Under cover...or all over? Meet the Karsts

Sound like a bad sitcom, huh?

Stay with me here...

Every once in a while, it's interesting to contemplate an aquatic habitat that we don't think about all that much in the hobby. Today, let's talk about one that's definitely a bit different than the ones we usually find ourselves working with.

Let's consider the habitats around the karsts!

The what?

The karsts.

"WTF is a 'karst', Fellman?

A karst is an area of land made up of limestone. Limestone, also known as chalk or calcium carbonate, is a soft rock that dissolves in water. This process produces geological features like ridges, towers, fissures, sinkholes and other characteristic landforms. Many of the world’s largest caves and underground rivers are located in karstlands.

Karstic terrain. Image by Jan Nyssen (used under CC BY-SA 4.0)

The porous limestone rock holds a lot of groundwater, ponds, and streams, sometimes located underground. And those cool  structures known as cenotes (closed basins)! Yeah, we'll revisit those some other time.

Karsts are characterised by the presence of caves, sink holes, dry valleys and "disappearing" streams. These landscapes are known for their groundwater flow and efficient drainage of surface water through a wide network of subterranean conduits, fractures and caves.

Karst are found throughout the world, including France, China, the Yucatán Peninsula; South America, and parts of the United States.

In typical karstic habitats, the water is very clear, becoming turbid after heavy rains. Flash floods occur several times during the rainy season. In this period the stream width increases, making available habitats to be colonized, called here "temporary stretches".

Are you thinking what I'm thinking? Yeah, these could be interesting aquarium subjects! 

Yeah. And since a bunch of 'em occur in South America, where some of our fave fishes come from...this could be really interesting!

A fascinating neotropical karst landscape is located in the São Francisco River basin, Minas Gerais State, in Brazil. The fish diversity in these waters is significant. One study that I stumbled upon identified 28 species distributed in 3 orders and 9 families in this one locale alone!

The pH values in the South American karst habitats I found studies on range from 6.3 to 8.2, and averaged around 7.2 (slightly alkaline). Water temperatures average around 75 degrees F ( 23.8C), conductivity averages .30mS/cm, and the ORP averages 178 mv. (lower than one might expect, right? In reef keeping, we shoot for around 300 mv, so...) It's thought that the low levels of ORP can be associated with environmental pollution and/or high concentrations of ions, which is consistent in waters with karstic origins.

(The Hanna pH/ORP checker...how players do it, yo!)

And these protected pools, streams and caves present highly unique habitats for fishes.

Caves? Did I say, caves? Yup. That's pretty interesting. Must be a very protective environment.


Of course, it's not all roses and unicorns in these caves!  The more-or-less permanent absence of light and limited food scarcity represent the most conspicuous ecological pressures posed on cave fauna.

We'll talk about those some other time. Lets focus on the protected streams and pools.

What kinds of fishes do you find in them?


(The "lovely" Hemmigrammus marginatus!)

Characins, like Astyanax fasciatus, Hemigrammus marginatus, Hyphessobrycon santae, and Serrapinnus piaba were all found in this one location in Brazil! Cichlids were represented with the well-known Geophagus brasiliensis, and Loricariids with Hypostomus lima. Other catfishes, such as Pimelodella lateristriga and Hoplias malabaricus were also found there. And lesser appreciated (in the aquarium world, at least) Characidium sp. are also found in the habitat.

The much-loved Geophagus brasiliensis! (Image by Cezary Porycki, used under CC BY 3.0)

Siluriformes (catfishes) are considered by ichthyologists to be the most common fish group showing the traits required for cave dwelling, and they're considered "pre-adapted" to the subterranean habitat because of their nocturnal habits, electronic orientation abilities, and omnivorous or generalist carnivorous diet.

It will come as no surprise to dedicated readers of "The Tint" to discover that the surrounding terrestrial habitat has a profound influence on the species richness of the fishes found in these locales!

Studies determined that the percentage of "channel canopy cover "has the strongest effect on fish assemblages and is related to the percentage of organic matter in the streambed.  Greater density of riparian vegetation is correlated by field studies to have a profound influence on fish community composition.

Ichthyologists have found that the canopy cover increased stream channel shade (oaky, that's kind of a no-brainer, right?), enhancing habitat use by certain fish groups. Light reduction also lowers what ecologists call "primary production" which decreases the density of algae-consuming species . Ichthyhologists  working on karst pools determined that "roots, arboreal and aquatic vegetation positively affected the species diversity of fish assemblages."

Another case of fish following the food, right?

Now, cave communities are usually dependent upon allochthonous organic matter that may enter the subterranean environment carried by different agents (wind, percolation, falling into the water, current, etc.). 

Wood and organic material substrates are significantly less abundant in the subterranean karst habitats who compared with surface sites. Ecologists feel that the presence of wood in rivers can potentially affect the biodiversity of aquatic ecosystems in many different ways. Wood is considered to be an important nutrient source for aquatic insects. And we all know how fishes feel about aquatic insects, right? 

Of course, this food for the insects is a relevant factor regarding the trophic structure and productivity in aquatic food webs! Cave environments  represent what ecologists call ‘harsh’ oligotrophic habitats that may prevent the formation of  populations sufficient to support fishes. Rotifers are quite common in these habitats, and no doubt form a substantial part of the diets of smaller fishes. Copepods were the next most abundant food items in one study I read.


Again, the food webs area primary contributor to the suitability of a given habitat for fishes! It's thought that permanent dark passages in caves may act as ‘filters’, selecting fish species with the necessary attributes to move into and exploit the food resources present in them. It is  thought by scientists that the presence of subterranean spaces in the karst habitats is responsible for shaping the fish community, typically favoring nocturnal and small-sized species.

So, from an aquarist's perspective, karstic habitats should be pretty easy to replicate in the aquarium, right? Lots of smooth stone and sand, with a scattering of leaves and a few branches. This is one instance where I'd tell you to use activated carbon or other chemical media, to keep the water more or less clear. I mean, in soem habitats, it's crystal clear! Check out this video by our pal, Tai Strietman, of a karstic river in Mato Grosso do Sul, Brazil. Look at the current in that sucker!  I think I now have a freshwater application for my Vortech MP10 pumps!

(Yeah, we'll have to have Tai back on to talk more about this habitat! He talked about his visits to them in his last visit to "The Tint" podcast.)

Lots of epiphytic algal growth, some broken up leaves, aggregations of rocks...sand...I mean, this is like aquarist paradise! You can pretty much use every trick in the book and still come up with a reasonably faithful biopic representation- functionally aesthetic, no less! And, for some of you, not to have to deal with super acidic water and dark tint could be a real win, huh?

I hope you are at least intrigued by this unique habitat Obviously, in a brief blog like this, we can only touch on the most cursory stuff. As I mention al the time, you CAN find out information about this stuff online and elsewhere- just not really in the aquarium hobby realm. You'll need to dig deeper. Scholarly articles/research papers are treasure troves of information about all of these unique habitats that we talk about here. Don't be intimidated by the technical stuff in these papers.

There's so much amazing stuff in them that it's well worth the read!

We'll be talking more about this unique habitat, for sure!

Stay curious. Stay resourceful. Stay intrigued. Stay creative...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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