Freshening rather than warming drives trematode transmission from periwinkles to mussels

Abstract

In the Western Baltic Sea, climate change is happening at much faster rate than in most other seas and organisms are additionally exposed to a steep and variable salinity gradient. Climate change has previously been shown to affect parasite transmission in other marine ecosystems, yet little is known about potential effects of warming and desalination on parasite-host interactions. In laboratory experiments, we determined the combined effects of projected seawater warming and freshening on the emergence, activity, survival, and infectivity of cercariae (free-swimming infectious stage) of the trematode Himasthla elongata (Mehlis 1831), shed from its first intermediate host, the periwinkle Littorina littorea (Linnaeus 1758), in the Baltic Sea. We also assessed the susceptibility of the second intermediate host, the mussel Mytilus edulis Linnaeus, 1758, to cercarial infections. Generally, salinity was the main driver, particularly of cercarial activity, infectivity, and mussel susceptibility to infection. At the lowest salinity (13), cercariae were 50% less active compared to the highest salinity (19). Infection success and host susceptibility followed a similar pattern, with 47% and 43% less metacercariae (encysted stage) present at salinity 13 than at salinity 19, respectively. In contrast, effects of simulated warming were found only for cercarial survival, with cercarial longevity being higher at 19 than at 23 °C. No significant interactions between temperature and salinity were found. In contrast to the literature, the results suggest that a climate change-driven freshening (partly also warming) may lead to a general decline of marine trematodes, with possible beneficial effects for the involved hosts.

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

“…The cumulative experimental results detected in previous studies (Studer and Poulin, 2012 ; Bommarito et al ., 2020 ) are matching with our field data, generally suggesting that at reduced salinity trematode transmission to the second intermediate host drastically decreases due to the combined osmotic stress experienced by both, the cercariae and their hosts. However, in the field the effect of salinity on transmission might be further enhanced or buffered by additional biotic components.…”

“…( 2003 ). This tight association among marine trematodes and habitat hydrology is confirmed by experimental studies, which detected cercarial emergence, survival and transmission negatively affected by lower salinities (Lei and Poulin, 2011 ; Studer and Poulin, 2012 ; Bommarito et al ., 2020 ). Negative effects of reduced salinity in cercarial emergence might be attributed to a stress response by the first intermediate host (Lei and Poulin, 2011 ; Bommarito et al ., 2020 ), whereas negative effects on survival and infectivity could be attributed to the vulnerability of the cercaria itself (Pietrock and Marcogliese, 2003 ).…”

“…In their experimental approach, Bommarito et al . ( 2020 ) simulated a salinity range of 13–19. The study found H. elongata emergence, activity and infectivity to be significantly reduced at low salinity (i.e.…”

“…Temperature, salinity, pH and light are all known to affect the transmission success of free-living stages of many trematode species and thus infection levels in their intermediate hosts (Pietrock and Marcogliese, 2003 ). In marine ecosystems, salinity plays a particularly important role, and experimental studies on trematode cercariae under laboratory conditions indicate that salinity affects various steps in the transmission from one host to the next (Koprivnikar and Poulin, 2009 and Koprivnikar et al 2010 ; Lei and Poulin, 2011 ; Studer and Poulin, 2012 ; Bommarito et al ., 2020 ). Emergence, survival and activity of cercariae, all generally decrease with a reduction in salinity (but see Koprivnikar and Poulin, 2009 ).…”

“…Emergence, survival and activity of cercariae, all generally decrease with a reduction in salinity (but see Koprivnikar and Poulin, 2009 ). Susceptibility of the target host also appears to be negatively affected by reduced salinity (Bommarito et al ., 2020 ), further decreasing the chances of transmission to the target host. Hence, based on the information provided by laboratory studies, decreasing salinity is expected to limit the transmission success of trematode parasites, resulting in lower prevalence and abundances in first intermediate hosts at lower salinities.…”