Um Ladaw Cave in India is a chamber 300 feet below the surface that fills to the brim with water in the rainy season, and is only accessible in the winter dry season…well, “accessible” is a relative term. This is how you get there.
Once you make the descent, there are pools of water filled with a large population of pale, blind cave fish. These are big fish, too. They are probably fed by the constant trickle of organic material flowing down from the surface. Deep caves are also a nesting place for bats that poop into the water, which just tells you that some animals can eat anything.
That kind of diet must leave them desperate — in the video, you can see them trying to gnaw on the camera lens. Anything is fair game.
The existence of this pattern of pigment and eye loss in multiple species around the world tells you that something as complex as vision requires constant maintenance via natural selection, though. It’s remarkably consistent that animal species living in the total darkness of deep caves tend to all become pallid and blind over time.
Would removal of selection “pressure” in the dark cave context cease mitigating against random errors that dissipate ordered structures such as eyes that no longer provide functional benefits AND/OR does constant ever scrutinizing selection (omnipotent/omniscient) “recognize” the inefficiency of maintaining eyes when internal resources could be better put toward other tasks?
Is mutation/drift OR efficiency the highest concern in downgrading of eyes in cave fish?
NatGeo Article:
OP:
This found footage was all that remained of the expedition.
Given that eyes are retained in deep-sea fish, even the ones living at the deepest, darkest parts of the oceans but almost universally are lost among cave fish (including species that live in the deep caves of the Congo), there’s probably a selective pressure against eyes in subterranean environments.
@3 The is bioluminescence in the deep ocean, and access to lit depths at all times, as even deep water fish can ascend. There may even be scant amounts of light filtering through that they can make use of. Caves are much more absolute when it comes to blocking out light than the deep ocean.
@1: This was discussed here a few years ago. To sum up it doesn’t look like efficiency, since some fish embryos actually grow eyes and then abandon them, but that they get disrupted by mutations enhancing the jaws instead.
@3: Except bioluminescence is actually very common in the deep sea, so if you want to eat things like other animals or even bacteria-covered marine snow eyes will still help. I’m not sure there are many blind fish even in ocean caves. In comparison, there’s almost no bioluminescence in freshwater.
Kept in the dark ,and fed on Bat sh1t .What a life .
@ 4 & 5: But there are fish that live at the deepest parts of the ocean and survive by probing the mud or simply sitting on the seabed facing the current and wait for food to passively drift into their mouths and they still retain eyes. If eyes are a useful feature for an organism, they get retained even in the most extreme conditions but with subterranean animals they tend to disappear quite rapidly whether you’re talking about cave fish or golden moles. To me that indicates that there’s a selective pressure against having eyes in animals with specific lifestyles.
@5- monad
Thanks for pointing to PZ’s previous coverage of this. Seems some sort of antagonistic offset of gene action that relates roughly to what Darwin called “correlation of growth”. But is that third way PZ puts forward from other’s work in the field necessarily mutually exclusive from an efficiency argument? Channelling Gould the historic origin of the shift in developmental emphasis from eyes and sight to jaws and taste was a product of pleiotropy in how the genes hedgehog and pax6 interrelate (huge Hemi handwave-3H). But given this origin, wouldn’t it be exaptatively beneficial that the fish isn’t devoting scarce bat guano resources (yummy) to adult eye structural maintenance and related brain processing, which can be allotted to taste and tactility? The energy allotment is not primary in the origin of the pleiotropic shift, but perhaps secondarily relates to misuse of calories not impacting the number of haploids the organisms can spew forward toward subsequent generations. And yes there is some early developmental recapitulation (oh no he didn’t) of now atavistic eye structure, but there are other vestiges organisms go through the trouble of forming because imperfection. It’s that adult eyes are not sapping resources that is important.
My preference for neutralistic explanations cannot be rescued because as PZ said in post you linked: “The genes involved in generating the eye all seem to be present and functional in the blind cave fish.”
Forgot to label my secondary fortuitous efficiency tangent with Gould’s notion of current utility (versus historic origin). Still a possible exaptive byproduct of pleiotropic origin.
The fish is described here:
https://www.researchgate.net/publication/338066161_The_world's_largest_known_subterranean_fish_a_discovery_in_Meghalaya_NE_India_of_a_cave-adapted_fish_related_to_the_Golden_Mahseer_Tor_putitora_Hamilton_1822
Most of the following comes from there:
The describers see the fish’s closest relative as the golden mahseer, Tor putitora, a large minnow or carp. An opportunistic omnivore.
They are the largest known cave fish, by far. Exceeding 42 cm in the largest individuals. The two next largest species are eel-like (thin) and get to about 32 to 35 cm. The remaining cave fish get to no more than about 22 cm, most being 3 to 7 cm.
Their size suggests that they have a pretty good protein source. Indeed the authors’ last sentence contains: “The large size … is probably related to a plentiful food supply.”
I’d suspect that unfortunate bats make up some of their diet. Young ones that are dropped and the occasional injured adult. They respond aggressively to surface disturbance, see them bite at the caver’s boots at the water’s edge in the video. Since seasonal flooding occurs there, these floods also probably wash in a variety of critters as well the vegetation that the cavers observed. PZ mentions the bat poop but the describers don’t mention any food besides the “biscuit crumbs” that they used to help capture a specimen. But I’d bet that assuming that the bats are not fruit bats but rather insectivorous spp., then their poop might retain a relatively high protein content. Something like cat poop, which is why dogs will eat it, I hope.
@5 monad
That discussion covers just one species of cave fish. There are actually about 250 very different species. The genetic changes may be very different between them. I don’t know if any others have actually had their genetics analyzed. They might all have the same mechanism, I don’t know for sure.