Shining new light on naupliar eyes: A novel molecular phylogeny for Pleuromamma (Family: Metridinidae) and the characterization of luciferase and opsin expression.

Pleuromamma (Giesbrecht, 1898) is a cosmopolitan genus of metridinid copepods, with species that perform remarkable diel vertical migrations (DVM) and emit a bioluminescent secretion when disturbed that varies both spectrally and kinetically. Copepod bioluminescence is autogenic and uses luciferase enzymes that catalyze a luciferin, coelenterazine, to produce light. Pleuromamma possess naupliar eyes, relatively simple photosensitive structures used for many visually-guided behaviors. Yet the fundamental molecular unit for vision, the opsin protein, has not been previously described for the family. The light producers and detectors are important to study because DVM is a behavior that mediates significant active elemental fluxes between the upper ocean and midwaters across vast stretches of oceanic habitat, and DVM is guided by visual behaviors, with animals tracking an isolume. Here we provide the first fully resolved molecular phylogeny for Pleuromamma (Family: Metridinidae) and describe the luciferase and opsin gene diversity and expression using de novo assembled transcriptomes. We successfully sequenced and assembled transcriptomes for 10 of 11 described species of Pleuromamma as well as two other metridinid species: Metridia longa and Gaussia princeps. In all species, we obtained coding sequences of one putative rhabdomeric middle wavelength sensitive visual opsin gene, as well as several non-visual opsins - a c-type pteropsin and a tetra-opsin type peropsin. Furthermore, Pleuromamma express luciferases from each of two main evolutionary clades (Luc1 and Luc2), and a single paralog (Luc2a) dominates expression throughout the group. The variation in luciferase number, sequence, and expression among species could lead to different spectral and kinetic properties of bioluminescence and aid in congener recognition.

A hidden lethal effect of long microplastic fibres on the coastal copepod Acartia erythraea.

We investigated the lethal effects of a long fibrous microplastic (the range of lengths, widths, and thicknesses were 1.5-3.6 mm, 10-16 µm, and 7-8.5 µm, respectively) made of polyethylene terephthalate textile, on the marine copepod Acartia erythraea. In laboratory, starved copepods were observed to take in a piece of fibrous microplastic sedimented on the bottom. While no individual ingested the entire fibre, the tip of the ingested fibre reached deep into the gut of the copepods. This suggests that ingestion was not accidental but purposeful behaviour to take in non-living organic matter as a supplementary food source. All copepods that had the fibre in their mouths eventually died within 24-h because the fibre penetrated deep into the gut, preventing feeding and potentially causing stress. Our finding implies that a single piece of microplastic fibre remaining at the bottom of coastal zones could continue to kill copepods owing to its non-degradability.

Caligus minimus Otto, 1821 (Copepoda: Caligidae): A commercially important but poorly described parasite of cultured European Sea Bass, Dicentrarchus labrax (Linnaeus, 1758).

Caligus minimus Otto, 1821 has been known for over two centuries and it is the second oldest of the approximately 275 species of Caligus O. F. Müller, 1985. Despite the numerous records of this species from European waters, it has never been fully described to modern standards. The lack of a comprehensive modern description has resulted in numerous misidentifications, even in recently published reports, and this is especially problematic for a species that is known to have a significant economic impact in aquaculture. This study presents a detailed description of both sexes and documents newly observed features of C. minimus collected from the buccal cavity of farmed European Sea Bass (ESB), Dicentrarchus labrax (Linnaeus, 1758). The morphology of C. minimus was examined using light microscope (LM), scanning electron microscope (SEM), and confocal laser scanning microscope (CLSM), and new details are revealed regarding the structure and ornamentation of the marginal membrane of the cephalothorax, maxilliped, antenna, sternal furca, abdomen, and legs 1, 3, 4, and 6. The ornamentation of the marginal membrane of the cephalothorax is unique and its impact on the functioning of the cephalothoracic sucker requires further investigation. Additionally, partial COI gene region sequences were obtained from four individuals of C. minimus and provided for future references. A phylogenetic analysis was conducted in conjunction with Caligus sequences available in the NCBI GenBank database.

Nutritional condition drives spatial variation in physiology of Antarctic lipid-storing copepods.

Lipid-rich copepods form an essential link between primary producers and higher trophic levels in high-latitude oceans. These zooplankton can take advantage of ephemeral phytoplankton blooms to fuel development and reproduction. However, we have limited understanding of how the physiological condition of these animals varies in relation to environmental factors such as food availability. Due to high advection, it is likely that physiological plasticity, rather than local adaptation, is primarily responsible for physiological differences within a region. We use transcriptomics and other physiological metrics to understand how two species of copepods (Calanoides acutus and Calanus propinquus) vary across environmental gradients along the West Antarctic Peninsula. For the primarily herbivorous C. acutus, physiological separation between sampling locations appears to be driven by feeding status, and gene expression differences indicate differential expression of genes regulating lipid metabolism, reproduction, aerobic metabolism, and protein translation. For the more omnivorous C. propinquus, physiology and gene expression did not segregate as clearly by location, showed minimal signs of food deprivation at any location, and had a weaker relationship with chlorophyll compared to C. acutus. By comparing these results with concurrent starvation experiments, we find that spatial variation in gene expression reflects short-term differences in food availability (particularly for C. acutus), and we identify genes whose expression indicates recent feeding status. Further examination of the relationships between food availability, copepod physiology, and population dynamics will ultimately improve our capacity to predict how copepod populations will respond to rapidly changing environmental conditions in the West Antarctic Peninsula ecosystem.

Modeling the differential functional responses and selectivity of a marine copepod to nano/microplastics in mixture.

Nano- and microplastics (NMPs) pollution is widespread in the oceans, posing potential risks to marine species. This study examined the accumulation capacity and selectivity potentials of NMPs by a marine copepod Parvocalanus crassirostris under different food mixtures by modeling the combined biokinetic and functional response. We investigated two sizes of NMPs (200 nm and 5 µm) across a concentration gradient (0 - 5000 µg/L) and varying diatom abundances (0, 104, 105 cells/mL). Fluorescence imaging and quantification revealed that P. crassirostris actively ingested NMPs at low concentration. Accumulation increased with NMPs concentration but eventually saturated due to gut capacity limits, following a Holling type II functional response (i.e., hyperbolic curve). Our novel functional response model estimated the key parameters and demonstrated that the maximum accumulation reached 5.3 % of dry weight with averaged half-saturation constants of 229 µg/L. The size of NMPs did not significantly affect the total accumulation or satiety levels. The presence of diatoms influenced the feeding selectivity and decreased the microplastic accumulation by 73 % at 105 cells/mL, while facilitating nanoplastic accumulation by 81 % at 104 cells/mL. This study enhanced our understanding of NMPs bioavailability and environmental fate in marine ecosystems.

Dissolved Barium Causes Toxicity to Groundwater Cyclopoida.

Barium (Ba) dissolution and mobilization in groundwater are predominantly controlled by sulfate because of the low solubility of barium sulfate (BaSO4) minerals. Naturally present at low concentrations in groundwater, elevated concentrations of Ba can occur as a result of anthropogenic activities, including use of barite in drill operations, and geogenic sources such as leaching from geological formations. No toxicity data exist for Ba with groundwater organisms (stygofauna) to assess the risk of elevated Ba concentrations. The present study measured Ba toxicity to two stygobiont Cyclopoida species: one collected from Wellington and the other from Somersby, New South Wales, Australia. Toxicity was measured as cyclopoid survival over 2, 4, 7, 14, 21, and 28 days in waters of varying sulfate concentration (<1-100 mg SO4/L). When sulfate was present, dissolved Ba concentrations decreased rapidly in toxicity test solutions forming a BaSO4 precipitate until dissolved sulfate was depleted. Barium in excess of sulfate remained in the dissolved form. The toxicity of Ba to cyclopoids was clearly attributed to dissolved Ba. Precipitated Ba was not toxic to the Wellington cyclopoid species. Toxicity values for dissolved Ba for the Wellington and Somersby cyclopoid species included a (21-day) no-effect concentration of 3.3 mg/L and an effective concentration to cause 5% mortality of 4.8 mg/L (at 21 days). Elevated dissolved Ba concentrations due to anthropogenic and/or biogeochemical processes may pose a risk to groundwater organisms. Further toxicity testing with other stygobiont species is recommended to increase the data available to derive a guideline value for Ba that can be used in contaminant risk assessments for groundwaters. Environ Toxicol Chem 2024;00:1-14. © 2024 The Author(s). Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Optimal phenology of life history events in Calanus finmarchicus: exit from diapause in relation to interannual variation in spring bloom timing and predation.

Respiration of lipids by copepods during diapause (overwintering dormancy) contributes to ocean carbon sequestration via the seasonal lipid pump (SLP). Parameterizing this flux in predictive models requires a mechanistic understanding of how life history adaptation in copepods shapes their timing of exit from diapause. We investigate the optimal phenology of Calanus finmarchicus in the Norwegian Sea using an individual-based model in which diapause exit is represented as a trait characterized by phenotypic mean and variance. Without interannual variability, optimal exit correlated with the onset of the spring phytoplankton bloom and phenotypic variance was of no benefit. In contrast, copepods endured reduced fitness and adopted bet-hedging strategies when exposed to interannual variability in bloom timing and predation: later exit from diapause and phenotypic variance maintained adult numbers in anomalous late-bloom years. Exit nevertheless remained well before the peak of the bloom which is a favorable strategy when low predation early in the year enhances survival of eggs and early developmental stages. Our work highlights the complex interactions between C. finmarchicus and its environment and the need for improved understanding of bet-hedging strategies and the cues of diapause exit to progress the representation of the SLP in global biogeochemical models.

A First Record on Microplastic Ingestion by Tropical Estuarine Copepods of Bangladesh.

Microplastics (MPs) pollution is a profound problem around the world yet it's study on the effect on zooplankton including copepods are very limited. The study was conducted between January 2021 and January 2022 in the Lower Meghna Estuary to investigate MPs ingestion in two different family of copepod: Calanoid and Cyclopoid. A method of acid digestion along with Scanning Electron Microscope (SEM) was used to identify MPs ingested by copepods from the conducted area. However, three types of MPs namely fiber, fragment and foam were extracted from this copepod biomass. Fibers represent highest (> 50%) of the ingested MPs from both group of copepod that exceed fragments and foams in all sampling stations. The overall ingestion rate of Calanoid was found higher (0.084 ± 0.002 particles/individual) compared to the Cyclopoid group (0.077 ± 0.001 particles/individual). The results of the study have effectively illustrated that copepod, obtained from multiple sampling sites within the Lower Meghna Estuary, display a propensity to ingest MPs and subsequently endangering the food security of seafood industry.

Remarkable shifts in the megabenthic community structure over four decades in Tokyo Bay, Japan, in relation to environmental variations.

We investigated long-term changes in the megabenthic community in Tokyo Bay, Japan, using data from fisheries-independent trawl surveys conducted from 1977 to 2023. In addition, we examined the potential relationship between changes in biotic communities and environmental conditions. The total abundance and biomass exhibited an increasing trend until 1987, followed by a substantial decline from the late 1980s to the 1990s due to a decrease in small to medium-sized fish and crustacean species. Meanwhile, a marked increase in the number of large fish (including elasmobranchs), mollusks, and echinoids, was observed in the 2000s. These shifts in the megabenthic community structure were correlated with an increase in water temperature and a decrease in nutrient concentrations and copepod densities. Cumulative evidence suggests that a remarkable shift in the megabenthic community structure occurred between the 1970s and the 2020s, which was possibly associated with variations in the environmental conditions in Tokyo Bay.

Distribution and bioaccumulation status of polycyclic aromatic hydrocarbons (PAHs) in Veraval coastal waters using copepods as bio-indicators.

The study investigates the pollution characteristics of 16 priority PAHs, accumulated in copepods from a major fishing harbour and its adjacent coastal waters of Veraval, west coast of India. The total PAH accumulation is in the range of 922.16-27,807.49 ng g-1 dw, with the mean concentration of 5776.59 ng g-1 dw. High concentrations of PAHs were present in the copepod samples from inside the harbour. Notably, there was no significant correlation between the lipid content of copepods and the accumulation of PAHs. The molecular diagnostic ratio method (MDR) indicates that the PAH sources are petrogenic in origin, while principal component analysis (PCA) points to petroleum, coal combustion and vehicular emission sources. Total cancerous PAHs (C-PAHs) in the study area dominate by 40% of the total PAHs identified; moreover, the bioaccumulation factor (BAF) is very high in the offshore area, which is also a fishing ground. The global relevance and magnitude of the present study in the Veraval, one of the prime seafood exporting hubs in India, should be dealt with utmost avidity as the accumulation status of PAHs in the zooplankton has never been explored in the Indian coastal waters. Moreover, the current study gives the foremost data on the bioaccumulation status of PAHs in copepods from the tropical waters of India.

First evidence of the induction of domoic acid production in Pseudo-nitzschia australis by the copepod Temora longicornis from the French coast.

Diatoms of the genus Pseudo-nitzschia are widespread in marine waters. Some of them can produce the toxin domoic acid (DA) which can be responsible for amnesic shellfish poisoning (ASP) when transferred into the food web. These ASP events are of major concern, due to their ecological and socio-economic repercussions, particularly on the shellfish industry. Many studies have focused on the influence of abiotic factors on DA induction, less on the role of biotic interactions. Recently, the presence of predators has been shown to increase DA production in several Pseudo-nitzschia species, in particular in Arctic areas. In order to investigate the relationship between Pseudo-nitzschia species and grazers from the French coast, exposures between one strain of three species (P. australis, P. pungens, P. fraudulenta) and the copepod Temora longicornis were conducted for 5 days. Cellular and dissolved DA content were enhanced by 1,203 % and 1,556 % respectively after the 5-days exposure of P.australis whereas no DA induction was observed in P. pungens and P. fraudulenta. T. longicornis consumed all three Pseudo-nitzschia species. The copepod survival was not related to DA content. This study is an essential first step to better understanding the interactions between planktonic species from the French coast and highlights the potential key role of copepods in the Pseudo-nitzschia bloom events in the temperate ecosystems.

Multigenerational effects of glyphosate-based herbicide and emamectin benzoate insecticide on the reproduction and gene expression of the copepod Pseudodiaptomus annandalei (Sewell, 1919).

This study investigates the effects of glyphosate-based herbicide (GLY) and pure emamectin benzoate (EB) insecticide on the brackish copepod Pseudodiaptomus annandalei. The 96h median lethal concentration (96 h LC50) was higher in the GLY exposure (male: 3420.96 ± 394.67 µg/L; female: 3093.46 ± 240.67 µg/L) than in the EB (male: 79.10 ± 7.30 µg/L; female: 6.38 ± 0.72 µg/L). Based on the result of 96h LC50, we further examined the effects of GLY and EB exposures at sub-lethal concentrations on the naupliar production of P. annandalei. Subsequently, a multigenerational experiment was conducted to assess the long-term impact of GLY and EB at concentrations 375 µg/L, and 0.025 µg/L respectively determined by sub-lethal exposure testing. During four consecutive generations, population growth, clutch size, prosome length and width, and sex ratio were measured. The copepods exposed to GLY and EB showed lower population growth but higher clutch size than the control group in most generations. Gene expression analysis indicated that GLY and EB exposures resulted in the downregulation of reproduction-related (vitellogenin) and growth-related (myosin heavy chain) genes, whereas a stress-related gene (heat shock protein 70) was upregulated after multigenerational exposure. The results of the toxicity test after post-multigenerational exposure indicated that the long-term GLY-exposed P. annandalei displayed greater vulnerability towards GLY toxicity compared to newly-exposed individuals. Whereas, the tolerance of EB was significantly higher in the long-term exposed copepod than in newly-exposed individuals. This suggests that P. annandalei might have greater adaptability towards EB toxicity than towards GLY toxicity. This study reports for the first time the impacts of common pesticides on the copepod P. annandalei, which have implications for environmental risk assessment and contributes to a better understanding of copepod physiological responses towards pesticide contaminations.

Peracetic acid treatment of squid eggs infected with parasitic copepod (Ikanecator primus gen. et sp. nov.).

Having been successfully bred in semi-intensive and intensive aquaculture systems, oval squids of the Sepioteuthis lessoniana species complex are emerging as promising candidates for research and industry. Nevertheless, information about pathogens and diseases that may affect squid aquaculture remains sparse. In this study, we identify new parasitic copepod species that causes squid mortality and decreases squid hatching rates, and we also offer a solution to eliminate the pathogen during incubation of squid eggs. The newly discovered copepod Ikanecator primus gen. et sp. nov. was identified on oval squid eggs for the first time using both morphological and molecular diagnostic markers. In the genomes of the copepod and associated microbiome, we identified multiple genes for enzymes involved in cephalopod eggshell degradation in genomes of the copepod and associated microbiome. Furthermore, we conducted experiments to assess efficacy of peracetic acid in inhibiting the I. primus gen. et sp. nov. both in vitro and in vivo using immersion treatment. We established that a 2-min exposure to a concentration of 250 µl/L of peracetic acid containing product (PAA-product; 35 mg/L PAA and 15 mg/L H2O2) inhibited the development of nauplii in vitro. All parasites exposed to a concentration of 500 µl/L of PAA-product (70 mg/L PAA and 30 mg/L H2O2) were eliminated within two minutes. On top of this, the immersion treatment with 500 µl/L of PAA-product (70 mg/L PAA and 30 mg/L H2O2) improved survival of squid embryos and increased size of squid hatchlings compared with control and the immersion treatment with 125 µl/L of PAA-product (17.5 mg/L PAA and 7.5 mg/L H2O2) and the immersion treatment with 250 µl/L of PAA-product (35 mg/L PAA and 15 mg/L H2O2). These findings suggest that PAA holds a great potential as inhibitor and controller of parasitic copepod infections and for overall health management in cephalopod culture.

Warming effects on the life cycles of two parasitic copepods with different invasion histories.

Climate change may exacerbate the impact of invasive parasites from warmer climates through pre-existing temperature adaptations. We investigated temperature impacts on two closely related marine parasitic copepod species that share the blue mussel (Mytilus edulis) as host: Mytilicola orientalis has invaded the system from a warmer climate <20 years ago, whereas its established congener Mytilicola intestinalis has had >90 years to adapt. In laboratory experiments with temperatures 10-26°C, covering current and future temperatures as well as heat waves, the development of both life cycle stages of both species accelerated with increasing temperature. In the parasitic stages, the growth of the established invader increased evenly from 10°C to 22°C, whereas the recent invader barely grew at all at 10°C and grew faster already at 18°C. In contrast, temperature had little effect on the transition success between life cycle stages. However, the highest temperature (26°C) limited the egg development success of the established invader and the host entry success of both species, whereas the infection success of the established invader increased at 18°C and 22°C. In general, our experiments indicate that the main effect of temperature on both species is through development speed and not life cycle stage transition success. Based on regional long-term temperature data and predictions, the numbers of completed life cycles per year will increase for both parasites. The established invader seems better adapted for low current temperatures (around 10°C), whereas the more recent invader barely develops at these temperatures but can cope in high temperatures (around 26°C). Hence, pre-existing temperature adaptations of the recent invader may allow the species to better cope with heat waves.

Comparative assessment of microplastics and microalgae as vectors of mercury and chlorpyrifos in the copepod Acartia tonsa.

Microplastics (MPs) raise concerns not only as pollutants themselves, but also due to their ability to act as vectors of pollutants adsorbed from seawater, transferring them to marine organisms. However, the relevance of MPs as carriers of pollutants compared to microalgae needs further exploration. This study compared the role of MPs (2-10 µm non-oxidized and 10-15 µm oxidized high-density polyethylene) and natural organic particles (Rhodomonas lens microalgae, MA) as carriers of mercury (Hg, 2.3 µg Hg/L) and chlorpyrifos (CPF, 1.0 µg CPF/L) to adult Acartia tonsa copepods, after 24-48 h exposure. Dose-response experiments were first performed with adult female copepods exposed to oxidized MPs (0.25-4.0 mg/L), waterborne Hg (0.01-10.0 µg/L) and Ox MPs + Hg (0.25-4.0 mg oxidized MPs/L + 0.50-8.0 µg Hg/L) for 48 h, to complement previous studies that focused on the pesticide CPF. Effects were evaluated with four replicates for physiological and reproductive responses (6 females/replicate), biochemical techniques (40 individuals/replicate) and Hg/CPF bioaccumulation measurements (1000 individuals/replicate). Copepods accumulated Hg/CPF similarly from dissolved pollutants (6204 ± 2265 ng Hg/g and 1251 ± 646 ng CPF/g) and loaded MPs (3125 ± 1389 ng Hg/g and 1156 ± 266 ng CPF/g), but significantly less from loaded MA (21 ± 8 ng Hg/g and 173 ± 80 ng CPF/g). After 24-48 h, copepods exposed to MPs + Hg/CPF showed generally greater biological effects than those exposed to dissolved Hg/CPF or to MA + Hg/CPF, although differences were not statistically significant. MA + CPF had significantly lower AChE inhibition (1073.4 nmol min-1 mg-1) and MA + Hg lower GRx induction (48.8 nmol min-1 mg-1) compared to MPs + Hg/CPF and dissolved Hg/CPF (182.8-236.4 nmol min-1 mg-1 of AChE and 74.1-101.7 nmol min-1 mg-1 of GRx). Principal component analysis suggested different modes of action for Hg and CPF.

Arsenic bioaccumulation and biotransformation in the marine copepod Tigriopus japonicus under chronic dietborne and waterborne exposure.

Arsenic (As) can be transferred along the food chain, while little is known about the toxic effects of dietborne As on marine copepods. In this study, we investigated the short-term and long-term effects of waterborne and dietborne As exposure on the bioaccumulation and biotransformation, as well as developmental toxicity of Tigriopus japonicus. Under acute As exposure, As bioaccumulation increased and reached a plateau with increasing exposure concentration. Moreover, As accumulation at dietborne exposure was 4.3 and 5.7 times greater than that at control group for AsIII and AsV, respectively. At chronic As exposure, As accumulation increased continuously with exposure time, with a 2.8-day extension of development time and a 45% reduction in 10-d fecundity under dietborne exposure compared to control, whereas 2.3-day extension of development time and a 20% reduction in 10-d fecundity were observed under waterborne exposure. Among As species, inorganic As had the highest concentrations, but the proportion of inorganic As decreased from 89% to 63% during 4 to 21 d of exposure, suggesting the conversion of inorganic As to organic As. The organic As was dominated by arsenobetaine (AsB, 13-25%), followed by monomethylarsenic (MMA, 8-25%). These results suggest that dietborne exposure has more pronounced toxic effects on T. japonicus, but the toxicity of As could be reduced through biotransformation under chronic exposure. Therefore, the arsenic species should be considered when assessing As toxicity.

The First Report of Pennella (Crustacea: Copepoda) Infesting Stenella coeruleoalba Stranded in Malta: Morphological and Genetic Analyses.

Here, we document the stranding of a striped dolphin Stenella coeruleoalba (Meyen, 1833) (Mammalia: Delphinidae), which was found dead in Maltese waters in July 2020. The stranded dolphin exhibited a severe infestation of the mesoparasitic copepod, Pennella balaenoptera Koren and Danielssen, 1877 (Copepoda: Pennelidae). Parasites of this genus represent the largest known mesoparasites to infest cetaceans. Under normal circumstances, cetaceans may have a few P. balaenoptera individuals attached to them, but cetaceans with compromised health are more prone to heavy infestations. The identification of the parasite was accomplished through morphological and genetic analyses. This incident highlights the significance of monitoring mesoparasitic infestations, offering valuable insights into the health of cetacean populations and emphasizing the potential implications for conservation efforts in the region.

Hydrostatic pressure impedes the degradation of sinking copepod carcasses and fecal pellets.

Fast-sinking zooplankton carcasses and fecal pellets appear to contribute significantly to the vertical transport of particulate organic carbon (POC), partly because of low temperature that decreases microbial degradation during the descent into the deep ocean. Increasing hydrostatic pressure could further reduce the degradation efficiency of sinking POC, but this effect remains unexplored. Here, the degradation of carcasses and fecal pellets of the abundant marine copepod Calanus finmarchicus was experimentally studied as a function of pressure (0.1-100 MPa). Samples were either exposed to elevated pressure in short 1-day incubations or a gradual pressure increase, simulating continuous particle sinking during a 20-day incubation. Both experiments revealed gradual inhibition of microbial respiration in the pressure range of 20-100 MPa, corresponding to 2-10-km depth. This suggests that hydrostatic pressure impedes carbon mineralization of fast-sinking carcasses and fecal pellets and enhances the deep-sea deposition rate of zooplankton-derived organic material.

Consistent Negative Correlations between Parasite Infection and Host Body Condition Across Seasons Suggest Potential Harmful Impacts of Salmincola markewitschi on Wild White-Spotted Charr, Salvelinus leucomaenis.

Assessing the impacts of parasites on wild fish populations is a fundamental and challenging aspect of the study of host-parasite relationships. Salmincola, a genus of ectoparasitic copepods, mainly infects salmonid species. This genus, which is notorious in aquaculture, damages host fishes, but its impacts under natural conditions remain largely unknown or are often considered negligible. In this study, we investigated the potential impacts of mouth-attaching Salmincola markewitschi on white-spotted charr (Salvelinus leucomaenis) through intensive field surveys across four seasons using host body condition as an indicator of harmful effects. The prevalence and parasite abundance were highest in winter and gradually decreased in summer and autumn, which might be due to host breeding and/or wintering aggregations that help parasite transmissions. Despite seasonal differences in prevalence and parasite abundance, consistent negative correlations between parasite abundance and host body condition were observed across all seasons, indicating that the mouth-attaching copepods could reduce the body condition of the host fish. This provides field evidence suggesting that S. markewitschi has a potential negative impact on wild white-spotted charr.

Assessment of toxic effects of imidacloprid on freshwater zooplankton: An experimental test for 27 species.

The neonicotinoid pesticide imidacloprid has been used worldwide since 1992. As one of the most important chemicals used in pest control, there have been concerns that its run-off into rivers and lakes could adversely affect aquatic ecosystems, where zooplankton play a central role in the energy flow from primary to higher trophic levels. However, studies assessing the effects of pesticides at the species level have relied on a Daphnia-centric approach, and no studies have been conducted using species-level assessments on a broad range of zooplankton taxa. In the present study, we therefore investigated the acute toxicity of imidacloprid on 27 freshwater crustacean zooplankton (18 cladocerans, 3 calanoid copepods and 6 cyclopoid copepods). The experiment showed that a majority of calanoid copepods and cladocerans were not affected at all by imidacloprid, with the exception of one species each of Ceriodaphnia and Diaphasoma, while all six cyclopoid copepods showed high mortality rates, even at concentrations of imidacloprid typically found in nature. In addition, we found a remarkable intra-taxonomic variation in susceptibility to this chemical. As many cyclopoid copepods are omnivorous, they act as predators as well as competitors with other zooplankton. Accordingly, their susceptibility to imidacloprid is likely to cause different responses at the community level through changes in predation pressure as well as changes in competitive interactions. The present results demonstrate the need for species-level assessments of various zooplankton taxa to understand the complex responses of aquatic communities to pesticide disturbance.