January 29, 2020

The Allure of Ephemeral aquatic habitats...And the fishes which reside in them.

There is something magical about unique aquatic habitats, isn't there?

Actually, let me clarify.

I find almost ANY aquatic habitat alluring, as most any fish geek does. And the interesting thing is that just about any body of water that fosters aquatic life is like a magnet, drawing the hardcore fish geek to it "like a moth to a flame."

Streams, creeks, rivers, ponds, lakes, flooded forests, and- damn, I love me some flooded forests (If you haven't guessed by now!)...Each and every one holds a special appeal. However, I'm really fascinated by aquatic habitats that are ephemeral in nature- fleeting bodies of water which form as a result of weather, floodwaters, or seasonal changes.

Yeah, the world is filled with all sorts of aquatic habitats which are not as well studied or even thought about by most hobbyists. One of my fave examples are the so-called "vernal pools", which although found in various parts of the world, hold special fascination for us when they occur in tropical locales like South America nd Africa.

Vernal pools are classified by ecologists as a type of wetland, although they are, as their name implies, temporary aquatic habitats. Certain fishes, such as annual killifish, have evolved to adapt and thrive in these environments over eons. This, of course, makes these unique aquatic ecosystems all the more fascinating to us as tropical fish hobbyists!

Typical vernal pools in the tropical locales mentioned above are dry for at least part of the year, and typically, but not always, fill with water during seasonal rain/flooding events. Some of these pools may stay partially filled with water during a given year- or longer- but all vernal pools dry up periodically. Sometimes, these pools empty and fill several times during the wet season. Movement of water between vernal pools also occurs.

Vernal pools are typically associated with plains or grasslands, and are typically small bodies of water- often just a few meters wide. The origin of the name, "vernal" refers to the Spring season. And, this makes a lot of sense, because most of these ephemeral habitats are at their maximum water depth during the Spring!

Vernal pools are typically found in areas comprised of various soil types that contain clays, sediments and silts. They can develop into what geologists call "hydric soils", which are defined as, “...a soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part.”

That's interesting!

A unique part of the vernal pools is what is an essentially impermeable layer of substrate called "clay pan." These substrates are hugely important to the formation of these habitats, as the clay soils bind so closely together that they become impermeable to water. Thus, when it rains, the water percolates until it reaches the "claypan" and just sits there, filling up with decaying plant material, loose soils, and water.

So, yeah- the substrate is of critical importance to the aquatic life forms which reside in these pools! Let's talk killies for a second! One study of the much-loved African genus Nothobranchius 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.

The resulting "mud-rich" substrate in these pools has a low degree of permeability, which enables water to remain in a given vernal pool even after the surrounding water table may have receded! And, of course, a lot of decaying materials, like plant parts and leaf litter is present in the water, which would impact the pH and other characteristics of the aquatic habitat.

Interestingly, it is known by ecologists that the water may stay alkaline despite all of this stuff, because of the buffering capacity of the alkaline clay present in the sediments!

And, to literally "cap it off"- if this impermeable layer were not present, the vernal pools would desiccate too rapidly to permit the critical early phases of embryonic development of the Nothobranchius eggs to occur. Yes, these fishes are tied intimately to their environment.

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

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!

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.

Okay, my head is about to explode here with this really interesting stuff!

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

Of course, when they're filled, vernal pools are literal oases of aquatic life, ranging from microorganisms and micro crustaceans (like Daphnia) to aquatic insects and their associated larvae (like mosquito larvae!), frogs, and in some instances...fishes! It makes sense that fishes would find their way into these habitats over eons- especially if they're literally filled with foods for the fishes during their wet season, right?

Interestingly, in the case of annual killifishes like Nothos, other species of (non-annual) fishes are occasionally found living with them, when these habitats might be connected temporarily to adjacent, more permanent bodies of water. Fishes as diverse as Lungfishes, Barbus sp., Clarias catfishes, mormyrids like Petrocephalus sp., Ctenopoma sp., non-annual killifishes such as Aplocheilichthys sp., and even some cichlids like Tilapia are found in these vernal pools!

Oh, that's pretty cool, right? What an eclectic group of fishes!

These fishes aggregate in these pools because of their connectivity to adjacent waters, and they feed and thrive off of the abundant food present in the vernal pools. And of course, the stomach contents of Nothobranchius and the species which occur with them include stuff like planktonic and benthic invertebrates, copepods, Daphnia, and insect larvae.

This confirms the remarkable abundance of life which helps sustain the fishes which reside in the vernal pools.

Let's think about this stuff in aquarium terms!

Killifish hobbyists have kept annual Notho species for many years, and have learned to utilize materials such as peat moss to incubate their resulting eggs. There has always been a more than causal interest among these hobbyists in creating the optimum levels of moisture and such within a given "batch" of peat moss- with the old "standby reference" that the peat should have the consistency of "moist pipe tobacco" being the "gold standard" for decades. Great if you're a pipe smoker- otherwise, it's just a guessing game, right?

After studying these vernal pools for some time now, I can't help but be drawn to the idea we've played with in our "Urban Igapo" tanks- utilizing soil/sediment mixes which simulate, to the most realistic extent possible, the substrates of the vernal pools. I'm fascinated by the idea of including alkaline clays, specifically those with the aforementioned smectites present, in a substrate mix intended to keep these fishes in a "permanent" setup.

Well, "permanent" in that we're not removing the substrate to dry it out. I suppose the water would be the "ephemeral" part, lol! We're removing the water (and the adult fishes, of course) to create a "dry period", while leaving the substrate intact during the process. Once the appropriate incubation period for a given species has passed, the substrate is wetted once again, and hopefully, you get some fry, which would be reared "in situ."

That's essentially the "gist" of the "Urban Igapo" concept as adapted to the killifish life cycle.

Now, I am not going to delude myself here and think that I've "invented" some new approach to keeping annual killies. This idea is not really "revolutionary", it's not exactly precise in nature, and is likely far less sophisticated, efficient and controllable than the "collect-the-peat-and-place-it-in-a-plastic-bag-to-incubate" methodology that's been used for generations.

I've already played with this idea with some South American annual killifish species, so I can't see why the idea wouldn't work with African annuals like Nothobranchius. Again, it's perhaps a bit less controlled as the more traditional approaches. It's not the key to propagating large numbers of these fishes in a predictable, guaranteed long-term sustainable manner.

I'll give you that.

However, I think it's an interesting experimental way to go.

It's certainly more fun than just throwing some peat in a plastic bag, right? It could give us some interesting insights its the life cycle of these fishes, and how their habitats impact their existence in tangible ways. It plays into our desire to recreate the habitats of fishes in a more realistic, more functional manner- and to understand how they work and the threats they face from mankind's encroachment.

And maybe, just maybe- they might help make killifish, and the killifish hobby, more interesting, appealing, and relevant to a new group of hobbyists- long a topic of concern among the gillie-keeping establishment!

A big win.

We've covered a fair amount of information on this rather obscure, yet fascinating ephemeral habitat and the fishes which reside there. In a future installment, I'll talk about my experiments with this process, and we can discuss the idea of creating one of these aquatic displays for Nothobranchius- and maybe for other fishes which are occasionally found with them in Nature.