Invader zebra mussels threaten our native Texas species
Zebra mussels, a species native to the Black and Caspian Seas, were first identified in Texas in 2009. Now, only eight years later, these mussels have spread over five different river basins and infested 11 Texas lakes: Belton, Bridgeport, Canyon, Dean Gilbert, Eagle Mountain, Lewisville, Randell, Ray Roberts, Stillhouse Hollow, Texoma, and Travis. As innocuous as these striped invaders may seem, their rapid expansion across Texas doesn’t just put a damper on boating – it puts our state’s native mussel species at risk.
Map of invasive zebra mussel findings in Texas. Courtesy of TPWD
Why do our native mussels matter?
Texas is home to 53 of the 300 species of mussels native to the United States. Fifteen of these are already threatened. Mussels are often the first species to disappear as the environment declines. When mussel populations are at risk, problems for fish, wildlife, and people are often just around the corner. Mussels are living proof that invertebrates and other small, seemingly insignificant creatures often play vital roles.
Zebra mussel invasions threaten native Texas species
Every female zebra mussel can lay from 30,000 to 40,000 eggs at a time, and up to half a billion in a lifetime. These larvae adhere to hard surfaces, unlike the larvae from native species, which live parasitically, but harmlessly, on native fish. They are known to occur in dense numbers and attach themselves to all available hard surfaces, including the shells of native mussels. This can cause shell deformity and impair native mussels’ movements, making it all the more difficult to compete with the zebra mussels for the food – phytoplankton, bacteria, silt, and other microorganisms – that mussels ingest as they filter water.
The spread of invasive zebra mussels in Texas threatens native mussel species. Photo courtesy of the US FWS.
High densities of zebra mussel populations combined with their high water filtration rates can have critical ecological effects. One zebra mussel colony can include trillions of mussels. The resulting highly filtered water has fewer available nutrients for the microscopic and nearly microscopic organisms living in it. Less available food leads a decrease in the population of these organisms, which in turn leads to a decrease in the number of the fish that feed on them. The existence of fewer fish is not only frustrating for fishing enthusiasts; it also mean fewer fish for the native mussel larvae to stick to.
Zebra mussel infestations can entirely change the ecosystems in Texas lakes. Lakes can change from turbid and phytoplankton-dominated to clear and full of aquatic plants. While clearer water doesn’t sound terribly threatening, it indicates a smaller amount of available food for fish, and it allows light to reach greater depths, which can result in higher water temperatures. Increased water temperatures threaten all aquatic species. Temperature affects metabolic rates, growth, and fertility of fish, mussels, snails, and other aquatic animals, as well as the distribution and virility of parasites and pathogens, potentially creating higher infection rates of aquatic animals.
All mussels, both native and invasive, cannot survive if their environment reaches a certain temperature. However, zebra mussels are at an advantage – native mussels live only in shallow water, which is warmer, while zebra mussels can live at greater depths, where the water is cooler. Increased temperatures resulting from greater water transparency can create temperatures too warm for the native mussels to survive. Warmer water also allows water pollutants to be taken up by fish faster, and with fewer native mussels filtering the pollutants from the water, the environment becomes even more toxic for fish.
A case study in Italy showed that zebra mussels can adapt quickly to new environments in as little as twenty years and are extremely ecologically flexible, living anywhere from zero to 197 foot depths. A model from the upper Mississippi River showed that the most common native mussel, the Threeridge mussel, would likely become extinct fifty years after a zebra mussel infestation1. Habitat destruction, such as that caused by a combination of water withdrawals and droughts, and expanded range of non-indigenous species are among the top threats to Texas freshwater mussels.
We can play a role in stopping the spread
Unlike the 53 native mussel species in Texas, zebra mussels can attach themselves to smooth, hard surfaces through small, sticky threads extending underneath their shells. They often colonize on top of one another and can severely clog water intake pipes and water filtration systems, resulting in serious economic loss. One analysis of the nation’s economic data projected costs for damage and controls to be around $1 billion per year.
The mussels hide in and on boats, using them to spread to new waters as well as damaging them. If their microscopic larvae get into a boat’s engine cooling system, the mussels can block intake screens, hoses, and strainers once they have grown. In affected areas, all boat owners are now required to completely drain their boats after use to prevent the spread of zebra mussels.
In addition to the boat drainage rule, boaters are now also required to remove any visible plants or animals from their boats or trailers before they leave the freshwater area. Conserving water is always good, but it is especially important to help keep water levels high so our native mussels don’t lose their habitats. Planting trees and other plants along banks also helps. Plants help control soil erosion and thus runoff, which decreases the amount of chemicals and harmful materials getting into the water, keeping it cleaner, and native mussels healthier. Everything in nature exists in a context of connected parts, and each must be protected to protect the whole.
- Hart, Rick A., et al. “Simulation Models of Harvested and Zebra Mussel Colonized Threeridge Mussel Populations in the Upper Mississippi River.” The American Midland Naturalist, vol. 151, no. 2, 2004, pp. 301–317. JSTOR, jstor.org/stable/3566747
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