Effects of first intermediate host density, host size and salinity on trematode infections in mussels of the south-western Baltic Sea

Abstract

Trematode prevalence and abundance in hosts are known to be affected by biotic drivers as well as by abiotic drivers. In this study, we used the unique salinity gradient found in the south-western Baltic Sea to: (i) investigate patterns of trematode infections in the first intermediate host, the periwinkle Littorina littorea and in the downstream host, the mussel Mytilus edulis, along a regional salinity gradient (from 13 to 22) and (ii) evaluate the effects of first intermediate host (periwinkle) density, host size and salinity on trematode infections in mussels. Two species dominated the trematode community, Renicola roscovita and Himasthla elongata. Salinity, mussel size and density of infected periwinkles were significantly correlated with R. roscovita, and salinity and density correlated with H. elongata abundance. These results suggest that salinity, first intermediate host density and host size play an important role in determining infection levels in mussels, with salinity being the main major driver. Under expected global change scenarios, the predicted freshening of the Baltic Sea might lead to reduced trematode transmission, which may be further enhanced by a potential decrease in periwinkle density and mussel size.

Heat sensitivity of first host and cercariae may restrict parasite transmission in a warming sea

“…These effects of trematodes on their gastropod host can go beyond the individual level and can result in a cascade of effects that compromises the structure and functioning of communities 45 . In the case of H. elongata , the prevalence of infection has been reported up to 40% in the southwestern Baltic Sea 46 . A high prevalence of infection combined with decreased thermal tolerance could translate into two possible (mutually non-exclusive) scenarios.…”

Combined effects of salinity and trematode infections on the filtration capacity, growth and condition of mussels

“…Around 500 were collected from Årøsund (Denmark) (55.25°N, 9.70°E) in November 2017 (see Table 1). From previous samplings, this site is known to contain a good prevalence of infected individuals (around 30 and 25% of individual snails are infected with Himasthla elongata and Renicola roscovita respectively in this area; Bommarito et al 2021). To determine their infection status, L. littorea individuals were placed in 6-well plates (1 snail well −1 ) filled with aerated filtered seawater and exposed to heating lamps at 25−27°C for 3−4 h to induce cercariae shedding.…”

“…In addition to salinity as a dominant abiotic driver in the Baltic Sea, the ecological performance of mussels can also be affected by biotic interactions such as parasitism. Mussels are often infected with trematodes that use them as second intermediate hosts in which the parasites form metacercariae that await being ingested by the final host (birds or fish, depending on the species; Lauckner 1983, Bommarito et al 2021. Metacercarial infections can cause structural tissue damage and inflammation that can compromise the functioning of mussels.…”

“…Metacercarial infections can cause structural tissue damage and inflammation that can compromise the functioning of mussels. Two common species of trematodes infecting the common periwinkle Littorina littorea as a first intermediate host and mussels as a second intermediate host in the Baltic Sea are Renicola roscovita and Himasthla elongata (Werding 1969, Bommarito et al 2021. Infections with R. roscovita can be found mainly in the labial palps and gill tissues where they can lead to a reduction in mussel filtration and growth rates (Thieltges 2006, Stier et al 2015.…”

Biotic and abiotic drivers affect parasite richness, prevalence and abundance inMytilus galloprovincialisalong the Northern Adriatic Sea

“…Previous experimental studies highlighted the prominent role of salinity in the transmission of trematodes’ free-living stages (Koprivnikar and Poulin, 2009; Koprivnikar et al ., 2010; Lei and Poulin, 2011; Studer and Poulin, 2012; Bommarito et al ., 2020 a ), showing reduced transmission at lower salinities caused by osmotic stress of both the parasite and its host. Lower transmission may, in turn, results in lower prevalence and abundance of parasites in the target host (Bommarito et al ., 2020 b ).…”