Parasitic infection alters the physiological response of a marine gastropod to ocean acidification

Parasites and Pollutants: Effects of Multiple Stressors on Aquatic Organisms

Parasites can affect their hosts in various ways, and this implies that parasites may act as additional biotic stressors in a multiple-stressor scenario, resembling conditions often found in the field if, for example, pollutants and parasites occur simultaneously. Therefore, parasites represent important modulators of host reactions in ecotoxicological studies when measuring the response of organisms to stressors such as pollutants. In the present study, we introduce the most important groups of parasites occurring in organisms commonly used in ecotoxicological studies ranging from laboratory to field investigations. After briefly explaining their life cycles, we focus on parasite stages affecting selected ecotoxicologically relevant target species belonging to crustaceans, molluscs, and fish. We included ecotoxicological studies that consider the combination of effects of parasites and pollutants on the respective model organism with respect to aquatic host-parasite systems. We show that parasites from different taxonomic groups (e.g., Microsporidia, Monogenea, Trematoda, Cestoda, Acanthocephala, and Nematoda) clearly modulate the response to stressors in their hosts. The combined effects of environmental stressors and parasites can range from additive, antagonistic to synergistic. Our study points to potential drawbacks of ecotoxicological tests if parasite infections of test organisms, especially from the field, remain undetected and unaddressed. If these parasites are not detected and quantified, their physiological effects on the host cannot be separated from the ecotoxicological effects. This may render this type of ecotoxicological test erroneous. In laboratory tests, for example to determine effect or lethal concentrations, the presence of a parasite can also have a direct effect on the concentrations to be determined and thus on the subsequently determined security levels, such as predicted no-effect concentrations. Environ Toxicol Chem 2023;42:1946-1959. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Filtration and respiration responses of mussels (Mytilus edulis) to trematode parasite infections (Renicola roscovita) and transient heat exposure

The mussel Mytilus edulis, a host to various trematode species, experiences performance decrements due to these infections. Yet, the impact magnitude and potential interactions with environmental stressors remain largely unexplored. This study scrutinizes the effect of Renicola roscovita infections on mussel filtration and respiration. We first assessed performance in both uninfected and lab-infected mussels at a mild temperature (16 °C), following an acute heat ramp to 30.5 °C and subsequent cooling. The experiment revealed neither a significant direct impact of the infection on the mussels' performance, nor any significant interplay between the infection and temperature variations. To account for possible infection effects obscured by low sample sizes or mussel size disparities, we conducted a reassessment at 16 °C using both small and large mussels. Infection notably hampered filtration in large mussels, with a marginal impact on smaller ones. A positive correlation was found between infection intensity and mussel filtration capacity, though the infection had no discernible impact on respiration. Our consistent finding of an 11-12 % infection effect size across all experiments indicates a slight reduction in mussel filtration due to trematode infections. While the exacerbating effect of transient heat stress on the infection's impact on filtration was not statistically significant, future investigations should explore potential interactions with prolonged heat stress. Our findings underscore the nuanced ways in which parasitic infections can influence marine bivalve physiology, emphasizing the need for more comprehensive studies that incorporate environmental stressors, such as heat stress, to fully elucidate the impact of parasitism on marine ecosystem health and resilience.

Parasitism by metacercariae modulates the morphological, organic and mechanical responses of the shell of an intertidal bivalve to environmental drivers

Environmental variation alters biological interactions and their ecological and evolutionary consequences. In coastal systems, trematode parasites affect their hosts by disrupting their life-history traits. However, the effects of parasitism could be variable and dependent on the prevailing environmental conditions where the host-parasite interaction occurs. This study compared the effect of a trematode parasite in the family Renicolidae (metacercariae) on the body size and the shell organic and mechanical characteristics of the intertidal mussels Perumytilus purpuratus, inhabiting two environmentally contrasting localities in northern and central Chile (ca. 1600 km apart). Congruent with the environmental gradient along the Chilean coast, higher levels of temperature, salinity and pCO₂, and a lower pH characterise the northern locality compared to that of central Chile. In the north, parasitised individuals showed lower body size and shell resistance than non-parasitised individuals, while in central Chile, the opposite pattern was observed. Protein level in the organic matter of the shell was lower in the parasitised hosts than in the non-parasitised ones regardless of the locality. However, an increase in polysaccharide levels was observed in the parasitised individuals from central Chile. These results evidence that body size and shell properties of P. purpuratus vary between local populations and that they respond differently when confronting the parasitism impacts. Considering that the parasite prevalence reaches around 50% in both populations, if parasitism is not included in the analysis, the true response of the host species would be masked by the effects of the parasite, skewing our understanding of how environmental variables will affect marine species. Considering parasitism and identifying its effects on host species faced with environmental drivers is essential to understand and accurately predict the ecological consequences of climate change.

Marine Parasites and Disease in the Era of Global Climate Change

Climate change affects ecological processes and interactions, including parasitism. Because parasites are natural components of ecological systems, as well as agents of outbreak and disease-induced mortality, it is important to summarize current knowledge of the sensitivity of parasites to climate and identify how to better predict their responses to it. This need is particularly great in marine systems, where the responses of parasites to climate variables are less well studied than those in other biomes. As examples of climate's influence on parasitism increase, they enable generalizations of expected responses as well as insight into useful study approaches, such as thermal performance curves that compare the vital rates of hosts and parasites when exposed to several temperatures across a gradient. For parasites not killed by rising temperatures, some simple physiological rules, including the tendency of temperature to increase the metabolism of ectotherms and increase oxygen stress on hosts, suggest that parasites' intensity and pathologies might increase. In addition to temperature, climate-induced changes in dissolved oxygen, ocean acidity, salinity, and host and parasite distributions also affect parasitism and disease, but these factors are much less studied. Finally, because parasites are constituents of ecological communities, we must consider indirect and secondary effects stemming from climate-induced changes in host-parasite interactions, which may not be evident if these interactions are studied in isolation.

How costly are metacercarial infections in a bivalve host? Effects of two trematode species on biochemical performance of cockles

Bivalve stocks have been decreasing in the last decades largely due to emergent diseases and consequent mass mortality episodes. Cerastoderma edule (the edible cockle) is one of the most exploited bivalves in Europe and is among the most common hosts for trematodes, the most prevalent macroparasites in coastal waters but yet poorly studied. Therefore, in the present study, this bivalve species was used as host model to determine if trematode infection exerts a negative effect on bivalve energy metabolism and balance and if the tissues targeted by different trematodes influence the metabolic cost, with physiological and biochemical consequences. Cockles were experimentally infected with two trematode species, Himasthla elongata and Renicola roscovitus, that infect the foot and palps, respectively. Trematode infection exerted a negative effect on the metabolism of C. edule, the second intermediate host, by reduction of oxygen consumption. A different host biochemical response was found depending on trematode species, especially in regard to the level of oxygen consumption decrease and the preferential accumulation of lipids and glycogen. This study represents a step towards the understanding of host-trematode relationships that can be used to better predict potential conservation threats to bivalve populations and to maximize the success of stock and disease management.

Oxygen consumption during and post-hypoxia exposure in bearded fireworms (Annelida: Amphinomidae)

Oxygen is necessary for all marine animals to support metabolic functions. When chronic low dissolved oxygen (DO) conditions occur, organisms must adjust to overcome this stressor's effect on metabolic rates. The bearded fireworm, Hermodice carunculata, is a widespread species frequently exposed to hypoxic conditions in areas within its broad distribution which may impact metabolism, wound healing, and regeneration. To study the impact of hypoxia on their metabolic rates, we exposed fireworms to two levels of lower than normal DO conditions (low 2.5 ± 0.25 mg O₂ L^-1 and mid 4.5 ± 0.25 mg O₂ L^-1) for 7 days by pumping nitrogen into their holding tanks. During a chronic hypoxia trial, we quantified oxygen consumption in each experimental group and subsequently determined post-hypoxia oxygen consumption of individuals from the lowest oxygen level. During the hypoxic exposure, the oxygen uptake rates declined in low and mid DO conditions, while remaining relatively constant for the normoxic (7.0 ± 0.25 mg O₂ L^-1) control. We then compared the oxygen consumption rates from the lowest DO condition to fireworms likely never exposed to hypoxia and fireworms from a location likely to be exposed to hypoxia. We found higher oxygen consumption rates in the experimentally hypoxia-exposed worms. These results suggest prolonged negative impacts of hypoxic exposure, leading to a lasting elevation of metabolic rates of these marine invertebrates. The increase in metabolic rates may lead to increased predation on their prey of choice, economically and commercially important coral, causing increased degradation of already threatened coral reef ecosystems.

Functional responses of intertidal bivalves to repeated sub-lethal, physical disturbances

In coastal habitats, physical disturbances of benthic organisms can be caused by natural events like wave-born objects and human activity like trampling, and these disturbances can be sub-lethal (e.g., resulting in the organism's displacement). We know little of how sessile organisms respond to physical disturbances such as displacements. Using Mytilaster minimus, a mussel that is native to the Mediterranean Sea, we tested how byssus production and oxygen uptake rates changed in response to different frequencies of disturbance events (10-60 events h^-1). Mussels increased oxygen uptake rates but not byssus production with increasing disturbance frequencies (50-60 events h^-1). Our results show that sub-lethal, physical disturbances can cause increased physiological rates in mussels if disturbances repeat rapidly. Therefore, sub-lethal, physical disturbances can have negative consequences for benthic organisms even if they do not cause immediate damage or mortality.

Trematode infection modulates cockles biochemical response to climate change

Resulting mainly from atmospheric carbon dioxide (CO₂) build-up, seawater temperature rise is among the most important climate change related factors affecting costal marine ecosystems. Global warming will have implications on the water cycle, increasing the risk of heavy rainfalls and consequent freshwater input into the oceans but also increasing the frequency of extreme drought periods with consequent salinity increase. For Europe, by the end of the century, projections describe an increase of CO₂ concentration up to 1120 ppm (corresponding to 0.5 pH unit decrease), an increase in the water temperature up to 4 °C and a higher frequency of heavy precipitation. These changes are likely to impact many biotic interactions, including host-parasite relationships which are particularly dependent on abiotic conditions. In the present study, we tested the hypothesis that the edible cockle, Cerastoderma edule, exposed to different salinity, temperature and pH levels as proxy for climate change, modify the infection success of the trematode parasite Himasthla elongata, with consequences to cockles biochemical performance. The results showed that the cercariae infection success increased with acidification but higher biochemical alterations were observed in infected cockles exposed to all abiotic experimental stressful conditions tested. The present study suggested that changes forecasted by many models may promote the proliferation of the parasites infective stages in many ecosystems leading to enhanced transmission, especially on temperate regions, that will influence the geographical distribution of some diseases and, probably, the survival capacity of infected bivalves.