An endangered species of aquatic beetle, Heterelmis comalensis, known as the Comal Springs riffle beetle (larva, pupa, and adult shown here left to right), inhabits just two spring systems in Texas and is at the center of ongoing protection efforts. A new study offers important insight into how to rear the species in captivity for conservation. (Image by Ely Kosnicki, Ph.D.)

By Paige Embry

Paige Embry

Riffle beetles (family Elmidae) get their name from the parts of rivers that suit them: areas where water flows fast and frothy over a bed of rocks. These areas have crannies where riffle beetles can hide and also provide water chock full of oxygen—something the tiny aquatic insects need, as most spend their entire lives submerged.

This underwater lifestyle is easy for the larvae because they have gills, but the adults must breath air—which they manage thanks to something akin to a never-empty SCUBA tank called a “plastron.” The plastron is an air bubble affixed to hairs on the insect’s body. Oxygen diffuses into the bubble from the surrounding water, which is why living in areas with well-oxygenated water is a necessity.

Riffles aren’t the only places where riffle beetles live, and burbling springs can make a fine home as well. Heterelmis comalensis, known as the Comal Springs riffle beetle, is an endangered species that lives in the headwaters of two spring systems near San Antonio, Texas: Comal and San Marcos Springs. Even though riffle beetles are aquatic, most have wings and can fly away if something goes awry in their watery home. But not the Comal Springs riffle beetle; its wings are too reduced to allow flight.

Ely Kosnicki, Ph.D., a researcher at Bio-West, Inc., studies these beetles. He says those inadequate wings mean, “these guys are relegated to there [the spring systems] … and that’s one of the reasons they are listed as endangered.”

Both of the spring systems that host H. comalensis are fed by the Edwards Aquifer, which supplies most of San Antonio’s drinking water as well as irrigation water for the surrounding agricultural lands. The aquifer, the springs, and their associated lakes and streams house eight threatened or endangered species, ranging from a blind salamander to a wild rice to the Comal Springs riffle beetle. A variety of stakeholders are working to keep the aquifer water readily available to users while also protecting the endangered species that live there.

Part of that protection includes preparing for a disaster like a toxic spill or the springs temporarily going dry.  For the beetles, this means learning how to rear and maintain a colony in captivity that could be used to re-populate the springs if needed. The list of need-to-knows for developing such a colony is long. What’s the best housing, appropriate food, optimal water flow, number of beetles needed to be self-sustaining, and so on?

A new study published this month in the open-access Journal of Insect Science investigates one small piece of the rearing puzzle: How many offspring can captive female H. comalensis produce? Prior to this study, no one even knew how many offspring an individual female could have. “They couldn’t separate the sexes,” Kosnicki says, “They kind of just threw a bunch of adults in a container and would come back later and try and count eggs or larvae.” In a 2019 study, Kosnicki found that the sexes could be determined using lateral illumination that revealed certain internal structures. That finding opened the door to a more precise understanding of the females’ fecundity.

For this experiment, one female and one male were together put into a production chamber—a tube 2.54 centimeters in diameter and 8 cm long—along with some food that took months to prepare. The beetles eat microbial biofilm, so the researchers took dried sycamore leaves and sycamore wood dowels and placed them in containers with flowing Edwards Aquifer water so that a delicious (to riffle beetles) biofilm could develop. The beetles and the food went into the tubes along with a constant flow of Edwards Aquifer water.

Once a month the researchers checked on the beetles and counted larvae. If the male was dead, a new one was provided. The 24 females that were tracked until death had between 0 and 121 offspring, with a mean of 29.3. Some females survived for almost a year, and longevity (rather than size) played a key role in how many offspring an individual produced. Kosnicki notes that survival times tended to increase as the experiment went on. He writes, “There was a learning curve with regard to properly packing resources into production chambers.”

A lot of money has been spent on these little beetles, but Kosnicki says, “They’ve become a surrogate species for monitoring the health of the aquifer.” When asked what he thought was important for people to know, either about the beetles or this study, Kosnicki says, “[The] springs are not some remote pristine place.” A lake was created by damming the river spawned by one of the springs. For years the lake housed a water park with underwater shows, and one can still take a glass-bottom boat trip there. People’s backyards go right down to the water. The point, Kosnicki says: “It’s a beautiful place … but at the same time it’s highly transformed and, wouldn’t you know it, there’s actually a whole bunch of endangered species that live in these springs. I think that’s a nice example to show you the biodiversity that really exists in our backyard. You never know what’s going to be in your backyard until you look there.”

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.