Close ties...

One of the most important takeaways from Nature is just how connected to their environment many species of fishes are. I'm fascinated by the relationship between fishes and their habitats. 

It's no secret that many habitats, such as our favorite flooded forests, fishes move in and out of them seasonally, not only because of the water level, but because of the food sources which are available to them. 

The flood cycle of the rivers into the igarapes are the dominant seasonal factor, and fish communities are found to fluctuate greatly over the year. During inundation, fish migrate into floodplain forests to feed on insects, fruits and seeds, among other things.

Studies of blackwater communities showed that, during these cycles, a greater diversity of fishes exists there. Many species were found to be specialized feeders. Fish, detritus and insects were the most important food resources supporting the fish community in both high and low water seasons, but the proportions of fruits, invertebrates and fish were reduced during the low water season.

Are there some "takeaways" here for us fish geeks?

Well, sure!

For one thing, fishes sort of "follow the food", right? And that the "seasonal availability" of some food sources actually dictates overall fish behavior. 

And, as we've discussed previously, the fish population and diversity in these igapo regions, long thought to be somewhat "impoverished", is actually very diverse and significant. Studies have revealed that many fishes are found in the submerged litter bank of these regions, forming dense local populations which are specialized and live on the allochthonous inputs (defined as material that imported into an ecosystem from outside of it) from the inundated forest floors.

The fishes have adapted to live in an environment with varying leaf and wood density, and seasonal variations in depth.

And the food production capacity of these habitats for the resident aquatic fauna is immense!

The  seasonal flooding brings fishes into contact with a greater abundance and diversity of allochthonous food resources, especially within forested watersheds, and this is significant to their life cycle. 

With regards to the types of fishes we find in these habitats, scientists have found repeatedly that the majority seem to be characins, followed by Loricariids, cichlids, and "everything else."

As we know by now, the allochthonous inputs (terrestrial leaves, seed pods, insects, flowers, etc.) that our fishes utilize in the wild, and can no doubt benefit from In the aquarium, as well.

One of the important food resources in natural aquatic systems are what are known as macrophytes- aquatic plants- which grow in and around the water, emerged, submerged, floating, etc.

Not only do macrophytes contribute to the physical structure and spatial organization of the water bodies they inhabit, they are primary contributors to the overall biological stability of the habitat, conditioning the physical parameters of the water.

Of course, anyone who keeps a planted aquarium could attest to that, right? 

One of the interesting things about macrophytes is that, although there are a lot of fishes which feed directly upon them, the plants themselves are perhaps most valuable as a microhabitat for algae, zooplankton, and other organisms upon which fishes feed on. Small aquatic crustaceans seek out the shelter of plants for both the food resources they provide (i.e.; zooplankton, diatoms) and for protection from predators (yeah, the fishes!). And leaf litter, botanical materials, etc. serve as perfect  shelter for these macrophytes in which to grow and multiply.

And of course, these interrelationships between the environment and the fishes are not limited to just South America.

Like many of you, I do a fair amount of research about fishes and the environments that they come from. Some of the most fascinating fishes, and ones which have an intimate connection to their habitat are the annual killifishes, specifically those from Africa, such as the much-loved Nothobranchius species.

(Image by Andrew Bogott, used under CC BY-S.A. 4.0)

Now, the typical environments which these fishes live in are small temporary savannah pools in sandy soils, with a layer of black mud on top. And that's where it gets kind of interesting. Nothos don't just live and reproduce in any old mud hole.


Rather, they're intimately tied to specific types of soils, muds, and sands. And there is good reason, too.

According to one study I read, "Nothobranchius never inhabit pools consisting only of orange-colored laterite soils (Reichard et al., 2009; Watters, 2009). Although these pools are very common in the African savannah, especially after heavy precipitation, they are characterized by kaoline-type clay minerals and are slightly acidic, and their substrate is not suitable for Nothobranchius embryo survival during the dry period."

And here is another huge takeaway: 

"The critical prerequisite of Nothobranchius occurrence in a particular pool is the specific composition of the substrate (Watters, 2009). Soil conditions are the primary drivers of habitat suitability for Nothobranchius, as the eggs can only survive the embryonic period and develop successfully on Quaternary vertisol and calcimorph soils."

In other words, the relationship between these fishes and their environment is supercritical.

These pools accumulate in a soil type called Vertisol.

This is a clay soil with little organic matter which occurs in regions having distinct wet and dry seasons. Alkaline clay minerals ( called smectites) are considered to be prerequisites to create suitable conditions during embryonic development in desiccated pool substrates.

An absolute relationship between the fish and their habitat.

And here's the other cool takeaway-one which actually can have some impact on the way we keep and breed these unique fishes:

The mud-rich layer in such pools has low permeability, a characteristic which enables water to remain in the pools after the surrounding water table has receded. Without the presence of this impermeable layer, the pools will rapidly desiccate. Visually, this substrate is dark brown to black, often forming a thick layer of soft mud on the bottom of the pool.

This, of course, makes these unique aquatic ecosystems all the more fascinating to us as tropical fish hobbyists!

(Image by HAL333- used under CC BY-SA 4.0)

In the dry part of  the range of the genus Nothobranchius in southwestern Mozambique, many pools inhabited by the well-known killies,  N. furzeri and N. orthonotus are usually isolated from more permanent bodies of water, and are filled exclusively by rainwater during periods of high precipitation. Some of these pools, however, may be occasionally connected, as they are essentially depressions in the dry savannah, in which water drained from these larger bodies of water, accumulates.

These pools and their cycles directly impact the life cycle and reproductive strategies of the annual fishes which reside in them.

The fascinating concept of embryonic diapause ( a form of prolonged, yet reversible developmental arrest) is well-known to scientists and lovers of annual killies. The occurrence and length of time of diapause varies from species to species, yet is considered by scientists to be an evolutionary adaptation and ecological trait in various populations of Nothobranchius, tied directly into the characteristics of the ephemeral habitats in which these fish reside!

(Image by Kils- used under CC BY-S.A. 3.0)

Diapause assures species survival by enabling the annual life cycle of these fish to be completed, and can even be affected by the presence of adult fishes in the habitat (not a good idea to hatch if potential predators are around, right?)- a fascinating adaptation! Since the embryonic phase of most Nothobranchius is a relatively long period of their lives- and in some species- the longest phase of their life, factors which impact embryonic development are extremely important. 

Oh, and the really interesting part:

"Organic material aggregates in the pool in the form of dead aquatic and terrestrial vegetation but does not cover the large part of the bottom, as is typical of water bodies in forested areas with leaf litter. Despite the presence of rapidly decaying material, the water stays alkaline due to the high buffering capacity of the alkaline clay in the sediment." (Reichard, 2009)

Something that we as botanical-style aquarists experience often- the buffering effect of substrate, despite a huge presence of decomposing leaves and botanical materials. It's part of the reason why some have trouble getting their pH down to very low levels in aquariums, I think!

That's where the idea of alternative substrates comes in! 

So, yeah- the substrate is of critical importance to the aquatic life forms which reside in them.  One study I read indicated that the soils are "the primary drivers of habitat suitability" for these fish, and that the eggs can only survive the embryonic period and develop in specific soil types containing alkaline clay minerals, known as "smectites", which create the proper soil conditions for this in desiccated pool substrates.

A takeaway for us as hobbyists.

I could go on and on and on talking about all sorts of relationships between fishes and their habitats, because there are so many!

And there are also killifish habitats covered in leaf litter-a component of the habitat which creates very specific conditions for the fishes' reproduction! Yeah- if you look hard enough, you can find examples for just about everything your looking for by observing Nature closely.

Oh, and we haven't really touched upon the relationship between land and water in this piece, but we've covered that a lot...and it's a relationship which we're just starting to consider in aquariums. A relationship which has vast implications for aquariums.

Just confining our research to hobby literature is to overlook the vast amount of information available to us via academic research. We as hobbyists should all "deep dive" now and then into the many resources available to us!

Again, the most important takeaway from today's little review is that our fishes found in specific habitats for a reason. They're often intimately tied to the environments in which they are found- not only benefitting from, but sometimes contributing to- the overall habitat.

An amazing relationship that we as hobbyists should look at very closely, as the potential for breakthroughs that can benefit our efforts is just out there...waiting for us to unpack.


Stay dedicated. Stay curious. Stay resourceful. Stay observant. Stay creative...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


Leave a comment