Consequences of eye fluke infection on anti-predator behaviours in invasive round gobies in Kalmar Sound

Evaluation of cataract formation in fish exposed to environmental radiation at Chernobyl and Fukushima

Recent studies apparently finding deleterious effects of radiation exposure on cataract formation in birds and voles living near Chernobyl represent a major challenge to current radiation protection regulations. This study conducted an integrated assessment of radiation exposure on cataractogenesis using the most advanced technologies available to assess the cataract status of lenses extracted from fish caught at both Chernobyl in Ukraine and Fukushima in Japan. It was hypothesised that these novel data would reveal positive correlations between radiation dose and early indicators of cataract formation. The structure, function and optical properties of lenses were analysed from atomic to millimetre length scales. We measured the short-range order of the lens crystallin proteins using Small Angle X-Ray Scattering (SAXS) at both the SPring-8 and DIAMOND synchrotrons, the profile of the graded refractive index generated by these proteins, the epithelial cell density and organisation and finally the focal length of each lens. The results showed no evidence of a difference between the focal length, the epithelial cell densities, the refractive indices, the interference functions and the short-range order of crystallin proteins (X-ray diffraction patterns) in lens from fish exposed to different radiation doses. It could be argued that animals in the natural environment which developed cataract would be more likely, for example, to suffer predation leading to survivor bias. But the cross-length scale study presented here, by evaluating small scale molecular and cellular changes in the lens (pre-cataract formation) significantly mitigates against this issue.

Boldness and physiological variation in round goby populations along their Baltic Sea invasion front

The round goby (Neogobius melanostomus) is a fish native to the Ponto-Caspian region that is highly invasive through freshwater and brackish habitats in northern Europe and North America. Individual behavioural variation appears to be an important factor in their spread, for example a round goby's personality traits can influence their dispersal tendency, which may also produce variation in the behavioural composition of populations at different points along their invasion fronts. To further analyze the drivers of behavioural variation within invasive round goby populations, we focused on two populations along the Baltic Sea invasion front with closely comparable physical and community characteristics. Specifically, this study measured personality within a novel environment and predator response context (i.e., boldness), and directly analyzed links between individuals' personality traits and their physiological characteristics and stress responses (i.e., blood cortisol and lactate, brain neurotransmitters). In contrast to previous findings, the more recently established population had similar activity levels but were less bold in response to a predator cue than the older population, which suggests that behavioural compositions within our study populations may be more driven by local environmental conditions rather than being a result of personality-biased dispersal. Furthermore, we found that both populations showed similar physiological stress responses, and there also appeared to be no detectable relationship between physiological parameters and behavioural responses to predator cues. Instead, body size and body condition were important factors influencing individual behavioural responses. Overall, our results reinforce the importance of boldness traits as a form of phenotypic variation in round goby populations in the Baltic Sea. We also highlight the importance of these traits for future studies specifically testing for effects of invasion processes on phenotypic variation in the species. Nonetheless, our results also highlight that the physiological mechanisms underpinning behavioural variation in these populations remain unclear.

Increased parasite load is associated with reduced metabolic rates and escape responsiveness in pumpkinseed sunfish

Wild animals have parasites that can compromise their physiological and/or behavioural performance. Yet, the extent to which parasite load is related to intraspecific variation in performance traits within wild populations remains relatively unexplored. We used pumpkinseed sunfish (Lepomis gibbosus) and their endoparasites as a model system to explore the effects of infection load on host aerobic metabolism and escape performance. Metabolic traits (standard and maximum metabolic rates, aerobic scope) and fast-start escape responses following a simulated aerial attack by a predator (responsiveness, response latency, and escape distance) were measured in fish from across a gradient of visible (i.e. trematodes causing black spot disease counted on fish surfaces) and non-visible (i.e. cestodes in fish abdominal cavity counted post-mortem) endoparasite infection. We found that a higher infection load of non-visible endoparasites was related to lower standard and maximum metabolic rates, but not aerobic scope in fish. Non-visible endoparasite infection load was also related to decreased responsiveness of the host to a simulated aerial attack. Visible endoparasites were not related to changes in metabolic traits nor fast-start escape responses. Our results suggest that infection with parasites that are inconspicuous to researchers can result in intraspecific variation in physiological and behavioral performance in wild populations, highlighting the need to more explicitly acknowledge and account for the role played by natural infections in studies of wild animal performance.

Eye fluke effects on Danish freshwater fish: Field and experimental investigations

Eye flukes in fish are common in freshwater lakes. Fish become infected by the penetration of cercariae released from freshwater snails, and high infection pressures may be associated with mortalities in a Danish lake. Examination of two other freshwater lakes, combined with laboratory study, supported the notion. We investigated 77 freshwater fish from two lakes and the infection level suggested the occurrence of a high cercarial infection pressure in the Danish lakes. Dominant genera were Tylodelphys and Diplostomum covering a range of species identified by PCR and sequencing of the 18S (partial)-ITS1-5.8S-ITS2-28S (partial) of the rDNA. Cercariae of the prevalent species Diplostomum pseudospathaceum were used to infect zebrafish Danio rerio for the elucidation of short-term effects on the fish host. Zebrafish did not display abnormal behaviour when exposed to 200-400 cercariae, but a dosage of 600 and 1,000 cercariae/fish proved lethal. When fish were exposed to sublethal dosages, 19 out of 27 immune genes were significantly regulated and three genes encoding cytokine (IL 4/13B, IL-6 and IL-8) were upregulated at 3 hr post-infection (hpi), whereas others were downregulated especially at a later time point. We suggest that direct massive cercarial penetration of fish surfaces may be detrimental and may represent a threat to fish populations.

Studies on the population biology of helminth parasites of fish species from the Caspian Sea drainage basin

The northern section of the Caspian Sea and lower reaches of the Zhaiyk (Ural) River is an important fishery for Kazakhstan. In the present study, a total of 1597 individuals of ten fish species were analysed. The fish were caught over three years, from 2018 to 2020. For each species studied - Abramis brama, Alosa saposchnikowii, Atherina boyeri caspia, Carassius gibelio, Chelon auratus, Cyprinus carpio, Leuciscus aspius, Rutilus caspius, Sander marinus and Sander volgensis - between 100 and 200 individuals were examined. A series of generalized linear models (GLMs) were used to examine the association between individual parasite intensity of infection and the Fulton index, age, year the fish was captured, where the fish was captured (northern Caspian or Zhaiyk River) and sex. For each GLM, the best-fitting probability distribution was used -either Poisson, zero-inflated Poisson, negative binomial or zero-inflated negative binomial. For some fish/parasite species, an increased Fulton index was associated with higher intensities of parasite infection, whilst, for others, the Fulton index decreased with the intensity of parasite infection. This was also true of age-related intensity of infection, with some parasites having an increased intensity of infection with age whilst others had a decreased intensity of infection with age. There was also some evidence of variation in intensity of parasite infection between different years when the fish were caught. For some species of fish that are endemic to both the fresh waters of the Zhaiyk River and the low-saline waters of the northern Caspian, there were variations in intensity of parasite infection between the two environments. The best-fitting probability distribution also gave some information about the dynamics of infection. No fish species had a Poisson distribution of parasites, which is consistent with an entirely random infection process, with all fish being potentially exposed. For some parasites, the distribution was a zero-inflated Poisson, which is consistent with either the fish being exposed to parasite infection or not; and, if exposed, infection was a random process. Other parasites had a negative binomial distribution, consistent with the entire fish population being exposed, but the infection process was clumped or there were variations in the susceptibility of infection between fish. Finally, some of the parasites had a zero-inflated negative binomial distribution, which can be interpreted as part of the fish population not being exposed and the remainder of the population being exposed to a clumped or aggregated infection process and/or a variation in individual susceptibility to infection.

Eye fluke infection changes diet composition in juvenile European perch (Perca fluviatilis)

Intraspecific diet specialization, usually driven by resource availability, competition and predation, is common in natural populations. However, the role of parasites on diet specialization of their hosts has rarely been studied. Eye flukes can impair vision ability of their hosts and have been associated with alterations of fish feeding behavior. Here it was assessed whether European perch (Perca fluviatilis) alter their diet composition as a consequence of infection with eye flukes. Young-of-the-year (YOY) perch from temperate Lake Müggelsee (Berlin, Germany) were sampled in two years, eye flukes counted and fish diet was evaluated using both stomach content and stable isotope analyses. Perch diet was dominated by zooplankton and benthic macroinvertebrates. Both methods indicated that with increasing eye fluke infection intensity fish had a more selective diet, feeding mainly on the benthic macroinvertebrate Dikerogammarus villosus, while less intensively infected fish appeared to be generalist feeders showing no preference for any particular prey type. Our results show that infection with eye flukes can indirectly affect interaction of the host with lower trophic levels by altering the diet composition and highlight the underestimated role of parasites in food web studies.

The Enemy Within: How Does a Bacterium Inhibit the Foraging Aptitude and Risk Management Behavior of Allenby's Gerbils?

AbstractMicrobes inhabiting multicellular organisms have complex, often subtle effects on their hosts. Gerbillus andersoni allenbyi are commonly infected with Mycoplasma haemomuris-like bacteria, which may cause mild nutrient (choline, arginine) deficiencies. However, are there more serious ecological consequences of infection, such as effects on foraging aptitudes and risk management? We tested two alternatives: the nutrient compensation hypothesis (does nutrient deficiency induce infected gerbils to make up for the shortfall by foraging more and taking greater risks?) and (2) the lethargy hypothesis (do sick gerbils forage less, and are they compromised in their ability to detect predators or risky microhabitats?). We compared the foraging and risk management behavior of infected and noninfected gerbils. We experimentally infected gerbils with the bacteria, which allowed us to compare between noninfected, acutely infected (peak infection loads), and chronically infected (low infection loads) individuals. Our findings supported the lethargy hypothesis over the nutrient compensation hypothesis. Infected individuals incurred dramatically elevated foraging costs, including less efficient foraging, diminished "quality" of time spent vigilant, and increased owl predation. Interestingly, gerbils that were chronically infected (lower bacteria load) experienced larger ecological costs than acutely infected individuals (i.e., peak infection loads). This suggests that the debilitating effects of infection occur gradually, with a progressive decline in the quality of time gerbils allocated to foraging and managing risk. These increased long-term costs of infection demonstrate how small direct physiological costs of infection can lead to large indirect ecological costs. The indirect ecological costs of this parasite appear to be much greater than the direct physiological costs.

Eye fluke (Tylodelphys clavata) infection impairs visual ability and hampers foraging success in European perch

Visual performance and environmental conditions can influence both behavioral patterns and predator-prey interactions of fish. Eye parasites can impair their host's sensory performance with important consequences for the detection of prey, predators, and conspecifics. We used European perch (Perca fluviatilis) experimentally infected with the eye fluke Tylodelphys clavata and evaluated their feeding behavior and competitive ability under competition with non-infected conspecifics, in groups of four individuals, for two different prey species (Asellus aquaticus and Daphnia magna). To test whether the effect of T. clavata infection differs at different light conditions, we performed the experiments at two light intensities (600 and 6 lx). Foraging efficiency of perch was significantly affected by infection but not by light intensity. The distance at which infected fish attacked both prey species was significantly shorter in comparison to non-infected conspecifics. Additionally, infected fish more often unsuccessfully attacked A. aquaticus. Although the outcome of competition depended on prey species, there was a general tendency that non-infected fish consumed more of the available prey under both light intensities. Even though individual prey preferences for either A. aquaticus or D. magna were observed, we could not detect that infected fish change their prey preference to compensate for a reduced competitive foraging ability. As infection of T. clavata impairs foraging efficiency and competitive ability, infected fish would need to spend more time foraging to attain similar food intake as non-infected conspecifics; this presumably increases predation risk and potentially enhances transmission success to the final host.

Impairment of retinal function in yellow perch (Perca flavescens) by Diplostomum baeri metacercariae

Histologic studies of fish from Douglas Lake, Cheboygan County, Michigan, USA show that Diplostomum spp. infect the lens of spottail shiners (Notropis hudsonius) and common shiners (Luxilus cornutus). In contrast, infection was confined to the choroidal vasculature of yellow perch (Perca flavescens), and the morphology of the pigment epithelium and retina in regions adjacent to the metacercariae was abnormal. The difference in location of metacercariae within the host suggested that different Diplostomum species may infect shiners and perch in Douglas Lake. Species diversity was investigated by sequencing the barcode region of the cytochrome oxidase I gene of metacercariae. Four species of Diplostomum were identified, all four of which were present in shiner lenses; however, only Diplostomum baeri was present in the perch choroid. To determine whether infection of perch eyes affects the response of the retina to a light stimulus, electroretinograms (ERG) were recorded. The amplitude of the b-wave of the ERG was reduced and the b-wave latency was increased in infected perch, as compared to uninfected eyes, and the flicker-fusion frequency was also reduced. Infection of the yellow perch choroid by Diplostomum baeri, which shows strong host and tissue specificity, has an adverse effect on retinal function, lending support to the hypothesis that parasite-induced impairment of host vision may afford Diplostomum baeri the evolutionary benefit of increasing the likelihood of transmission, via host fish predation, to its definitive avian host.