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Abstract Immune responses can be energetically expensive and subject to trade-offs. Prior work on the freshwater zooplankton, Ceriodaphnia cornuta, demonstrated an association between eye size and infection, leading to questions about whether investment in eyes trades off against investment in immunity. We used the crustacean host, Daphnia dentifera, and its fungal parasite, Metschnikowia bicuspidata, to investigate the relationships between eye size, parasite resistance and infection. In the field, we found a negative correlation between size-corrected eye area (SCEA) and Metschnikowia infection, suggesting that either SCEA decreases infection (thereby indicating resistance) or that infection decreases SCEA. Controlled laboratory experiments reinforced the latter result: exposure to the fungal parasite decreased a host’s SCEA, regardless of the parasite dose or host genotype. We also uncovered significant plasticity in this trait—both host age and resource level increased SCEA. Identifying causality in physiological correlations is challenging. Our results suggest that negative associations between parasitism and energetically-expensive traits can arise through plasticity. 

Abstract The levels of the hormone melatonin fluctuate daily, with higher concentrations often found at night. These fluctuations likely influence multiple aspects of physiology, including the immune response. We demonstrated that the addition of exogenous melatonin increased the proportion of the freshwater zooplanktonDaphnia dentiferathat became infected by the fungal pathogenMetschnikowia bicuspidata, during the day but not at night. To determine the stage of this host–pathogen interaction at which melatonin may increase susceptibility, we conducted a series of laboratory experiments in which we raisedDaphniain the presence and absence of exogenous melatonin. To complete its life cycle,Metschnikowiamust encounter a foraging host, overcome the host's barrier resistance (gut wall), and evade the host's immune response (internal clearance). We quantified encounter rate by measuring the gut passage time and the number of spores that entered the gut. We also measured the number of spores that successfully entered the body cavity (barrier resistance) and the hemocyte response to spores entering the body cavity (one metric of internal clearance). Finally, we quantified the effect of exogenous melatonin on triggering molting. The addition of exogenous melatonin lengthened gut passage time and decreased the number of spores present in the gut. We found no effect of melatonin on the percentage of gut spores successfully entering the host's body cavity, nor on the hemocyte response. Melatonin is known to influence the timing of molting and hosts that molted during exposure were more likely to become infected, likely due to a decrease in barrier resistance. In a fully factorial experiment, there was a high death rate, low infection rate, and therefore no discernible effect of melatonin on molting, nor molting or melatonin on infection. Our results provide insight into the stages of infection where melatonin does and does not have significant effects. 

Abstract While vertebrate immune systems are appreciated for their complexity and adaptability, invertebrate immunity is often considered to be less complex. However, immune responses in many invertebrates likely involve sophisticated processes. Interactions between the crustacean hostDaphnia dentiferaand its fungal pathogenMetschnikowia bicuspidataprovide an excellent model for exploring the mechanisms underlying crustacean immunity. To explore the genomic basis of immunity inDaphnia, we used RNA‐sequencing technology to quantify differential gene expression between individuals of a single host genotype exposed or unexposed toM. bicuspidataover 24 h. Transcriptomic analyses showed that the number of differentially expressed genes between the control (unexposed) and experimental (exposed) groups increased over time. Gene ontology enrichment analysis revealed that differentially expressed genes were enriched for immune‐related molecules and processes, such as cuticle development, prostaglandin, and defense response processes. Our findings provide a suite of immunologically relevant genes and suggest the presence of a rapidly upregulated immune response involving the cuticle inDaphnia. Studies involving gene expression responses to pathogen exposure shine a light on the processes occurring during the course of infection. By leveraging knowledge on the genetic basis for immunity, immune mechanisms can be more thoroughly understood to refine our understanding of disease spread within invertebrate populations. 

1. Understanding the factors underlying the abundance and distribution of species requires the consideration of a complex suite of interacting biotic and abiotic factors operating on multiple spatial and temporal scales. Larval mosquitoes inhabiting small human‐constructed ponds represent a unique opportunity to investigate the relative importance of these structuring mechanisms while simultaneously generating applied knowledge on mosquito control. 2. A multi‐year field survey of 32 stormwater ponds was conducted in central Illinois (Champaign County). From each pond, data were collected on pond structure type and hydroperiod, the presence/absence of cattails (Typhaspp.), and measures of total nitrogen, phosphorus, and organic carbon, and chlorophylla. The communities of crustacean zooplankton and aquatic insects were characterised, and these taxa were assigned into two main groups: predators and competitors of larval mosquitoes. Structural equation modelling was used to explore the direct and indirect effects of these biotic and abiotic factors on larval density for three species of culicine mosquitoes (Culex pipiens,Culex restuans, andAedes vexans). 3. Hydroperiod had an indirect negative effect onC. pipiensdensity. However, this effect was mediated by predator density; more permanent ponds had more predators, which therefore reduced the density ofC. pipienslarvae.Aedes vexansdensity was positively correlated with predator density. No predictor variables were found that explained variation inC. restuansdensity. 4. This study show that the relative importance of these biotic and abiotic factors varies among species of culicine mosquitoes inhabiting stormwater ponds.