Parasitism of an intertidal foundation species: Allometric scaling and environmental constraints on trematode distribution in barnacles across biological scales

Parasites of foundation species, such as fucoids, mussels, and barnacles, can shape the intertidal communities that depend on them, yet their effects remain poorly understood. On rocky shores of the Northeast Pacific, the acorn barnacle Balanus glandula forms dense aggregations that provide habitat for other species. We investigated infection by the microphallid trematode Maritrema gratiosum in its second intermediate host, B. glandula, in British Columbia, Canada. First, we described the metacercarial cysts and examined how host morphology (in spring and summer) and reproductive output relate to infection. Second, we examined how infection varies with host position on live mussel shells, substratum type (rock versus mussel shells), and shore height. Third, we related prevalence and parasite load (number of cysts per host) to local air and sea-surface temperatures. Cysts had a mean diameter of 604 ± 46 µm and 80% of them occurred in fewer than 20% of hosts. Host size predicted infection differently at two scales: within sites, larger barnacles were more likely to be infected, but among sites, infection was most likely in populations of intermediate mean size. Among infected hosts, parasite load was associated with host shape and became strongly size-dependent later in the season. Brood mass scaled hyper-allometrically with somatic mass (b = 1.27) and was unrelated to parasite load, while the scaling of parasite load with host mass differed among sites. Infection intensity (cysts per g of somatic tissue) did not vary with host position on mussels or between substrata, but varied unimodally with shore height, peaking at ~2.9 m above chart datum. At a larger spatial scale around Vancouver Island, 70.7% of barnacles were infected with site prevalence ranging from 0 to 100%. Infection probability increased with host size but was unrelated to temperature. Parasite load, however, increased with host size and with warmer-than-usual sea-surface temperatures, but decreased with warmer-than-usual air temperatures. Relationships among individual hosts did not consistently hold across sites, seasons, or thermal environments, suggesting that host demography, immersion time, and parasite supply jointly structure infection in this intertidal foundation species.