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.

穴居生物对沉积物的生物扰动（bioturbation）在海岸生态系统的功能中起着关键作用。穴居被认为是一项耗能高的活动，但穴居的能量成本以及多压力源（如盐度胁迫和波浪作用）对海洋生物扰动者的生物能学（bioenergetics）和穴居性能的潜在影响尚不清楚。我们研究了机械扰动和盐度胁迫对一种常见海洋生物扰动者——来自波罗的海的软壳蛤Mya arenaria（对照盐度15）的穴居行为、有氧能力（aerobic capacity）和挖掘能耗的影响。Mya arenaria的穴居效率存在显著个体差异，平均每次埋栖会消耗约7%的体能量储备。线粒体能力（mitochondrial capacity）较高且每次埋栖能耗较低的蛤表现出更强的耐力。经低盐度（5）或波动盐度（5-15）驯化3-4周后，蛤的穴居速度和重埋次数均降低，但全身或鳃的线粒体能力未受影响。波动盐度驯化使穴居的主要燃料利用从蛋白质转向脂质。我们的数据表明，盐度胁迫下蛤穴居性能的下降并非源于能量可获得性或有氧能力的限制，而必定涉及其他机制（如肌肉性能受损）。盐度胁迫导致的蛤穴居能力下降可能对浅海海岸生态系统中蛤的生存、活动和生态功能具有重要意义。