Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA 92697, USA.
J Exp Biol. 2018 Apr 13;221(Pt 8):jeb168039. doi: 10.1242/jeb.168039.
Fishes acclimated to hypoxic environments often increase gill surface area to improve O uptake. In some species, surface area is increased via reduction of an interlamellar cell mass (ILCM) that fills water channels between gill lamellae. Amphibious fishes, however, may not increase gill surface area in hypoxic water because these species can, instead, leave water and breathe air. To differentiate between these possibilities, we compared wild amphibious mangrove rivulus from two habitats that varied in O availability - a hypoxic freshwater pool versus nearly anoxic crab burrows. Fish captured from crab burrows had less gill surface area (as ILCMs were enlarged by ∼32%), increased rates of normoxic O consumption and increased critical O tension compared with fish from the freshwater pool. Thus, wild mangrove rivulus do not respond to near-anoxic water by decreasing metabolism or increasing O extraction. Instead, fish from the crab burrow habitat spent three times longer out of water, which probably caused the observed changes in gill morphology and respiratory phenotype. We also tested whether critical O tension is influenced by genetic heterozygosity, as is one of only two hermaphroditic vertebrate species that can produce both self-fertilized (inbred) or out-crossed (more heterozygous) offspring. We found no evidence for inbreeding depression, suggesting that self-fertilization does not impair respiratory function. Overall, our results demonstrate that amphibious fishes that inhabit hypoxic aquatic habitats can use a fundamentally different strategy from that used by fully aquatic water-breathing fishes, relying on escape behaviour rather than metabolic depression or increased O extraction ability.
适应低氧环境的鱼类通常会增加鳃表面积以提高氧气摄取量。在某些物种中，通过减少填充鳃片之间水通道的板间细胞质量 (ILCM) 来增加表面积。然而，两栖鱼类在低氧水中可能不会增加鳃表面积，因为这些物种可以离开水并呼吸空气。为了区分这些可能性，我们比较了来自两个栖息地的野生两栖红树林若鲉，这两个栖息地的氧气供应情况不同——一个是低氧淡水池，另一个是几乎缺氧的螃蟹洞穴。与来自淡水池的鱼相比，从螃蟹洞穴中捕获的鱼的鳃表面积较小（因为 ILCM 增大了约 32%），在正常氧消耗和临界氧张力方面的速率增加。因此，野生红树林若鲉不会通过降低新陈代谢或增加氧气提取来应对近缺氧水。相反，来自螃蟹洞穴栖息地的鱼离开水的时间长了三倍，这可能导致了观察到的鳃形态和呼吸表型的变化。我们还测试了临界氧张力是否受到遗传杂合性的影响，因为它是仅有的两种可以产生自交（近交）或杂交（更多杂合）后代的两性异形脊椎动物之一。我们没有发现近交衰退的证据，这表明自交不会损害呼吸功能。总的来说，我们的研究结果表明，栖息在低氧水生栖息地的两栖鱼类可以使用与完全水生呼吸鱼类不同的基本策略，依赖于逃避行为而不是代谢抑制或增加氧气提取能力。
