Invasive zebra mussels continue their takeover of Texas lakes and waterways: They've now spread to four more lakes in Central Texas, pushing the boundary of their southern expansion.

The lakes – Granger, east of Georgetown; Walter E. Long, also known as Decker Lake, in East Austin; Dunlap, outside New Braunfels; and Placid, near Seguin – are fed by rivers upstream where zebra mussels were already present. It was likely a matter of time before they showed up, says Patrick Ireland, a fisheries biologist with the Texas Parks and Wildlife Department.

Does that mean they'll continue moving farther south?

“There’s still a lot we don’t know yet,” Ireland says. “We are in kind of the southern boundary of what’s known for them to be able to inhabit.”

The mussels are known to prefer colder water with high alkalinity. Ireland says those conditions become less suitable for them as they move farther south.

But research has shown that the mussels, originally from the cold waters of Eurasia, are evolving.

“They produce hundreds of thousands of young per individual when they reproduce,” Robert McMahon, a biology professor emeritus at UT Arlington, told KUT in 2017. So even if only a few evolve to withstand the hot water, they produce young that are more heat-tolerant.

The mussels disrupt ecosystems and clog water intake pipes. That could be a problem at Decker Lake, which is fed by water pumped in from the Colorado River and serves as a cooling lake for the Decker Power Plant, a natural gas plant owned by the City of Austin.

Zebra mussels have been present in the Highland Lakes since at least 2017. Many Austin residents first became aware of them earlier this year when their presence in the city water supply gave tap water a funky smell and flavor.

While the mussels in Texas' river systems are moving downstream with the flow of water, Parks and Wildlife officials say they're also spread by boat owners.

They urge anyone moving a watercraft between Texas lakes to thoroughly drain, dry and wash their boats beforehand to avoid spreading the mussels.