Below the karstic Jaintia Hills in the northeastern Indian state of Meghalaya there is a cave system reachable only by a 100‐m descent down a vertical shaft. Leave your ropes behind and you soon come across a crystal‐clear stream that runs the system's length. Here and there it forms pools that, until February 2019, had managed to keep a secret: in them live the world's largest cave fish (Cave Karst Sci 2019; 46: 121–26). And they are whoppers. Most of the 250+ species of known cave fish are just 2–13 cm long, fewer than ten reach 23 cm, and just a couple with skinny eel‐like bodies reach 35 cm. The blind, pigment‐less, and very much “fish‐bodied” creatures in these Indian pools, however, can measure nearly 43 cm. And they have the bulk to match. From their photographs, these likely relatives of the (very!) much larger golden mahseer (Tor putitora) would not be embarrassed on a fishmonger's scale (Figure 1). Harder to tell is whether they themselves harbor secrets – secrets that may help explain something of our own evolution and even improve our health.

Some cave fish are already giving such secrets up. The Somalian blind cavefish (Phreatichthys andruzzii), for example, has been providing hints about why placental mammals are not the best at fixing damaged DNA (Curr Biol 2018; 28: 3229–43). Every summer we're reminded (if not exactly in the following words) via some source or other that UV light can cause mutagenic and cytotoxic lesions in our DNA. But most organisms don't need sunscreen; they can fix the DNA‐scrambling pyrimidine (usually thymine) dimers induced by UV light via a violet (or blue) light‐driven DNA repair system known as photoreactivation. The latter wavelengths cause the photolyase‐managed breakdown of these dimers, repairing the damage. Placental mammals, however, no longer have this knack – and neither do Somalian blind cavefish. It's been postulated that the fish lost it over millions of years of evolution in darkness, and that our early forerunners may have lost it in a similar way: by being nocturnal to avoid becoming dinosaur snacks! T rex: not only terrifying, but ultimately carcinogenic.

The blind, cave‐dwelling Mexican tetra (Astyanax mexicanus) has meanwhile been busy trying to tell us when diabetes is not diabetes (Nature 2018; 555: 647–51). Some populations of these creatures have a mutation in their insulin receptors that reduces insulin binding (ie these fish are insulin resistant and consequently have high blood sugar). When the very same mutation occurs in humans it causes diabetes. Yet these cavefish do not suffer from the glycated proteins found in diabetic humans, they live for the same 14 years as their light‐dwelling relatives, and the mutation even appears to let them grow better on their scanty food supply. It's not hard to see why there is interest in knowing how they manage these things.

But wait – there's more! Like us, the members of one population of Mexican cavefish cannot regenerate their hearts after surgically induced injury. Their surface‐living cousins, however, can and do (Cell Rep 2018; 25: 1997–2007). Understanding the genetic, cytological, and physiological reasons for this discrepancy could lead to advances in regenerative medicine. Yes, fish could mend your broken heart. And they might be able to help you at bedtime, too. Cave‐dwelling Mexican tetras hardly ever sleep (perhaps to maximize foraging time) and it's been revealed that their brains are full of neurons that make hypocretin, the dysregulation of which in humans is known to be involved in narcolepsy. If you block their hypocretin, these fish drop everything, chill out, and take a nap (see eLife 2018; 7: e32808 and e32637), suggesting they may hold the secret to the control of insomnia, too.

Actually, the current cavefish discovery rate is one thing keeping biologists awake. Alongside the Meghalaya monsters, recent years have seen the first European cave fish – a type of loach (Barbatula) – reported from southern Germany, and a subterranean loach (Eidinemacheilus proudlovei) encountered in Kurdistan (northern Iraq); the largest known habitat (more than 30 km × 160 km) for cave fish has been discovered in neighboring Iran; a Thai cave fish (Cryptotora thamicola) has been shown capable of climbing up waterfalls; and a previously undescribed type of snakehead, the Gollum snakehead (Aenigmachanna gollum), was found after floods flushed it out from the caverns below Kerala, India. Although it does look a little pale, this slender creature still has normal eyes (most cave fish do not), and its other sensory systems do not yet seem to have undergone the modifications expressed in other types of cave fish – a new arrival to the underground life and an open window on evolution.

Who knows how many more species of fish live unseen in waterways below our feet, or upon what mysteries they may one day throw light from their dark, dark world.

Adrian Burton

在印度东北部梅加拉亚邦（Meghalaya）的喀斯特（Jaintia）山下，有一个洞穴系统，只有沿垂直竖井下降100 m才能到达。抛开绳索，您很快就会遇到像水晶一样清澈的溪流，流淌着整个系统。它到处都是水池，直到2019年2月，它一直设法保密：在其中生活着世界上最大的洞穴鱼（Cave Karst Sci 2019; 46：121–26）。他们是whoppers。250多种已知的洞穴鱼类中，大多数大多只有2-13厘米长，不到10只达到23厘米，仅有一对具象鳗鱼的瘦骨鱼达到35厘米。但是，这些印度水池中的盲人，无色素且非常“鱼体”的生物可以测量近43厘米。并且他们有很多可以匹配。从他们的照片中，这些（非常！）大得多的金马赫（Tor putitora）的亲戚不会以鱼贩的规模为难（图1）。很难说的是它们本身是否包含秘密-这些秘密可能有助于解释我们自己的进化甚至改善我们的健康。

一些山洞鱼已经在泄露这样的秘密。例如，索马里盲cave（Phreatichthys andruzzii）一直在暗示为什么胎盘哺乳动物并非最擅长修复受损的DNA（Curr Biol 2018; 28：3229–43）。每年夏天，都会通过某种方式或其他方式提醒我们（如果不是以下话），紫外线会在我们的DNA中引起诱变和细胞毒性损伤。但是大多数生物不需要防晒霜。它们可以通过称为光再活化的紫色（或蓝色）光驱动DNA修复系统，修复由紫外线引起的扰乱DNA的嘧啶（通常是胸腺嘧啶）二聚体。后面的波长导致这些二聚体的光解酶控制分解，修复了损伤。但是，胎盘哺乳动物不再具有这种诀窍了，索马里的盲穴洞穴鱼也没有。据推测，这条鱼在黑暗中经过了数百万年的进化而失去了它，而我们的早期先行者也可能以类似的方式失去了它：通过夜间活动来避免成为恐龙零食！霸王龙：不仅令人恐惧，而且最终致癌。

与此同时，盲人居住在洞穴中的墨西哥四虫（Astyanax mexicanus）一直在忙于告诉我们什么时候糖尿病不是糖尿病（Nature 2018; 555：647-51）。这些生物的某些种群在其胰岛素受体中具有降低胰岛素结合的突变（即，这些鱼具有胰岛素抵抗力，因此血糖很高）。当人类发生完全相同的突变时，就会引起糖尿病。然而，这些穴居鱼并未患有糖尿病人中发现的糖基化蛋白质，它们与居住在轻型亲戚中的人生活了14年，而这种突变甚至使它们在缺乏食物的情况下生长得更好。不难看出为什么有兴趣了解他们如何管理这些事情。

但是，等等–还有更多！像我们一样，一个墨西哥洞穴鱼种群的成员在手术诱发的伤害后无法再生自己的心脏。然而，他们的表面生活表亲是可以做到的（Cell Rep 2018; 25：1997–2007）。了解这种差异的遗传，细胞学和生理学原因可能会导致再生医学的进步。是的，鱼可以修补你破碎的心。他们也许也可以在就寝时为您提供帮助。居住在洞穴中的墨西哥四肢几乎从未入睡（也许可以最大限度地延长觅食时间），而且据发现，他们的大脑充满了神经元，这些神经元会产生促胰高血糖素，人体中的这种调节失调与发作性睡病有关。如果您阻止它们的降钙素，这些鱼会掉落所有东西，让其放松，然后小睡一下（请参阅eLife 2018; 7：e32808和e32637），表明它们也可能是控制失眠的秘密。

实际上，当前的洞穴鱼发现率是使生物学家保持清醒的一件事。除了梅加拉亚邦（Meghalaya）的怪兽外，近年来还出现了德国南部报道的第一条欧洲山fish鱼（一种泥ach（Barbatula）），以及在库尔德斯坦（伊拉克北部）遇到的一种地下泥ach（Eidinemacheilus Charminglovei）。在邻国伊朗发现了最大的已知洞穴鱼栖息地（超过30公里×160公里）。泰国洞穴鱼（Cryptotora thamicola）已被证明能够攀登瀑布。以及之前没有描述过的蛇头类型，即Gollum蛇头（Aenigmachanna gollum）是在洪水从印度喀拉拉邦下方的洞穴中冲出后发现的。尽管看上去有些苍白，但这种细长的生物仍具有正常的眼睛（大多数洞穴鱼没有），其其他感觉系统似乎尚未经历过其他类型的洞穴鱼所表现出的改变-这是新出现的洞穴鱼。地下生活和进化的开放窗口。

谁知道在我们脚下的水道中还生活着多少种鱼类，或者有一天他们会从黑暗的黑暗世界中找到光明的秘密。

阿德里安·伯顿（Adrian Burton）