Effect of salinity and mechanical disturbance on the energy status and mitochondrial respiration of soft shell clam Mya arenaria

Bioturbation of sediments by burrowing organisms plays a key role in the functioning of coastal ecosystems. Burrowing is considered an energetically expensive activity, yet the energy costs of burrowing and the potential impacts of multiple stressors (such as salinity stress and wave action) on bioenergetics and burrowing performance of marine bioturbators are not well understood. We investigated the effects of mechanical disturbance and salinity stress on the burrowing behavior, aerobic capacity and energy expense of digging in a common marine bioturbator, the soft-shell clam Mya arenaria from the Baltic Sea (control salinity 15). Mya arenaria showed large individual variability in the burrowing efficiency, with an average of ~7% of the body energy reserves used per burial. Clams with higher mitochondrial capacity and lower energy expenditure per burial showed higher endurance. Acclimation for 3-4 weeks to low (5) or fluctuating (5-15) salinity reduced the burrowing speed and the number of times the clams can rebury but did not affect the mitochondrial capacity of the whole body or the gill. Acclimation to the fluctuating salinity shifted the predominant fuel use for burrowing from proteins to lipids. Our data indicate that the reduced burrowing performance of clams under the salinity stress is not due to the limitations of energy availability or aerobic capacity but must involve other mechanisms (such as impaired muscle performance). The reduction in the burrowing capacity of clams due to salinity stress may have important implications for survival, activity and ecological functions of the clams in shallow coastal ecosystems.掘穴生物对沉积物的生物扰动（bioturbation）在海岸生态系统的功能运转中发挥核心作用。掘穴行为被认为是一项高能耗活动，但目前学界对掘穴的能量成本，以及盐度胁迫（salinity stress）、波浪作用等多重胁迫因子对海洋生物扰动者（marine bioturbators）的生物能学（bioenergetics）和掘穴表现的潜在影响仍缺乏深入认知。本研究以采自波罗的海的常见海洋生物扰动者——软壳蛤（Mya arenaria）为实验对象，设置对照盐度为15，探究了机械扰动与盐度胁迫对其掘穴行为、有氧能力（aerobic capacity）以及掘穴能量消耗的影响。软壳蛤的掘穴效率存在显著的个体差异，单次掘穴平均消耗约7%的机体能量储备；线粒体容量更高、单次掘穴能量消耗更低的个体，其耐力表现更佳。将软壳蛤经3-4周的低盐度（盐度5）或波动盐度（盐度范围5-15）驯化（acclimation）后，其掘穴速度与可重复掘穴的次数均出现下降，但机体整体及鳃组织的线粒体容量（mitochondrial capacity）未受影响。经波动盐度驯化后，软壳蛤掘穴的主要供能底物从蛋白质转变为脂质。本研究数据表明，盐度胁迫下软壳蛤掘穴表现的下降并非源于能量供给或有氧能力的限制，而是由其他机制（如肌肉功能受损）所导致。盐度胁迫引发的软壳蛤掘穴能力下降，可能对浅海海岸生态系统中该物种的存活、活动及生态功能产生重要影响。