Treberg Jason R, Speers-Roesch Ben
Department of Biological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Department of Ocean Sciences, Memorial University of Newfoundland, St John's, Newfoundland, Canada A1C 5S7.
J Exp Biol. 2016 Mar;219(Pt 5):615-25. doi: 10.1242/jeb.128108.
The deep sea is the largest ecosystem on Earth but organisms living there must contend with high pressure, low temperature, darkness and scarce food. Chondrichthyan fishes (sharks and their relatives) are important consumers in most marine ecosystems but are uncommon deeper than 3000 m and exceedingly rare, or quite possibly absent, from the vast abyss (depths >4000 m). By contrast, teleost (bony) fishes are commonly found to depths of ∼ 8400 m. Why chondrichthyans are scarce at abyssal depths is a major biogeographical puzzle. Here, after outlining the depth-related physiological trends among chondrichthyans, we discuss several existing and new hypotheses that implicate unique physiological and biochemical characteristics of chondrichthyans as potential constraints on their depth distribution. We highlight three major, and not mutually exclusive, working hypotheses: (1) the urea-based osmoregulatory strategy of chondrichthyans might conflict with the interactive effects of low temperature and high pressure on protein and membrane function at great depth; (2) the reliance on lipid accumulation for buoyancy in chondrichthyans has a unique energetic cost, which might increasingly limit growth and reproductive output as food availability decreases with depth; (3) their osmoregulatory strategy may make chondrichthyans unusually nitrogen limited, a potential liability in the food-poor abyss. These hypotheses acting in concert could help to explain the scarcity of chondrichthyans at great depths: the mechanisms of the first hypothesis may place an absolute, pressure-related depth limit on physiological function, while the mechanisms of the second and third hypotheses may limit depth distribution by constraining performance in the oligotrophic abyss, in ways that preclude the establishment of viable populations or lead to competitive exclusion by teleosts.
深海是地球上最大的生态系统，但生活在那里的生物必须应对高压、低温、黑暗和食物稀缺等问题。软骨鱼类（鲨鱼及其近亲）是大多数海洋生态系统中的重要消费者，但在深度超过3000米的地方并不常见，在广阔的深渊（深度>4000米）中极其罕见，甚至很可能不存在。相比之下，硬骨鱼类通常能在深度约8400米的地方被发现。为什么软骨鱼类在深渊深度稀缺是一个主要的生物地理学谜题。在这里，在概述了软骨鱼类中与深度相关的生理趋势之后，我们讨论了几个现有的和新的假说，这些假说认为软骨鱼类独特的生理和生化特征可能是其深度分布的潜在限制因素。我们强调三个主要的、并非相互排斥的可行假说：（1）软骨鱼类基于尿素的渗透调节策略可能与低温和高压对深海中蛋白质和膜功能的相互作用效应相冲突；（2）软骨鱼类依靠脂质积累来实现浮力有独特的能量成本，随着深度增加食物可用性降低，这可能会越来越限制生长和繁殖产出；（3）它们的渗透调节策略可能使软骨鱼类异常缺乏氮，这在食物匮乏的深渊中可能是一个潜在的不利因素。这些假说共同作用可能有助于解释软骨鱼类在深海中稀缺的现象：第一个假说的机制可能对生理功能设定一个与压力相关的绝对深度限制，而第二个和第三个假说的机制可能通过限制在贫营养深渊中的表现来限制深度分布，其方式是排除可行种群的建立或导致被硬骨鱼类竞争排斥。
