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Energy & Environment

This tiny fish in the San Marcos River is probably extinct. So, what killed it?

A pair of tiny, silvery/yellow fish
Texas Parks and Wildlife
The San Marcos gambusia hasn't been seen since 1983. Next month, it'll be officially declared extinct.

Farewell to the San Marcos gambusia. We barely knew you.

This tiny species of fish, once found only in a half-mile stretch of the San Marcos River, hasn’t been seen in decades. And now the U.S. Fish and Wildlife Service wants to make it official: It’s extinct.

Fully-grown, this species of gambusia was about an inch long. It had lemon yellow median fins and a dark stripe along the upper edge of its dorsal fin. It loved to dine on small invertebrates that shared its habitat just east of where I-35 crosses over the river. It preferred the quiet backwaters of the river, away from the hustle and bustle of the main current.

The San Marcos gambusia was discovered by humans — or described, as a biologist would say — in the late 1960s.

“That’s kind of late for a species to be described,” says Tim Bonner, a biology professor at Texas State University in San Marcos. “Most of the Texas fishes were described going back to the 1800s.”

It was first identified by the late Dr. Clark Hubbs — a legend in Texas zoology circles. He estimated a population of about only 1,000 at the time. The San Marcos gambusia was declared endangered about a decade after it was identified.

This species is very similar to other types of gambusia out there, except for one thing.

“One of the big metrics that we use is the nature of its gonadal structure,” says Bonner.

The female San Marcos gambusia gives birth to live young, rather than laying eggs. The male of the species has what’s called an anal fin that it uses to deliver sperm to fertilize the eggs inside the female — effectively, a fish penis.

“It’s the shape of that structure that helps us to identify the different types of gambusia,” Bonner says.

The San Marcos gambusia was last seen in 1983.

So, what killed this fish?

The official line goes that reduced flow of water from the springs at the river’s headwaters and water pollution threatened, and ultimately ended, this noble fish’s time on this mortal plane.

But Bonner isn’t so sure about that. He notes that the water quality of the San Marcos River is much better than it was even 100 years ago.

“Back in the 1930s, San Marcos’ wastewater effluent was dumped in the middle of town,” he says, not far from the San Marcos gambusia’s habitat.

This species’ genetic cousins live on. There are plenty of other species of gambusia out there, such as the western mosquitofish and the largespring gambusia, which are thriving in the San Marcos River.

“So, it really doesn’t make sense,” Bonner says of the San Marcos gambusia’s presumed extinction. “This is not one that I’d hang my hat on to sit there and talk about conservation needs and ‘we need to do a better job at our relationship with the environment.’ I wouldn’t offer up the San Marcos gambusia as the poster fish for this argument.”

Still, he mourns the loss of this unique fish, which he says could have helped scientists learn more about how species evolve.

“I’m sad that I don’t have that species still existing [so] that we can ask, what I think, are the cool questions.”

But if anyone finds one, they've got until Nov. 29 to tell the federal government before it's officially declared extinct.

R.I.P. San Marcos gambusia (????-1983?).

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