July 15, 2020

Temporary beauty- permanent inspiration: The allure of vernal pools

As we push further into attempting to replicate unusual ecological niches in our aquariums, the allure of small, temporary bodies of water is something that we can and should devote more time to studying and recreating in our tanks.

Consider the unique habitats which arise in vernal pools.

A great definition of this habitat is found in a study I located by researchers Keely and Zedler:

"We define vernal pools as precipitation-filled seasonal wetlands inundated during periods when temperature is sufficient for plant growth, followed by a brief waterlogged-terrestrial stage and culminating in extreme desiccating soil conditions of extended duration."

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!

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

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.

The high degree of variability of vernal pools is a prominent attribute of them. This is a result of year-to-year-fluctuations in rain, the length of the flooding period and time which it takes for "shorelines" to emerge in these habitats.

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)

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!

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 Nothobranchiusand the species which occur with them include stuff like planktonic and benthic invertebrates, copepods, Daphnia, and insect larvae.

Understanding this type of habitat has lots of implications for creating very cool biotope-inspired aquariums.

And why not make 'em for killifish?

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 killie-keeping "establishment!"

A big win, if you ask me!

These pools are surprisingly productive, with significant crustacean and insect life- a truly remarkable abundance of life- which helps sustain the fishes which reside in the vernal pools. Most Nothobranchius species are what ecologists classify as "generalized carnivores", feeding on a range of planktonic and benthic invertebrates and insects found in these habitats.

How would you effectively replicate this habitat in the aquarium?

A shallow, wide aquarium, filled with an appropriate mix of aquatic soils, leaves, and botanical materials could be employed. The unique dimensions of such tanks gives you interesting possibilities to create simple, yet utterly fascinating displays.

You would keep water depth fairly shallow, and then you could slowly lower the water level as the "dry season" comes...Just like our "Urban Igapo" idea, this is really sort of similar- an aquatic habitat which ebbs and flows, and eventually returns to its dry, terrestrial state.

Playing with this stuff would not only be great for species like killies- it would go a long way towards recreating many of the aspects of this unique habitat in the aquarium.

We need to see more of this...

I mean, on the surface, this is hardly a revolutionary idea...

Now, this is NOT exactly the same idea as the "biotope aquarium" crowd plays with, IMHO. That's more of a physical and arguably "superficial" attempt at replicating the aesthetic aspects of the natural habitats from which our fishes hail. Very, very cool- but different, I believe, than what we're talking about here.

This is function at its most "raw" and literal.

And that's kind of what our mission here is all about, isn't it?

Stay creative. Stay thoughtful. Stay engrossed. Stay observant...

And Stay Wet.

Scott Fellman

Tannin Aquatics