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1. Prey taxonomy rather than size determines salp diets

   https://doi.org/10.1002/lno.11165  

   Dadon‐Pilosof, Ayelet ; Lombard, Fabien ; Genin, Amatzia ; Sutherland, Kelly R. ; Yahel, Gitai ( April 2019 , Limnology and Oceanography)

   Salps are gelatinous planktonic suspension feeders that filter large volumes of water in the food‐dilute open ocean. Their life cycle allows periodic exponential growth and population blooms. Dense swarms of salps have a high grazing impact that can deplete the photic zone of phytoplankton and export huge quantities of organic matter to the deep sea. Previous studies described their feeding manner as mostly nonselective, with larger particles retained at higher efficiencies than small particles. To examine salp diets, we used direct in situ sampling (InEx method) of undisturbed solitarySalpa maxima. Aggregates (“chains”) ofSalpa fusiformisandThalia democraticawere studied using in situ incubations. Our findings suggest that in situ feeding rates are higher than previously reported and that cell removal is size independent with ∼ 1μm picoeukaryotes preferentially removed over both larger eukaryotes and smaller bacteria. The prey : predator size ratios we measured (1 : 10⁴–1 : 10⁵) are an order of magnitude smaller than previously reported values and to the best of our knowledge, are the smallest values reported so far for any planktonic suspension feeders. Despite differences among the three species studied, they had similar prey preferences with no correlation between salp body length and prey size. Our findings shed new light on prey : predator relationships in planktonic systems—in particular, that factors other than size influence filtration efficiency—and suggest that in situ techniques should be devised and applied for the study of suspension feeding in the ocean.

    

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2. Weak spatiotemporal response of prey to predation risk in a freely interacting system

   https://doi.org/10.1111/1365-2656.12968  

   Cusack, Jeremy J. ; Kohl, Michel T. ; Metz, Matthew C. ; Coulson, Tim ; Stahler, Daniel R. ; Smith, Douglas W. ; MacNulty, Daniel R. ; Börger, ed., Luca ( March 2019 , Journal of Animal Ecology)

   The extent to which prey space use actively minimizes predation risk continues to ignite controversy. Methodological reasons that have hindered consensus include inconsistent measurements of predation risk, biased spatiotemporal scales at which responses are measured and lack of robust null expectations.

   We addressed all three challenges in a comprehensive analysis of the spatiotemporal responses of adult female elk (Cervus elaphus) to the risk of predation by wolves (Canis lupus) during winter in northern Yellowstone,USA.

   We quantified spatial overlap between the winter home ranges ofGPS‐collared elk and three measures of predation risk: the intensity of wolf space use, the distribution of wolf‐killed elk and vegetation openness. We also assessed whether elk varied their use of areas characterized by more or less predation risk across hours of the day, and estimated encounter rates between simultaneous elk and wolf pack trajectories. We determined whether observed values were significantly lower than expected if elk movements were random with reference to predation risk using a null model approach.

   Although a small proportion of elk did show a tendency to minimize use of open vegetation at specific times of the day, overall we highlight a notable absence of spatiotemporal response by female elk to the risk of predation posed by wolves in northern Yellowstone.

   Our results suggest that predator–prey interactions may not always result in strong spatiotemporal patterns of avoidance.

    

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3. Selection of food patches by sympatric herbivores in response to concealment and distance from a refuge

   https://doi.org/10.1002/ece3.1940  

   Crowell, Miranda M. ; Shipley, Lisa A. ; Camp, Meghan J. ; Rachlow, Janet L. ; Forbey, Jennifer S. ; Johnson, Timothy R. ( March 2016 , Ecology and Evolution)

   Small herbivores face risks of predation while foraging and are often forced to trade off food quality for safety. Life history, behaviour, and habitat of predator and prey can influence these trade‐offs. We compared how two sympatric rabbits (pygmy rabbit,Brachylagus idahoensis; mountain cottontail,Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush‐steppe of westernNorthAmerica respond to amount and orientation of concealment cover and proximity to burrow refuges when selecting food patches. We predicted that both rabbit species would prefer food patches that offered greater concealment and food patches that were closer to burrow refuges. However, because pygmy rabbits are small, obligate burrowers that are restricted to sagebrush habitats, we predicted that they would show stronger preferences for greater cover, orientation of concealment, and patches closer to burrow refuges. We offered two food patches to individuals of each species during three experiments that either varied in the amount of concealment cover, orientation of concealment cover, or distance from a burrow refuge. Both species preferred food patches that offered greater concealment, but pygmy rabbits generally preferred terrestrial and mountain cottontails preferred aerial concealment. Only pygmy rabbits preferred food patches closer to their burrow refuge. Different responses to concealment and proximity to burrow refuges by the two species likely reflect differences in perceived predation risks. Because terrestrial predators are able to dig for prey in burrows, animals like pygmy rabbits that rely on burrow refuges might select food patches based more on terrestrial concealment. In contrast, larger habitat generalists that do not rely on burrow refuges, like mountain cottontails, might trade off terrestrial concealment for visibility to detect approaching terrestrial predators. This study suggests that body size and evolutionary adaptations for using habitat, even in closely related species, might influence anti‐predator behaviors in prey species.

    

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4. Harmful alga trades off growth and toxicity in response to cues from dead phytoplankton

   https://doi.org/10.1002/lno.11414  

   Brown, Emily R. ; Kubanek, Julia ( January 2020 , Limnology and Oceanography)

   Organisms are under selection pressure to recognize predators and assess predation risk to avoid becoming prey. In some cases, the presence of injured competitors alerts individuals to the likelihood that predators are nearby. Previous studies have shown that the marine dinoflagellateAlexandrium minutumresponds to chemical cues from copepods by dramatically upregulating sodium channel‐blocking toxins that appear to function as defenses against copepod grazing. However, it is unknown whetherA. minutumuses other cues, such as damaged phytoplankton, some of which are its competitors, to assess predation risk and subsequently increase its resistance to predators. To investigate the role of dead phytoplankton cues in chemical defense plasticity,A. minutumwas exposed for 3 days to chemical cues from six different phytoplankton. Chemical cues from dead, unrelated, historically co‐occurring phytoplankton species induced toxin production inA. minutumcoincident with a decrease in growth. In contrast, exposure to chemical cues from more closely related dead phytoplankton, either conspecific or congeneric, suppressed toxin production inA. minutumrelative to their absence. This was coupled with a modest, yet significant, increase in growth. The consistent inverse relationship between toxin production and growth suggests thatA. minutumexperiences a trade‐off. Together, these results reveal that relatedness of dead phytoplankton is important in howA. minutumutilizes resources for growth and defense.

    

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5. microRNA regulation in an ancient obligate endosymbiosis

   https://doi.org/10.1111/mec.14464  

   Feng, Honglin ; Wang, Lingyu ; Wuchty, Stefan ; Wilson, Alex C. C. ( January 2018 , Molecular Ecology)

   Although many insects are associated with obligate bacterial endosymbionts, the mechanisms by which these host/endosymbiont associations are regulated remain mysterious. While microRNAs (miRNAs) have been recently identified as regulators of host/microbe interactions, including host/pathogen and host/facultative endosymbiont interactions, the role miRNAs may play in mediating host/obligate endosymbiont interactions is virtually unknown. Here, we identified conserved miRNAs that potentially mediate symbiotic interactions between aphids and their obligate endosymbiont,Buchnera aphidicola. Using smallRNAsequence data fromMyzus persicaeandAcyrthosiphon pisum, we annotated 93M. persicaeand 89A. pisummiRNAs, among which 69 were shared. We found 14 miRNAs that were either highly expressed in aphid bacteriome, theBuchnera‐housing tissue, or differentially expressed in bacteriome vs. gut, a non‐Buchnera‐housing tissue. Strikingly, 10 of these 14 miRNAs have been implicated previously in other host/microbe interaction studies. Investigating the interaction networks of these miRNAs using a custom computational pipeline, we identified 103 miRNA::mRNAinteractions shared betweenM. persicaeandA. pisum. Functional annotation of the sharedmRNAtargets revealed only two over‐represented cluster of orthologous group categories: amino acid transport and metabolism, and signal transduction mechanisms. Our work supports a role for miRNAs in mediating host/symbiont interactions between aphids and their obligate endosymbiontBuchnera. In addition, our results highlight the probable importance of signal transduction mechanisms to host/endosymbiont coevolution.

    

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