How guppies can swim stably against the current.

Watching fish swimming around is experienced by us as completely normal and natural. We also know how they're able to swim. But are there also circumstances that raise questions in which stable swimming can still be considered normal?

Moving forward in still water takes little to no effort as the fish encounter virtually no resistance, making them appear to float through the water. Swimming with the current is even easier because they themselves are carried by the current. Especially if the force of the current is greater than the mass of the fish itself. Logical of course… But what if a fish has to swim against the current and is not hindered by the speed and force of the countercurrent in relation to the mass of the fish in question?

With a swimming direction opposite to the current direction, we can speak of a smooth stream and a downward current. There are fish that usually swim with the current and fish that swim against the current. In general, most fish that swim with the current are weaker than fish that swim against the current. And why? Simply because those fish don't have to swim against the current. If they did, they would simply be pushed away by the water current because of the resistance they find in it. Or they waddle against the current due to the massive resistance they encounter. What about fish swimming against the current? And that in a stable way that makes it clear that they are not affected by the resistance of the water flow?

Many species of freshwater fish have an innate response to orienting their bodies in water currents. This phenomenon is better known as “positive rheotaxis” (Northcutt 1997). In addition to freshwater fish, this ability is also found in crustaceans with mechanical and sensory setae (which can be small protrusions, hairs, spines, or spatulas that serve as receptors). In fish, this can be the whiskers, antennae, or bristles, which are often located near the jaw. 

Catfish species are good examples of such fish. But primarily, most fishes will use the lateral line on both sides of the body as a current receptor. Crustaceans can even have them on other body parts. On the other hand, fishes that swim with the direction of the current is called "negative rheotaxis". The rheotaxis is active after the fishes have indirectly oriented themselves. This means by observing the shift from the solid ground via the receptors on their bodies. This can be the eyes or the other receptors. Rheotaxis is basically the same as the same behavior in air currents (birds in the air flying against the wind), which is called "anemotaxis".

As already been mentioned, most fish use the lateral line to detect changes in the upcoming flow pattern of a body of water. And the corresponding orientation of the fish towards or away from the current. The sensory part of the lateral line consists of mechanosensory hair cells that detect the movement of water.

In contrast to species with no active swimming ability or other body adaptations, this innate swimming response prevents the extinction of, among other things, closed fish populations subject to dominant downstream migration (described as a drift paradox by Müller, 1954). Rheotaxis also maximizes chemical signal perception and intercepts prey without expending too much energy on the fish. From an evolutionary perspective, rheotaxis allows fish to maintain a position in a water stream (station keeping), minimizing the body energy associated with emigration found in a wide variety of habitats from river environments to lakes. A good example is guppies found in the mountainous area of ​​the Caroni drainage in Trinidad, where guppies are exposed to seasonal ash flooding due to the rains that fall in the wet season. The fish then have to deal with the hydrodynamic habitats. Flooding of a body of water in this area during a wet season can cause parts of guppy populations to move to another body of water, which may close off after the wet season. Depending on the slope of a water current, they could swim back against the current. But if that is not the case, then they are still in a reasonable tidal water area. This in contrast to guppies that occur in the Pitch Lake in Trinidad. The Pitch Lake is a flat crater with pitch and asphalt folds that creates several freshwater pools with an area of ​​about 0.8 km². The guppies that live here experience little or no water flow. Research on these guppies led to the conclusion that they have lost their innate rheotactic behavior. Simply because there is no urgency for this in reasonably stagnant water.

Based on these two areas, a rheotaxis study was performed in 2009 by simulating the hydrodynamics of both areas. And take advantage of three different guppy populations, including an aquarium population from the Instanbul Tribe (research strain), an Upper Naranjo population, and a Pitch Lake population. All three populations were tested for rheotactic and stationary behavior. Stationary behavior means, among other things, that there is little or no migratory behavior relative to its own location. The Upper Naranjo is an upstream tributary of the Aripo River

This test was designed to test the hypothesis that riverine populations in the wild, show stronger habitat retention than guppies living in stable water conditions. This test showed that guppies from Upper Naranjo were significantly less likely to be flushed downstream than guppies from Pitch Lake and the aquarium species used. One of the assumptions, therefore, was that wild guppies experiencing seasonal flooding exhibit stronger rheotaxis, or stationary behavior, compared to guppies living in habitats with little or no natural water currents. Why can certain wild guppies that come from a lowland have a certain rheotaxis? This is usually because there is more predation in the lowlands than in the highlands. To be able to react quickly to avoid the threat of predation, such a fish must also be able to swim against the water current if that's the only alternative to get away from the predators. Guppies with lower to no rheotaxis hunt faster than guppies with rheotaxis ability. Guppies with low or no rheotaxis also waste more energy compared to guppies with rheotaxis. It was also concluded that guppies with rheotaxis from the base of the tail have a firmer hull compared to guppies with lower or no rheotaxis due to a better developed pendulum muscle. They have developed this to be able to move more stably in the countercurrent.

Above: River flow speed changes seasonally.

The rheotaxis is then activated by the escape response in a high predation environment. This escape response is also referred to as "fast-start evasion response" or "c-start" (Ghalambor et al. C2004). However, wild guppies are more likely to become a prey to predators in the deeper parts of a river. Because most predators also spend more time in the deeper water layers. This may explain why most guppies are found more on the side of a river, with the exception of really shallow pools or streams.  For they spread all over the width of the water in shallow waters. What also emerged during the test was that guppy females moved more to the deeper layers than guppy males. This is partly because males who show color can become easier a prey in comparison to gray females. But in this way the females could also better escape the violent attempts of the males. The strongest water flow arises in the upper layer of the deepest part of the water.