including Polychaeta (40.71%), Bivalvia (25.48%), Gastropoda (22.7%), and Arthropoda (8.3%) as the most prevalent clades. Multivariate analysis based on the macrofauna data showed that benthic community composition and abundance differed along the year, depending on the sampling site. Cockles' density, salinity, temperature, and sediment descriptors (namely median grain size and total organic matter content) were the best predictors of the differences found. Cockles were infected by six different trematode species represented by 1432 individuals. The most prevalent species were Gymnophallus minutus (70.74%), Himasthla quissetensis (25.14%) and Himasthla spp. (3.63%). The environmental factors influencing the trematode community composition were the median grain size, salinity, dissolved oxygen, and oxidation-reduction potential. It was demonstrated that the environmental background (cockles' density and related physicochemical differences) promotes in opposite ways the macrofauna and the parasitic diversity/abundance. The findings of this study, although preliminary and lacking validity in other areas, can have implications on decision making when stablishing cockle catch limits and resource protective measures.

Automated summary

Biotic and abiotic factors play important roles in parasite communities. Host density affects parasite transmission, while high host density can also reduce individual burden. This study investigated the effects of cockle density and environmental factors on the structure of free-living and parasitic communities. Analysis showed differences in benthic community composition and abundance based on sampling site and environmental factors. Different trematode species infected the cockles, with environmental factors influencing parasite community composition.