By Ed Kluender, guest blogger

Hybrid creatures are one of the most common tropes of both goofy and great horror movies (think The Fly or countless werewolf movies), and plenty of them are at least half fish – Creature, Piranhaconda, and the Sharktopus franchise. Most of those films have a pretty shaky basis in reality, but they do contain a few truths: hybrids do occur in nature, and they can be surprising to see.

Hybridization is more common among fishes than other vertebrates, due in part to frequent habitat overlap between very genetically similar species and comparatively indiscriminant reproductive behavior. It’s actually fairly common for several species to spawn at the same time in the same small patch of habitat. Hybridization between fishes in their native habitat is rare, though. As a systematics expert once told me, “fish can tell the difference between each other pretty well.”

Enter the nonnative species factor. When one of the hybridizing species is a non-native, hybridization can be an unwelcome threat to native organisms. Yet despite invasive species being one of the most heavily-studied issues in fisheries ecology, we know much less how hybridization with invasive species affects populations of native fishes.

At the simplest level, hybridization between nonnative and native fishes suppresses native reproduction; every native egg fertilized by a nonnative is one fewer native fish in the water. In some cases, hybridization poses little threat to native fishes. Many hybrids are truly sterile, like tiger trout or tiger muskellunge. For functionally sterile hybrids like saugeye, splake, or wipers, the threat is limited to individual fish.

However, a much more insidious form of hybridization threatens native species: introgression, or “back crossing” (Rhymer and Simberloff 1996, Quist et al 2009). Some hybrids come from much more closely-related species and are fertile (Mandeville et al 2015). These fertile hybrids can reproduce with each other and either or both parental forms generation after generation ad infinitum, which dilutes the genetics of the parent species…sometimes to the point of extinction. Fertile hybrids can even reproduce with a third species, forming a three-species hybrid!

Sustained introgression can produce a “hybrid swarm” that can completely eliminate one or both parental species. In some rivers of the Colorado River Basin, introduced White Suckers hybridize with several native species. The Flannelmouth Sucker is probably the most threatened by introgression (McDonald et al 2008), as the current ranges of the two species overlap almost entirely. Hybrid swarms have replaced the native Flannelmouths in some of their native streams to the point that no sustainable population of the native species remains (Compton 2007). Genetic analyses show that repeated backcrossing can happen so often that a fish that appears to an experienced biologist to be a pure Flannelmouth or White Sucker can have a substantial amount of the other species’ genes (Quist et al 2009, Mandeville et al 2015).

Hybridization between nonnatives and native fish is usually amplified by our own impacts on the rivers. White suckers, a species native to the Midwest and Great Lakes region, thrive in the cold, clear waters below large mainstem dams like Flaming Gorge Dam on the Green River. Before dams, the lower reaches of desert rivers would have been the warm, turbid, and flashy habitat in which Flannelmouth Suckers evolved. This is generally not hospitable to white suckers. Though white suckers were introduced into the Colorado River basin before most of the major dams were built, the dams are very likely a major cause of the growing problem of white sucker hybridization we see today.

For better or worse, any given aspect of freshwater use and the resulting fish conservation is part of a vast network of actions and consequences for other species and habitat. To quote Kevin Bestgen, Director and senior scientist of the Larval Fish Laboratory at Colorado State University, “the Colorado River Basin is like a balloon; you can’t poke it anywhere without it bulging out somewhere else.”

As long as the need for water for human uses exists in the West (and the world), we will continue creating unexpected consequences for the fishes that live in it. With that comes the implicit duty to study and understand those consequences and to conscientiously manage them to reduce negative impacts.

Ed  Kluender is a Research Associate II in the  Larval Fish Laboratory at Colorado State University, and a regular contributor to The Fisheries Blog

References

Compton, R. I. 2007. Population fragmentation and white sucker introduction affect populations of bluehead suckers, flannelmouth suckers, and roundtail chubs in a headwater stream system, Wyoming. Master’s Thesis. Department of Zoology and Physiology, University of Wyoming. Laramie, WY.

Mandeville, E. G., T. L. Parchman, D. B. McDonald, and C. A. Buerkle. 2015. Highly variable reproductive idolation among pairs of Catostomus species. Molecular Biology. 24:1856 – 1872

McDonald, D. B., T. L. Parchman, M. R. Bower, W. A. Hubert, and F. J. Rahel. 2008. An introduced and a native vertebrate hybridize to form a genetic bridge to a second native species. Proceedings of the National Academy of Sciences. 105(31):10837 – 10842

Rhymer, J. M. and D. Simberloff. 1996. Extinction by hybridization and introgression. Annual Review of Ecology and Systematics. 27:83 – 109

Quist, M. C., M. R. Bower, W. A. Hubert, T. L. Parchman, and D. B. McDonald. 2009. Morphometric and meristic differences among bluehead suckers, flannelmouth suckers, white suckers, and their hybrids: tools for the management of native species in the upper Colorado River basin. North American Journal of Fisheries Management. 29:460 – 467