Effects of Intertidal Position on the Capacity for Anaerobic Metabolism and Thermal Stress Response in the Common Acorn Barnacle, Balanus glandula

Intertidal habitats are characterized by dynamic, tidally-driven fluctuations in abiotic and biotic factors. Many of the environmental stressors that vary across the intertidal (e.g., temperature, oxygen, food availability, predation pressure) are strong drivers of metabolic rate in ectotherms. As such, we predicted that there may be pronounced differences in the metabolic and stress physiology of conspecific sessile invertebrates occupying at different relative tidal heights. The common acorn barnacle Balanus glandula represents an ideal model organism in which to investigate the possibility of tidal height-dependent physiological differences, owing to their wide distribution in the intertidal zone and their eurytolerant nature. In the first chapter of my thesis, we investigate the hypothesis that B. glandula anchored in the low intertidal have a greater capacity for anaerobic metabolism than conspecifics in the high intertidal, and that this is due to increased predation pressure during submersion. Further, we explore the temporal and spatial fidelity of certain tidal-height driven trends in lactate dehydrogenase activity previously observed in our lab (i.e., higher LDH activity in low intertidal barnacles; Horn et al., 2021), and attempt to identify environmental variables that drive plasticity in LDH activity. We found that, in general, there were higher densities of B. glandula and gastropod whelk predators in the low intertidal compared to the high intertidal, but follow-up studies in the lab revealed that opercular closure in B. glandula was induced by predator exposure (Acanthinucella spirata) for less than 24h. This time frame for shell closure is unlikely to result in internal hypoxia or enhance capacity for anaerobic metabolism. We were therefore not surprised to find that LDH activity in B. glandula was likewise not affected by predator exposures (48h) carried out in the lab. After failing to find an effect of predators on LDH activity in B. glandula, we attempted to replicate the previous finding that LDH activity was highest in low intertidal populations of B. glandula. We did this at the original location in San Luis Obispo Bay, CA as well as at three novel field sites and across seasons and years. While we did observe variation in LDH activity over time and between sites, we did not consistently observe the same trend in LDH activity whereby low intertidal barnacles had the highest activity. In response to these variable patterns, we attempted to identify what environmental parameters, other than predation, might be responsible for plasticity in LDH activity. Unfortunately, neither temperature nor emersion stress – the two variables we examined – had any significant an effect on LDH activity in B. glandula. These data suggest that there must be multiple, interacting stressors – including tidal position - that influence the anaerobic metabolic capacity of B. glandula. In the second chapter of my thesis, we went on to investigate how the response to thermal stress might differ between populations of B. glandula from different vertical heights in the intertidal zone. To this end, we assessed how aerial temperature stress affected oxygen consumption rates (MO2), superoxide dismutase (SOD) activity, and time to mortality in B. glandula collected from both low and high intertidal positions. We found that barnacles from the low intertidal showed a significant increase in MO2 with higher temperature, while MO2 was unaffected by temperature in B. glandula from the high intertidal. We also observed that SOD activity levels were higher in the high intertidal barnacles compared to the low intertidal barnacles, although neither group was increasing SOD activity under higher temperature. Finally, we observed significantly longer survival times during thermal stress in barnacles from the high intertidal zone (e.g., LT50 = 8.75 h vs 5 h at 33˚C for the high and low barnacles, respectively), although this advantage seemed to be lost with the addition of desiccation stress at these same temperatures. It is evident that life in highest reaches of the intertidal zones is physiologically challenging, and this has resulted in a population of B, glandula barnacles that are less sensitive to and better suited to tolerate temperature extremes than conspecifics in the lowest intertidal regions. Understanding how habitat variation may differentially impact the metabolic and thermal stress physiology of B. glandula is increasingly important as climate change progresses. This is particularly significant considering that organisms in the intertidal already reside within a relatively stressful environment and may be living closer to their thermal tolerance limits than animals from less extreme habitats