Scalloped hammerhead sharks (Sphyrna lewini) routinely perform rapid dives to forage on mesopelagic prey. These deep dives consist of intensive swimming followed by recovery periods in the surface mixed layer. Swimming muscle temperature profiles suggest that S. lewini suppresses gill function as a means to reduce convective heat loss during dives into cool water. Such intensive swimming behavior coupled with reduced respiration prompted us to test whether the aerobic and anaerobic metabolic capacities of the white swimming muscle tissue of this species are greater than those of other shark species from the same region. The activities of key enzymes used in aerobic and anaerobic metabolism provide an indirect indicator of the metabolic potential (“poise”) of a tissue. Here we measured the maximal activities [international units (µmol substrate converted to product per min, U) per gram of wet tissue mass at 10°C] of the citric acid cycle enzymes citrate synthase (CS) and malate dehydrogenase (MDH) and glycolytic enzymes pyruvate kinase (PK) and lactate dehydrogenase (LDH) from white swimming muscle of S. lewini. Enzyme activities, and ratios of these enzyme activities that indicate relative indexes of aerobic to anaerobic capacity, were compared to those measured in three sympatric coastal carcharhinid sharks and two deep-dwelling species, Echinorhinus cookei and Hexanchus griseus. This is the first report of swimming-muscle enzyme activity for these deep-dwelling species. In comparison to the other species, S. lewini had significantly higher activities of both LDH and MDH in the white muscle, and a higher MDH/CS ratio. The high LDH activities suggest that the white muscle of S. lewini relies on relatively high rates of anaerobic ATP production, with would result in build up of high lactate levels, during deep foraging dives. High MDH activity in S. lewini white muscle suggests the potential for lactate levels to be rapidly reduced when aerobic conditions are restored while in the surface mixed layer between dives. These biochemical characteristics may enable S. lewini to swim rapidly while suppressing gill function during deep dives and thereby exploit a very different ecological niche from sympatric shark species (e.g., coastal carcharhinids) and hunt more rapidly via faster swimming for deep-water prey compared to species that permanently inhabit deep depths.

中文翻译：

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深潜扇形双髻鲨白色游泳肌肉的有氧和厌氧姿势：与同域沿海和深水物种的比较


扇形双髻鲨 （Sphyrna lewini） 经常进行快速俯冲以觅食中上层猎物。这些深潜包括密集的游泳，然后在表面混合层中进行恢复期。游泳肌肉温度曲线表明，S. lewini 抑制鳃功能，以减少潜入冷水期间的对流热量损失。这种密集的游泳行为加上呼吸减少，促使我们测试该物种白色游泳肌肉组织的有氧和厌氧代谢能力是否大于同一地区的其他鲨鱼物种。需氧和厌氧代谢中使用的关键酶的活性提供了组织代谢潜力（“平衡”）的间接指标。在这里，我们测量了柠檬酸循环酶柠檬酸合酶 （CS） 和苹果酸脱氢酶 （MDH） 以及糖酵解酶丙酮酸激酶 （PK） 和乳酸脱氢酶 （LDH） 在 10°C 下每克湿组织质量的最大活性 [国际单位（μmol 底物转化为每分钟产物，U）] 来自雷氏链球菌白色游泳肌。将酶活性和这些酶活性的比率（表明有氧能力与厌氧能力的相对指标）与三种同域沿海 carcharhinid 鲨鱼和两种深栖物种 Echinorhinus cookei 和 Hexanchus griseus 中测量的酶活性和比率进行了比较。这是这些深栖物种的游泳肌酶活性的首次报道。与其他物种相比，S. lewini 在白肌中 LDH 和 MDH 的活性均显著升高，MDH/CS 比值较高。高 LDH 活性表明 S 的白色肌肉。 lewini 依赖于相对较高的厌氧 ATP 产生率，在深觅食潜水期间会导致高乳酸水平的积累。S. lewini 白色肌肉的高 MDH 活性表明，在潜水之间处于表面混合层时，当有氧条件恢复时，乳酸水平可能会迅速降低。这些生化特性可能使 S. lewini 能够在深潜时快速游泳，同时抑制鳃功能，从而利用与同域鲨鱼物种（例如，沿海 carcharhinids）截然不同的生态位，并通过更快的游泳更快地捕食深水猎物与永久栖息在深处的物种相比。