Climate change imposes unusual long‐term trends in environmental conditions, plus some tremendous shifts in short‐term environmental variability, exerting additional stress on marine ecosystems. This paper describes an empirical method that aims to improve our understanding of the performance of benthic filter feeders experiencing changes in environmental conditions, such as temperature, on time scales of minutes to hours, especially during daily cycles or extreme events such as marine heatwaves or hypoxic upwelling. We describe the Fluorometer and Oximeter equipped Flow‐through Setup (FOFS), experimental design, and methodological protocols to evaluate the flood of data, enabling researchers to monitor important energy budget traits, including filtration and respiration of benthic filter‐feeders in response to fine‐tuned environmental variability. FOFS allows online recording of deviations in chlorophyll and dissolved oxygen concentrations induced by the study organism. Transparent data processing through Python scripts provides the possibility to adjust procedures to needs when working in different environmental contexts (e.g., temperature vs. pH, salinity, oxygen, biological cues) and with different filter‐feeding species. We successfully demonstrate the functionality of the method through recording responses of Baltic Sea blue mussels (Mytilus) during one‐day thermal cycles. This method practically provides a tool to help researchers exposing organisms to environmental variability for some weeks or months, to relate the observed long‐term performance responses to short‐term energy budget responses, and to explain their findings with the potential to generalize patterns. The method, therefore, allows a more detailed description of stress‐response relationships and the detection of species' tolerance limits.

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.

Background: Avian schistosomes comprise a diverse and widespread group of trematodes known for their surprising ability to switch into new hosts and habitats. Their cercariae are the causative agents of the waterborne allergic disease cercarial dermatitis. There are 13 recognised genera of avian schistosomes with majority of the extant species with freshwater-based life-cycles. Despite the considerable research attention on avian schistosomes much less it is known about the diversity, geographical range and host associations of the marine representatives. Here, we provide novel data on the species diversity and host-parasite relationships of the marine schistosomes in respect to their intermediate gastropod host from the Persian Gulf. Methods: A total of 1,745 horn snails of Pirenella cingulata (Gmelin), were examined during a survey from December 2019 to February 2020 from eight distinct locations along the coast of Iran. Partial sequences of the mitochondrial cytochrome c oxidase subunit 1 (cox1) and the nuclear 28S rRNA gene were generated for the schistosome isolates recovered and used for molecular identification and phylogenetic reconstruction. Results: Our molecular analyses inferred from both molecular markers revealed presence of two schistosome species, Ornithobilharzia canaliculata (Rudolphi, 1819) Odhner, 1912 and a putative new species of Austrobilharzia Johnston, 1917. Molecular elucidation of the life-cycle of O. canaliculata was achieved for the first time via matching novel and published sequence data from adult and larval stages. This is the first record of Ornithobilharzia from the Persian Gulf and globally the first record of this genus in a potamidid snail host.Conclusions: The present study provides new host and distribution records for major etiological agents of cercarial dermatitis and contributes important information on host-parasite relationships. Our study further highlights the importance of the molecular systematics in the assessment of schistosome diversity and calls for further surveys in order to reach a better understanding of the schistosome diversity and patterns of relationships among them, host associations, transmission strategies and distribution coverage.

Predators can affect parasite–host interactions when directly preying on hosts or their parasites. However, predators may also have non‐consumptive indirect effects on parasite–host interactions when hosts adjust their behaviour or physiology in response to predator presence. In this study, we examined how chemical cues from a predatory marine crab affect the transmission of a parasitic trematode from its first (periwinkle) to its second (mussel) intermediate host. Laboratory experiments revealed that chemical cues from crabs lead to a threefold increase in the release of trematode cercariae from periwinkles as a result of increased periwinkle activity. This positive effect on transmission was contrasted by a 10‐fold reduction in cercarial infection rates in the second intermediate host when we experimentally exposed mussels to cercariae and predator cues. The low infection rates were caused by a substantial reduction in mussel filtration activity in the presence of predator cues, preventing cercariae from entering the mussels. To assess the combined net effect of both processes, we conducted a transmission experiment between infected periwinkles and uninfected mussels. Infection levels of mussels in the treatments with crab cues were sevenfold lower than in mussels without crab chemical cues. This suggests that predation risk effects on mussel susceptibility can counteract the elevated parasite release from first intermediate hosts, with negative net effects on parasite transmission. These experiments highlight that predation risk effects on parasite transmission can have opposing directions at different stages of the parasite's life cycle. Such complex non‐consumptive predation risk effects on parasite transmission may constitute an important indirect mechanism affecting prevalence and distribution patterns of parasites in different hosts across their life cycle.

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The blue mussel (Mytilus species complex) is an important ecosystem engineer, and salinity can be a major abiotic driver of mussel functioning in coastal ecosystems. However, little is known about the interactive effects of abiotic drivers and trematode infection. This study investigated the combined effects of salinity and Himasthla elongata and Renicola roscovita metacercarial infections on the filtration capacity, growth, and condition of M. edulis from the Baltic Sea. In a laboratory experiment, groups of infected and uninfected mussels were exposed to a wide range of salinities (6-30, in steps of 3) for 1 mo. Shell growth was found to be positively correlated with salinity and optimal at 18-24 at the end of the experiment, imposed by constraints in shell calcification under lower salinities. Mussel shell growth was not affected by H. elongata infection. While salinity had only a minor effect on tissue dry weight, infected mussels had a significantly lower tissue dry weight than uninfected mussels. Most interestingly, the combination of salinity and trematode infections negatively affected the mussels’ condition indices at lower salinity levels (6 and 9), suggesting that trematode infections are more detrimental to mussels when combined with freshening. A significant positive effect of salinity on mussel filtration was found, with an initial optimum at salinity 18 shifting to 18-24 by the end of the experiment. These findings indicate that salinity and parasite infections act as synergistic stressors for mussels, and enhance the understanding of potential future ecosystem shifts under climate change-induced freshening in coastal waters.

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Background Avian schistosomes comprise a diverse and widespread group of trematodes known for their surprising ability to switch into new hosts and habitats. Their cercariae are the causative agents of the waterborne allergic disease cercarial dermatitis. There are 13 recognised genera of avian schistosomes with majority of the extant species with freshwater-based life-cycles. Despite the considerable research attention on avian schistosomes much less it is known about the diversity, geographical range and host associations of the marine representatives. Here, we provide novel data on the species diversity and host-parasite relationships of the marine schistosomes in respect to their intermediate gastropod host from the Persian Gulf. Methods A total of 1,745 horn snails of Pirenella cingulata (Gmelin), were examined during a survey from December 2019 to February 2020 from eight distinct locations along the coast of Iran. Partial sequences of the mitochondrial cytochrome c oxidase subunit 1 (cox1) and the nuclear 28S rRNA gene were generated for the schistosome isolates recovered and used for molecular identification and phylogenetic reconstruction. Results Our molecular analyses inferred from both molecular markers revealed presence of two schistosome species, Ornithobilharzia canaliculata (Rudolphi, 1819) Odhner, 1912 and a putative new species of Austrobilharzia Johnston, 1917. Molecular elucidation of the life-cycle of O. canaliculata was achieved for the first time via matching novel and published sequence data from adult and larval stages. This is the first record of Ornithobilharzia from the Persian Gulf and globally the first record of this genus in a potamidid snail host. Conclusions The present study provides new host and distribution records for major etiological agents of cercarial dermatitis and contributes important information on host-parasite relationships. Our study further highlights the importance of the molecular systematics in the assessment of schistosome diversity and calls for further surveys in order to reach a better understanding of the schistosome diversity and patterns of relationships among them, host associations, transmission strategies and distribution coverage.