No one was quite prepared for what the Doc Ricketts , a remotely operated submersible vehicle, would reveal when, in late March 2015, it illuminated the gloomy depths of the Gulf of California. Between 600 and 900 m below the surface, in one of the most extreme low‐oxygen environments anywhere in the world's oceans, it found fish. Lots of fish. In fact, the seabed was littered with cusk eels (Cherublemma emmelas ) and lollipop catsharks (Cephalurus cephalus ) ( https://youtu.be/ZjIUe​9K4NMU). The instruments read a partial pressure of oxygen value for the surrounding water of just 0.12 kilopascals – almost an order of magnitude more hypoxic than any hypoxia‐tolerant fish was thought able to stand, and 27 times lower than the average for such fish. And yet here, where there was virtually no oxygen at all, some fish were even resting with their heads dug into the mud, ostensibly reducing their oxygen supply even further. Their abundance even tapered off as the vehicle reached more oxygen‐rich water. How could they survive here? No one is sure. But with the ocean's dissolved oxygen levels falling and hypoxic dead zones expanding in size and number, natural selection may smile kindly on animals able to pull off this kind of trick, and perhaps even more so on those that require no oxygen at all.

Oxygen, is, of course, handy stuff. If you have the mitochondria for it, you can use it to make lots of ATP. Indeed, for most animals, oxygen is very difficult to do without; just try holding your breath (warning: do not attempt to beat static apnea world‐record holder Stephane Mifsud's staggering 11 min 35 s). In environments where oxygen becomes meager or unavailable, one solution is to take it with you. Whales, for example, take into their bloodstream some 80–90% of the oxygen contained in a single breath (compared to our 15%), helping them stay underwater for an hour or more. You could also make more of less: those lollipop catsharks (WebFigure 1) have oversize heads with oversize gills, likely increasing their ability to absorb oxygen. Fancy hemoglobin helps too. For instance, some hummingbirds in the Andes can flit about at altitudes of around 4000 m because their hemoglobin has a much greater affinity for oxygen. You could also make do with less, forget those mitochondria for a while, switch to anaerobic respiration, and slow down. Common carp (Cyprinus carpio ) marooned in puddles of liquid mud during a drought can survive severe hypoxia in this way. Goldfish (Carassius auratus ) and crucian carp (Carassius carassius ) can do even better, going for months with no oxygen at all, running on glycogen stored in their liver and getting rid of the troublesome lactic acid (an end‐product of glycolysis) by converting it into ethanol and shedding it at the gills. Naked mole rats (Heterocephalus glaber ) have a different metabolic trick. When the going gets tough they use fructose instead of glucose to avoid feedback inhibition of glycolysis via phosphofructokinase, allowing them to live in hypoxic underground colonies and even to survive some 18 minutes of full‐on anoxia. Despite these impressive feats, however, no animal can do without oxygen forever…or can it?

It is becoming ever clearer that some can. In 2010, Roberto Danovaro and his colleagues found three species of loriciferan (Figure 1a), all new to science, on the bottom of the L'Atalante Basin, a hypersaline, hydrogen sulfide‐rich, and completely anoxic “lake” over 3500 m down in the Mediterranean Sea about 190 km west of Crete (BMC Biol 2010; 8 : 30). Loriciferans are tiny – usually well under 1 mm – but they are definitely multicellular eukaryotes with a head, a gut, and a protective coat. They look a bit like a vase full of science fiction‐inspired flowers. Experiments on the collected creatures suggested them to be metabolically active, even reproducing in their chemically hostile deep‐sea world under huge pressure. Electron microscopy confirmed them to have no mitochondria; what use would they be in a completely anoxic medium? Rather, they had hydrogenosomes, organelles which, until then, had only been seen in certain single‐celled protists restricted to anaerobic metabolism. Danovaro's loriciferans, in contrast, were multicellular animals.

And there are others. In February 2020, news broke of a Cnidarian (think jellyfish and corals) – Henneguya salminicola (Figure 1b), a parasite that infests the white muscles of salmon and perhaps an annelid worm – with no mitochondrial genome (P Natl Acad Sci USA 2020; doi.org/10.1073/pnas.1909907117). In fact, it has also lost nearly all the nuclear genes required for aerobic respiration. Although oxygen must arrive at salmon muscles, these parasites either fail to get it (perhaps they can't compete for it with the host cells and no longer try) or just don't want it.

There are likely more undescribed obligate anaerobic animals waiting to be discovered, stuck to their patches of anoxic marine mud, and more places where cusk eels and lollipop catsharks meet to party at the bottom of the sea. If the growth of dead zones continues we may not have to look quite so hard to find them. I am not sure, however, whether that is much compensation for the loss of all those creatures that cannot cope with a deoxygenated ocean.

Adrian Burton

谁也不为了什么准备文件里基茨的时候，在2015年3月下旬，它照亮加利福尼亚湾的灰暗深处，一个遥控潜水器，将显露。在海面以下600至900 m之间，在世界海洋中最极端的低氧环境之一中，它发现了鱼类。很多鱼。实际上，海底到处都是小（Cherublemma emmelas）和棒棒糖cat（Cephalluus cephalus））（https://youtu.be/ZjIUe 9K4NMU）。仪器读取的周围水的氧气分压仅为0.12千帕斯卡-比认为耐缺氧的鱼类能够忍受的缺氧量高近一个数量级，比此类鱼类的平均水平低27倍。然而，在这里几乎根本没有氧气的地方，有些鱼甚至还把头埋在泥里休息，表面上甚至进一步减少了氧气供应。随着车辆注入更多的富氧水，它们的丰度甚至逐渐降低。他们如何在这里生存？没有人确定。但是随着海洋中溶解氧水平的下降和缺氧死区的大小和数量的增加，自然选择可能会对那些能够实现这种技巧的动物表现出友好的笑容，甚至对那些根本不需要氧气的动物来说，可能会更加友好。

氧气当然是方便的东西。如果您具有线粒体，则可以使用它来制造许多ATP。实际上，对于大多数动物来说，没有氧气是很难做到的。只需屏住呼吸即可（警告：请勿尝试击败静态呼吸暂停世界纪录保持者Stephane Mifsud惊人的11分35秒）。在氧气贫乏或无法使用的环境中，一种解决方案是随身携带氧气。例如，鲸鱼一次呼吸会吸收大约80-90％的氧气（相比之下，我们的15％是氧气），帮助他们在水下停留一个小时或更长时间。您也可以少花钱多：棒棒糖猫鲨（WebFigure 1）的头部较大，g片也较大，可能增加了它们吸收氧气的能力。花式血红蛋白也有帮助。例如，安第斯山脉中的一些蜂鸟可以在大约4000 m的高度飞来飞去，因为它们的血红蛋白对氧气的亲和力更大。您也可以少花钱，忘记那些线粒体一会儿，切换到无氧呼吸，然后放慢脚步。鲤鱼（鲤鱼）在干旱液体泥水坑被困在这种方式生存严重缺氧。金鱼（Carassius auratus）和（鱼（Carassius carassius）甚至可以做得更好，可以连续几个月完全没有氧气地运转，依靠储存在他们肝脏中的糖原运行，并通过消除麻烦的乳酸（糖酵解的最终产品）。将其转化为乙醇并在at处脱落。裸mole鼠（Heterocephalus glaber）有不同的代谢技巧。当事情变得艰难时，他们使用果糖代替葡萄糖，以避免通过磷酸果糖激酶抑制糖酵解的反馈，使他们生活在低氧的地下菌落中，甚至可以在全缺氧状态下存活约18分钟。尽管有这些令人印象深刻的壮举，但是没有动物可以永远缺氧……还是可以吗？

有些人可以变得越来越清楚。2010年，Roberto Danovaro及其同事在L'Atalante盆地底部发现了3种新发现的Loriciferan（图1a），这是一种超盐分，富含硫化氢且完全缺氧的“湖”，覆盖了3500 m在克里特岛以西约190公里处的地中海下（BMC Biol 2010; 8：30）。狼蛛非常小，通常小于1毫米，但它们绝对是具有头部，内脏和保护层的多细胞真核生物。它们看上去有点像一个装满科幻风格花朵的花瓶。对收集的生物进行的实验表明，它们具有代谢活性，甚至在巨大的压力下在化学不利的深海世界中繁殖。电子显微镜检查证实它们没有线粒体。它们在完全缺氧的培养基中有什么用？相反，它们具有氢核小体，细胞器，直到那时，仅在某些限于厌氧代谢的单细胞生物中才看到它们。相反，达诺瓦罗的狼蛛是多细胞动物。

还有其他。2020年2月，有消息人士（想像水母和珊瑚）Henneguya salminicola（图1b）爆发了消息，这种寄生虫感染了鲑鱼的白肌肉，也许还感染了无性蠕虫，没有线粒体基因组（P Natl Acad Sci USA， 2020； doi.org/10.1073/pnas.1909907117）。实际上，它也失去了有氧呼吸所需的几乎所有核基因。尽管氧气必须到达鲑鱼肌肉，但是这些寄生虫要么无法获取氧气（也许它们无法与宿主细胞竞争，并且不再尝试），要么就是不想要氧气。

可能还有更多未描述的专性厌氧动物等待发现，粘在它们的缺氧海洋泥块上，还有更多的地方，海鳗和棒状黄sh鲨在海底相遇。如果死区的增长继续下去，我们可能不必费劲寻找它们。但是，我不确定这是否能弥补所有无法应对缺氧海洋的生物的损失。

阿德里安·伯顿（Adrian Burton）