Abstract Sessile invertebrates in the nearshore coastal and rocky intertidal habitats can experience oxygen limitation during low tide air exposure and environmental hypoxia events. For some organisms, unique morphological characteristics may make these events especially challenging. The giant acorn barnacle, Balanus nubilus, has the largest muscle fibers in the animal kingdom (diameters can exceed 3 mm in adults). At these extreme sizes, muscle cells may already be at the brink of insufficient oxygen delivery owing to low SA:V ratios and long intracellular diffusion distances. We are interested in characterizing the internal oxygen dynamics of B. nubilus during air exposure and seawater anoxia so that we can better understand how they maintain function of their giant muscle fibers during environmentally-induced oxygen limitation. To this end, we examined the following: 1) hemolymph pO2, pCO2, pH and ion (Na+, Cl−, K+, Ca2+) concentrations across 9 h exposure to air emersion, anoxic immersion and normoxic immersion (control), 2) oxygen consumption rates (MO2) of barnacles held in water and air for 6 h (at 10, 15 and 20 °C), and 3) respiratory behaviors (e.g., % time operculum open or cirri extended, cirral beat frequency) of barnacles during acute (6 h) exposure to emersion, anoxic immersion and normoxic immersion. Our data revealed that hemolymph pO2 declined significantly (by 3 h) in the anoxic barnacles, whereas the air-exposed barnacles maintained hemolymph oxygen levels that were intermediate to the control and anoxia barnacles. We also found that MO2 values for B. nubilus were very similar in seawater and air at a common temperature. These results suggest that B. nubilus can effectively acquire oxygen and support aerobic metabolism while in the air. This assumption was corroborated by our behavioral data, which revealed that air exposed (and anoxic) barnacles spent significantly more time engaged in cirral beating than control barnacles. Such a behavioral preference should enhance oxygen delivery to the internal respiratory surfaces inside the shell. Finally, we found significantly higher hemolymph [K+] in the emersed and anoxic barnacles, which - when coupled to the relatively stable pH values we observed in all treatments - may suggest involvement of K+ ions in an effective acid-base buffering system. In sum, we predict that muscle function would be preserved in B. nubilus during periods of low tide emersion, whereas environmental hypoxia events, which are increasing in frequency and duration as global climates change, have the potential to diminish functionality of their giant muscle fibers.

中文翻译：

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巨型橡子藤壶 Balanus nubilus 对限氧环境的生理反应

摘要 近岸沿海和岩石潮间带栖息地的无柄无脊椎动物在低潮空气暴露和环境缺氧事件中会经历氧气限制。对于某些生物，独特的形态特征可能使这些事件特别具有挑战性。巨大的橡子藤壶 Balanus nubilus 拥有动物界中最大的肌肉纤维（成人的直径可以超过 3 毫米）。在这些极端尺寸下，由于 SA:V 比率低和细胞内扩散距离长，肌肉细胞可能已经处于氧气输送不足的边缘。我们有兴趣表征 B. nubilus 在空气暴露和海水缺氧期间的内部氧气动力学，以便我们可以更好地了解它们在环境引起的氧气限制期间如何保持其巨大肌肉纤维的功能。为此，我们检查了以下内容：1) 暴露于空气中、缺氧浸泡和常氧浸泡（对照）9 小时内的血淋巴 pO2、pCO2、pH 和离子（Na+、Cl−、K+、Ca2+）浓度，2）耗氧率（MO2 ) 的藤壶在水和空气中保持 6 小时（在 10、15 和 20 °C 下），和 3) 藤壶在急性期（6 小时）的呼吸行为（例如，鳃盖打开的时间百分比或 crill 扩展，循环搏动频率）浸泡、缺氧浸泡和常氧浸泡。我们的数据显示，缺氧藤壶中的血淋巴 pO2 显着下降（3 小时），而暴露在空气中的藤壶保持的血淋巴氧水平介于对照和缺氧藤壶之间。我们还发现 B. nubilus 的 MO2 值在相同温度下的海水和空气中非常相似。这些结果表明 B. 在空气中，nubilus 可以有效地获取氧气并支持有氧代谢。我们的行为数据证实了这一假设，该数据表明暴露于空气（和缺氧）的藤壶比对照藤壶花更多的时间参与 circral 跳动。这种行为偏好应该会增强氧气输送到壳内的内部呼吸表面。最后，我们发现浸出的和缺氧的藤壶中血淋巴 [K+] 显着升高，这与我们在所有处理中观察到的相对稳定的 pH 值相结合时，可能表明 K+ 离子参与了有效的酸碱缓冲系统。总而言之，我们预测在低潮出现期间，B. nubilus 中的肌肉功能将得以保留，而环境缺氧事件，