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1. Experimental ocean acidification and food limitation reveals altered energy budgets and synergistic effects on mortality of larvae of a coastal fish

   https://doi.org/10.3389/fmars.2023.1240404  

   Siegfried, Emma; Johnson, Darren W. (August 2023, Frontiers in Marine Science)

   Ocean acidification (OA) presents a unique challenge to early life stages of marine species. Developing organisms must balance the need to grow rapidly with the energetic demands of maintaining homeostasis. The small sizes of early life stages can make them highly sensitive to changes in environmental CO₂levels, but studies have found wide variation in responses to OA. Thus far most OA studies have manipulated CO₂only, and modifying factors need to be considered in greater detail. We investigated the effects of high pCO₂and food ration on rates of growth and mortality of a coastal fish, the California Grunion (Leuresthes tenuis). We also examined how CO₂and food levels affected feeding success, metabolic rate, and swimming activity – processes reflective of energy acquisition and expenditure. In general, exposure to high CO₂decreased energy intake by reducing feeding success, and increased energy expenditure by increasing metabolic rate and routine swimming speed, though the magnitudes of these effects varied somewhat with age. Despite these changes in energetics, growth of biomass was not affected significantly by pCO₂level but was reduced by low ration level, and we did not detect an interactive effect of food ration and pCO₂on growth. However, under OA conditions, larvae were in poorer condition (as evaluated by the mass to length ratio) by the end of the experiment and our analysis of mortality revealed a significant interaction in which the effects of OA were more lethal when food energy was limited. These results are consistent with the idea that although energy can be reallocated to preserve biomass growth, increased energetic demand under ocean acidification may draw energy away from maintenance, including those processes that foster homeostasis during development. Overall, these results highlight both the need to consider the availability of food energy as a force governing species’ responses to ocean acidification and the need to explicitly consider the energy allocated to both growth and maintenance as climate changes. 

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2. How modularity and heterotrophy complicate the understanding of the causes of thermal performance curves: the case of feeding rate in a filter-feeding animal

   https://doi.org/10.1242/jeb.247776  

   Powell, Jackson A; Burgess, Scott C (June 2024, Journal of Experimental Biology)

   ABSTRACT Warming global temperatures have consequences for biological rates. Feeding rates reflect the intake of energy that fuels survival, growth and reproduction. However, temperature can also affect food abundance and quality, as well as feeding behavior, which all affect feeding rate, making it challenging to understand the pathways by which temperature affects the intake of energy. Therefore, we experimentally assessed how clearance rate varied across a thermal gradient in a filter-feeding colonial marine invertebrate (the bryozoan Bugula neritina). We also assessed how temperature affects phytoplankton as a food source, and zooid states within a colony that affect energy budgets and feeding behavior. Clearance rate increased linearly from 18°C to 32°C, a temperature range that the population experiences most of the year. However, temperature increased algal cell size, and decreased the proportion of feeding zooids, suggesting indirect effects of temperature on clearance rates. Temperature increased polypide regression, possibly as a stress response because satiation occurred quicker, or because phytoplankton quality declined. Temperature had a greater effect on clearance rate per feeding zooid than it did per total zooids. Together, these results suggest that the effect of temperature on clearance rate at the colony level is not just the outcome of individual zooids feeding more in direct response to temperature but also emerges from temperature increasing polypide regression and the remaining zooids increasing their feeding rates in response. Our study highlights some of the challenges for understanding why temperature affects feeding rates, especially for understudied, yet ecologically important, marine colonial organisms. 

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3. Estrogen Receptor Beta Mediates the Dipsogenic Effect of Estradiol in Ovariectomized Female Rats

   https://doi.org/10.1159/000530962  

   Santollo, Jessica; Edwards, Andrea A. (May 2023, Neuroendocrinology)

   Although the fluid inhibitory effects of estradiol are well characterized, a dipsogenic role of the hormone was recently identified. In ovariectomized (OVX) rats, unstimulated water intake, in the absence of food, was increased after estradiol treatment. The goals for these experiments were to further characterize the fluid enhancing effects of estradiol by determining the estrogen receptor subtype mediating the dipsogenic effect, examining saline intake, and testing for a dipsogenic effect of estradiol in male rats. Pharmacological activation of estrogen receptor beta (ERβ) increased water intake, in the absence of food, and was associated with changes in postingestive feedback signals. Surprisingly, activation of ERα reduced water intake even in the absence of food. A follow-up study demonstrated that when food was available, co-activation of ERα and ERβ reduced water intake, but when food was not available water intake was increased. In addition, in OVX rats, estradiol increased saline intake through changes in postingestive and orosensory feedback signals. Finally, although estradiol decreased water intake in male rats with access to food, estradiol had no effect on water intake in the absence of food. These results demonstrate that the dipsogenic effect is mediated by ERβ, the fluid enhancing effects of estradiol generalize to saline, and is limited to females, which implies that a feminized brain is necessary for estradiol to increase water intake. These findings will aid in guiding future studies focused on elucidating the neuronal mechanisms that allow estradiol to both increase and decrease fluid intake. 

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4. Effects of increased temperature on arctic slimy sculpin Cottus cognatus is mediated by food availability: Implications for climate change

   https://doi.org/10.1111/fwb.13659  

   Pennock, Casey A.; Budy, Phaedra; Atkinson, Carla L.; Barrett, Nick (December 2020, Freshwater Biology)

   Abstract Lakes are vulnerable to climate change, and warming rates in the Arctic are faster than anywhere on Earth. Fishes are sensitive to changing temperatures, which directly control physiological processes. Food availability should partly dictate responses to climate change because energetic demands change with temperature, but few studies have simultaneously examined temperature and food availability.We used a fully factorial experiment to test effects of food availability and temperature (7.6, 12.7, and 17.4°C; 50 days) on growth, consumption, respiration, and excretion, and effects of temperature (12 and 19.3°C; 27 days) on habitat use and growth of a common, but understudied, mid‐level consumer, slimy sculpinCottus cognatus, in arctic lakes. We also used bioenergetics modelling to predict consumptive demand under future warming scenarios.Growth rates were 3.4× higher at 12.7°C in high food compared to low food treatments, but the magnitude of differences depended on temperature. Within low food treatments, there was no statistical difference in growth rates among temperatures, suggesting food limitation. Consumption, respiration, and nitrogen excretion increased with temperature independent of food availability. Lower growth rates coincided with lower phosphorus excretion at the highest temperature, suggesting that fish selectively retained phosphorus at high temperatures and low food. In habitat choice experiments, fish were more likely to use the 12°C side of the tank, closely matching their optimal temperature. We predicted a 9% increase in consumption is required to maintain observed growth under a 4°C warming scenario.These results highlight considering changes in food resources and other associated indirect effects (e.g. excretion) that accompany changing temperatures with climate change. Depending on how food webs respond to warming, fish may cope with predicted warming if density‐dependent feedback maintains population sizes. 

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5. Sensory perception plays a larger role in foraging efficiency than does heavy-tailed movement strategies.

   https://doi.org/doi:10.1016/j.ecolmodel.2019.02.015  

   LaScala-Gruenewald, DE; Mehta, RS; Denny, MW (January 2019, Ecological modelling)

   Animals must balance their rates of energetic intake and expenditure while foraging. Several mathematical models have been put forward as energetically optimal foraging strategies when the food environment is sparse (i.e., the distance between food patches in the environment is much larger than the distance from which the forager can perceive food). In particular, Lévy walks with a power law exponent approaching 1 are considered optimal for destructive foragers. However, these models have yet to explore the role of sensory perception in foraging success as the distance between food patches approaches the distance from which the forager can perceive food. Here, we used an agent-based modeling approach to address this question. Our results concur that lower values of the power law exponent (i.e. values approaching 1) result in the most food found, but in contrast to previous studies, we note that, in many cases, lower exponents are not optimal when we consider food found per unit distance traveled. For example, higher values of the exponent resulted in comparable or higher foraging success relative to lower values when the forager's range of sensory perception was restricted to an angle±30° from its current heading. In addition, we find that sensory perception has a larger effect on foraging success than the power law exponent. These results suggest that a deeper examination of how animals perceive food sources from a distance may affect longstanding assumptions regarding the optimality of Lévy walk foraging patterns, and lend support to the developing theoretical shift towards models that place increasing emphasis on how organisms interact with their environments. 

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