GAPE-LIMITED PREDATORS AS AGENTS OF SELECTION ON THE DEFENSIVE MORPHOLOGY OF AN INVASIVE INVERTEBRATE

Can young-of-the-year invasive fish keep up with young-of-the-year native fish? A comparison of feeding rates between invasive sticklebacks and whitefish

Invasion of non-native species might alter food web structure and the strength of top-down control within lake ecosystems. As top-down control exerted by fish populations is often dominated by young of the year fish, the impact of new fish species might depend on the feeding rates of the juvenile fish. Here we provide comparative analyses of feeding rates of juvenile whitefish (Coregonus wartmanni) - a native and specialised planktivore and an invasive generalist (sticklebacks, Gasterosteus aculeatus). We studied feedings rates of whitefish and sticklebacks in aquaria experiments using 2 cm to 8 cm fish feeding on seven zooplankton species common to Lake Constance. As whitefish hatch several months earlier than sticklebacks, 0+ whitefish are larger than 0+ sticklebacks throughout the year and hence are predicted to have higher feeding rates on especially large zooplankton species. We show that sticklebacks as small as 2 cm were able to feed on the largest zooplankton species of Lake Constance. Further, stickleback feeding rates were similar to both the same size 0+ whitefish and the larger 0+ whitefish co-occurring with smaller 0+ sticklebacks. Hence, 0+ sticklebacks will compete with 0+ whitefish for the same zooplankton species, therefore the invasion of sticklebacks is unlikely to change the relative feeding pressure by individual 0+ fish on zooplankton species.

Metabarcoding of native and invasive species in stomach contents of Great Lakes fishes

As aquatic invasive species (AIS) proliferate worldwide, a better understanding of their roles in invaded habitats is needed to inform management and introduction prevention strategies and priorities. Metabarcoding of stomach content DNA (scDNA) shows considerable promise in such regard. We thus metabarcoded scDNA from two non-native fish species (alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax)), and three native ones (bloater (Coregonus hoyi), ninespine stickleback (Pungitius pungitius), and slimy sculpin (Cottus cognatus)). Fishes (N = 376) were sampled in spring 2009 and 2010 from 73–128 m depths at three Lake Michigan sites. Four mitochondrial cytochrome oxidase 1 (CO1) primer sets designed to target five potential AIS prey, and a universal aquatic invertebrate CO1 primer set targeting both native and AIS prey were used. Quality controlled prey amplicons were matched to three AIS prey: Bythotrephes longimanus (mean percent frequency occurrence, all samples = 7%), Cercopagis pengoi (5%), and Dreissena rostriformis bugensis (11%). Neither invasive prey Dreissena polymorpha nor Hemimysis anomala were detected. Native prey Leptodiaptomus sicilis, Limnocalanus macrurus, and Mysis diluviana were relatively common in scDNA (respective mean percent occurrences, all samples: 48%, 25%, 42%). Analysis of variation in prey occurrences for sample site, predator species, sample year, sample depth, and predator total length (TL) indicated site and predator species were most important. However, B. longimanus occurrence in scDNA depended upon predator TL, perhaps indicative of its unique defensive spine limiting susceptibility to predation until fishes exceed species-specific gape-based limitations. Our analysis of native and invasive prey species indicated possible indirect AIS impacts such as native predators switching their diet due to AIS-driven losses of preferred native prey. Metabarcoding demonstrated that AIS are integrated components of the offshore Lake Michigan food web, with both native and non-native predators, and both invasive and native prey are affecting species interactions across multiple trophic levels.

Does a bigger mouth make you fatter? Linking intraspecific gape variability to body condition of a tropical predatory fish

In gape-limited predators, gape size restricts the maximum prey size a predator is capable to ingest. However, studies investigating the energetic consequences of this relationship remain scarce. In this study, we tested the hypothesis that gape-size variability influences individual body condition (a common proxy for fitness) in one of the largest freshwater teleost predators, the barramundi. We found that individual barramundi with larger gapes relative to body size had higher body condition values compared to conspecifics with smaller gapes. Body condition was highest soon after the wet season, a period of high feeding activity on productive inundated floodplains, and body condition decreased as the dry season progressed when fish were restricted to dry season remnant habitats. The increased condition obtained during the wet season apparently offsets weight loss through the dry season, as individuals with large gapes were still in better condition than fish with small gapes in the late-dry season. Elucidation of the links between intraspecific variability in traits and performance is a critical challenge in functional ecology. This study emphasizes that even small intraspecific variability in morphological trait values can potentially affect individual fitness within a species' distribution.

Trade-offs between larval survival and adult ornament development depend on predator regime in a territorial dragonfly

Trade-offs between juvenile survival and the development of sexually selected traits can cause ontogenetic conflict between life stages that constrains adaptive evolution. However, the potential for ecological interactions to alter the presence or strength of these trade-offs remains largely unexplored. Antagonistic selection over the accumulation and storage of resources could be one common cause of environment-specific trade-offs between life stages: higher condition may simultaneously enhance adult ornament development and increase juvenile vulnerability to predators. We tested this hypothesis in an ornamented dragonfly (Pachydiplax longipennis). Higher larval body condition indeed enhanced the initial development of its intrasexually selected wing coloration, but was opposed by viability selection in the presence of large aeshnid predators. In contrast, viability selection did not oppose larval body condition in pools when aeshnids were absent, and was not affected when we manipulated cannibalism risk. Trade-offs between larval survival and ornament development, mediated through the conflicting effects of body condition, therefore occurred only under high predation risk. We additionally characterized how body condition influences several traits associated with predator avoidance. Although body condition did not affect burst distance, it did increase larval abdomen size, potentially making larvae easier targets for aeshnid predators. As high body condition similarly increases vulnerability to predators in many other animals, predator-mediated costs of juvenile resource accumulation could be a common, environment-specific limitation on the elaboration of sexually selected traits.

Size selection by a gape-limited predator of a marine snail: Insights into magic traits for speciation

The intertidal snail Littorina saxatilis has repeatedly evolved two parallel ecotypes assumed to be wave adapted and predatory shore crab adapted, but the magnitude and targets of predator‐driven selection are unknown. In Spain, a small, wave ecotype with a large aperture from the lower shore and a large, thick‐shelled crab ecotype from the upper shore meet in the mid‐shore and show partial size‐assortative mating. We performed complementary field tethering and laboratory predation experiments; the first set compared the survival of two different size‐classes of the crab ecotype while the second compared the same size‐class of the two ecotypes. In the first set, the large size‐class of the crab ecotype survived significantly better than the small size‐class both on the upper shore and in the laboratory. In the second set, the small size‐class of the crab ecotype survived substantially better than that of the wave ecotype both on the upper shore and in the laboratory. Shell‐breaking predation on tethered snails was almost absent within the lower shore. In the laboratory shore crabs (Pachygrapsus marmoratus) with larger claw heights selected most strongly against the small size‐class of the crab ecotype, whereas those with medium claw heights selected most strongly against the thin‐shelled wave ecotype. Sexual maturity occurred at a much larger size in the crab ecotype than in the wave ecotype. Our results showed that selection on the upper shore for rapid attainment of a size refuge from this gape‐limited predator favors large size, thick shells, and late maturity. Model parameterization showed that size‐selective predation restricted to the upper shore resulted in the evolution of the crab ecotype despite gene flow from the wave ecotype snails living on the lower shore. These results on gape‐limited predation and previous ones showing size‐assortative mating between ecotypes suggest that size may represent a magic trait for the thick‐shelled ecotype.

Evolutionary stasis despite selection on a heritable trait in an invasive zooplankton

Invasive species are one of the greatest threats to ecosystems, and there is evidence that evolution plays an important role in the success or failure of invasions. Yet, few studies have measured natural selection and evolutionary responses to selection in invasive species, particularly invasive animals. We quantified the strength of natural selection on the defensive morphology (distal spine) of an invasive zooplankton, Bythotrephes longimanus, in Lake Michigan across multiple months during three growing seasons. We used multiple lines of evidence, including historic and contemporary wild-captured individuals and palaeoecology of retrieved spines, to assess phenotypic change in distal spine length since invasion. We found evidence of temporally variable selection, with selection for decreased distal spine length early in the growing season and selection for increased distal spine length later in the season. This trend in natural selection is consistent with seasonal changes in the relative strength of non-gape-limited and gape-limited fish predation. Yet, despite net selection for increased distal spine length and a known genetic basis for distal spine length, we observed little evidence of an evolutionary response to selection. Multiple factors likely limit an evolutionary response to selection, including genetic correlations, trade-offs between components of fitness, and phenotypic plasticity.