Browsing by Author "Charpentier, Corie L."
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Item Fish kairomone-induced defenses in crustacean zooplankton: on the diversity of strategies, mechanism, and environment(University of Delaware, 2017) Charpentier, Corie; Charpentier, Corie L.Predator chemical cues, or kairomones, induce behavioral and morphological defense strategies in zooplankton. For example, several species become more responsive to light following exposure to fish kairomones. These behavioral changes likely impact diel vertical migration (DVM). A nearly universal behavior amongst zooplankton, DVM generally describes the nighttime ascent to feed and daytime descent to avoid visual predators. Although predator avoidance was proposed as the adaptive significance of DVM decades ago, we know little about the mechanism by which kairomones alter photobehavior. In addition, several zooplankton species exhibit kairomone-induced morphological defenses (e.g. spines), although previous reports of morphological plasticity have been generally limited to freshwater zooplankters. Despite laboratory and field evidence of kairomone-induced defenses, further work is needed to quantify the effective concentration of kairomone molecules in marine environments. ☐ Here, I first evaluated the mechanism by which kairomones enhance photoresponses in zooplankton. Comparing behavior and visual physiology (electrophysiology, eye structure), I found that kairomones altered photobehavior by increasing visual sensitivity at the photoreceptor level in the larvae of two marine crab species, Rhithropanopeus harrisii and Hemigrapsus sanguineus, as well as in a branchiopod crustacean, Artemia spp. Using similar behavioral experiments, I also found that signaling by a ubiquitous neurotransmitter, γ-aminobutyric acid (GABA), is required for detection and/or processing of predator kairomones in larval H. sanguineus. In addition, larvae lost the ability to detect kairomones after exposure to low pH/high CO2, over timescales that reflect diel/tidal pH cycles in coastal ecosystems (12 h). The mechanism of this pH effect differs from that described in fish, and the loss of kairomone-induced photosensitivity at low pH may be adaptive in these zooplankton under diel-cycling pH. ☐ In addition to behavioral defenses, crab larvae have spines. In some species, including R. harrisii, these spines are known to aid in predator defense. The freshwater crustacean zooplankton, Daphnia, exhibit drastic spine elongation after exposure to predator kairomones, but similar phenotypic plasticity has not been observed in estuarine or marine crustacean zooplankton. Here, I assessed the effect of developmental kairomone exposure on the morphology of crab larvae (R. harrisii and H. sanguineus). Kairomone exposure resulted in slightly elongated spines in larval R. harrisii as well as decreased consumption by a planktivorous fish, Menidia mendia, suggesting that the observed spine elongation is an inducible defense. ☐ The described behavioral and morphological defenses are induced by fish kairomones. In estuarine and marine systems, these signaling molecules are derived from body mucus and include sulfated glycosaminoglycans (sGAGs). Little work has been done to quantify these cues in marine environments or to assess whether kairomone concentrations used in laboratory studies reflect natural scenarios. During a two-year field study, I applied quantitative chemical and behavioral assays to measure these kairomones in an estuarine environment. I found that sGAG concentration and zooplankton photosensitivity were positively related to fish quantity. Further, sGAG concentration was similar between field samples and mucus extracts used in previous laboratory experiments, supporting that kairomone-induced changes to zooplankton behavior are not an artifact of laboratory experiments. ☐ The results of this dissertation have important implications to evolutionary biology, namely the evolution of sensory systems. For example, we can now compare the neural mechanisms that mediate kairomone-induced photobehavior in crustaceans to other zooplankton and ancestral defense mechanisms. More broadly, DVM impacts biogeochemical cycles and ecological interactions in aquatic environments. Hence, assessing the response of migrating zooplankton to environmental change (e.g. acidification, predator abundance) is essential in predicting ecosystem health. Although previous laboratory work has provided immense insight to zooplankton behavior in a controlled environment, there is still a great need for complementary fieldwork.Item Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night(PLOS Biology, 2021-10-19) Cohen, Jonathan H.; Last, Kim S.; Charpentier, Corie L.; Cottier, Finlo; Daase, Malin; Hobbs, Laura; Johnsen, Geir; Berge, JørgenLight plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only “twilight” periods defined by the sun’s elevation below the horizon at midday; we term this “midday twilight.” Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light environment is relevant to behavioral and visual processes. We reveal from acoustic field observations that the zooplankton community is undergoing diel vertical migration (DVM) behavior. Furthermore, using electroretinogram (ERG) recording under constant darkness, we show that the main migratory species, Arctic krill (Thysanoessa inermis) show endogenous increases in visual sensitivity during the subjective night. This change in sensitivity is comparable to that under exogenous dim light acclimations, although differences in speed of vision suggest separate mechanisms. We conclude that the extremely weak midday twilight experienced by krill at high latitudes during the darkest parts of the year has physiological and ecological relevance.