The murky world of cavefish

Speaking on an episode of the BBC Earth podcast, Chakrabarti explained that for him, these places are caves. From enormous hollowed out caverns, to tiny caves the width of a helmet, Chakrabarti’s ventures have led to the discovery of 15 new species of fish.

“There are all kinds of things that can happen in a cave. You can get hypothermia because it’s too cold, or you can overheat because it’s too hot, and that can happen in the same cave.”

We currently know of almost 300 species of cavefish, and roughly a third of them have been discovered in China. While each cavefish species has evolved in a different way, over time they will develop distinct characteristics that set them apart from their above ground ancestors.

Notably, over multiple generations cavefish will develop reductions in eyesight as well as melanin pigment. In other words, they evolve to become both blind and paler, as they adapt to a dark cave environment.

Exactly what leads fishes into these caves in the first place varies from location to location: “Each time one lineage goes into a cave, it could be for different reasons,” explains Chakrabarti. “One group could get flushed into a cave and get stuck there and find a way to survive over thousands or millions of years. Others got there pretty recently, or are able to even escape and so may use that as a refuge.”

While there’s still some mystery around how and why each group entered these caves, Chakrabarti’s risky ventures provide us with important information about the biology of how we see, and the geology of our planet.

On his first cave dive in Madagascar, Chakrabarti and a field team discovered a new species of cavefish that they named Typhleotris mararybe, which means ‘big sickness’ in Malagasy. It measured 38mm in length and - unlike many other cavefish - was darkly coloured.

“It was one of the few species that I’ve seen and upon first seeing it I knew it was new. There are no other darkly coloured pigmented cavefishes,” says Chakrabarti.

Upon further study of this species, the team found that its closest relatives live over 6,000 kilometres away in Australia. However, such small freshwater cavefish could not have swum across the Indian Ocean, so how did they get there? The team concluded that they didn’t move at all; the continents did.

It’s believed the two species were separated more than 100 million years ago, which is roughly the time that the southern continents were last together. So, such a discovery also allows us to measure with more accuracy the date and time of ancient geological events.

But descending into the sinkhole in Madagascar by way of chain and ladder to discover a new cavefish species was no easy feat. And some of the team who spent four hours snorkelling around in the dark caught something else previously undiscovered.

In the days following the cave expedition, members of the team who had stayed snorkelling for hours developed a debilitating viral sickness that nobody could properly identify. They called the sickness sinkhole fever.

“Swimming around basically a pit where things fall and don’t come out isn’t the best place to be accidentally drinking the water, and we probably picked up not just a new species but some new illnesses unknown to man previously.”

And yet, despite all these challenges Chakrabarti finds an unlikely sense of solace in the dark and hostile cave environment.

“There is no darkness you can experience like that. If you shut your eyes or open them you can’t tell the difference. And then the sounds... sometimes you can hear water dripping or flowing, but it can be quiet and that frightens people. But it’s also just really calming and meditative.”

Featured image © Brent Durand | Getty