Save the Monkeyface!
Despite their funny names, freshwater mussels are important indicators of a river’s overall health.
By Wendee Holtcamp
The southeast United States is a global hotspot for freshwater mussels; in fact, it’s the most species-diverse region in the world. Nearly 300 mussel species live in our rivers, compared to a slim dozen in all of Europe. They may claim funny names like rock pocketbook, monkeyface, and fuzzy pigtoe, but our freshwater mussels are seriously imperiled. The American Fisheries Society survey found that more than 70 percent of the nation’s mussels are endangered, threatened or of special concern, compared to only 7 percent of the nation’s mammals and birds. Thirty-five freshwater mussels have already gone extinct in the U.S., with 56 species listed as federally endangered or threatened, and as many additional candidate species awaiting review for listing. We stand at the brink of an unparalleled mollusk extinction, and the declines can be directly attributed to human-induced river modifications. If “Save the Whales” was yesteryear’s swan song, perhaps today it’s time to proclaim, “Save the Monkeyface!”
I can’t often resist the call of a river, so I’m thrilled when offered the chance to kayak the Navasota to look for mussels with a Navasota junior high teacher and five science camp students. Ronald Rushing, or Mr. Rushing, as he’s known to his students, has led exploratory kayak trips for the last five summers and has included TPWD’s Texas Mussel Watch on the trip for the past three. Nothing inspires a budding interest in science like getting kids muddy and grimy and sampling wild creatures the way biologists do.
“You have to get kids outside to experience nature if you want them to appreciate the natural world,” Rushing says of his annual kayak trips. “It’s the best way for me to make ecology relevant to a handful of kids.”
Rushing is one of a few dozen teachers and individuals trained to search for and monitor mussels along Texas riverbanks for TPWD’s Texas Mussel Watch, one of several Texas Nature Tracker programs. TPWD staff train volunteers like Rushing in day-long workshops. They learn about mussel biology, about mussels’ imperiled status and the reasons for their decline. Then workshop participants try out their mussel identification skills in a field excursion. After training, volunteers monitor a local stream and report their findings back to headquarters.
“Texas Mussel Watch has been a challenge,” program director Marsha Reimer tells me. “It’s not a warm and fuzzy watch like some of the other programs. They’re not cute like horned lizards, they’re not beautiful like butterflies, and they’re difficult to get to. It’s not something that you can hand someone a monitoring packet and say, ‘Here, go monitor mussels.’”
The real possibility of finding a rare species of mussel in a watershed overshadows the challenges. Biologists are concerned about over half of Texas’ 52 mussel species, and several species are found nowhere else in the world, including the Texas fatmucket, golden orb, Texas pimpleback, Texas fawnsfoot and triangle pigtoe. Since biologists don’t have complete information on distribution and population levels for many species, monitoring any Texas stream could reveal vital information. Determining what species reside where, monitoring their populations, and watching for mass die-offs are crucial first steps in conservation.
Texas Mussel Watch volunteers have already increased the scientific knowledge of mussel distribution throughout Texas. Junior high teacher Melba Sexton found a golden orb in the San Marcos River, a Texas-endemic species known only from a handful of populations around the state. Rushing’s Navasota River expeditions have also met with success. Reimer went with Rushing last year on his annual trip, and knew they’d found a mussel haven. “There are so many mussels in that river it’s absolutely amazing,” Reimer says. “So many rivers where we’ve collected, the mussels are gone.”
We paddle down the muddy brown Navasota for a few miles. There have been heavy rainfalls this year, which can churn up sediment, but I wonder whether the river was always this muddy, or whether human activities have significantly changed the watershed. When erosion, runoff or dam construction cause increased amounts of sediment to flow into or pile up in streams that used to run clear, the fauna within often can’t adapt to the rapid changes.
“Habitat degradation is probably one of the biggest concerns when it comes to things like mussels because they can’t get up and run away,” says TPWD aquatic biologist Dick Luebke. “So many rivers have impoundments on them, and that changes it from flowing to standing water. Historically, we didn’t have lakes. The entire state had flowing water. And that throws a curveball at species.”
The Navasota has a single dam, upstream of where we’re paddling. Dams create a multitude of problems for mussels. Upstream, dams cause decreased dissolved oxygen, reduced water flow, and lower algae populations, which mussels filter-feed on. In a reservoir, water flow ceases and sediment piles up. Many mussels can’t live in mushy soft substrates. Besides losing habitat, mussels can suffocate in the increased sediment load. For organisms that adapted over millions of years for flowing water, changes to a river’s flow regime can spell disaster.
“Many of the species that are ‘of concern’ need flowing water at some point in their life,” says TPWD aquatic biologist John Prentice, who works with Luebke at Heart of the Hills Fisheries Research Science Center. As sensitive species disappear, a region becomes dominated by a few common species. Exotic species get a foothold, like the Asian clam, which has invaded all Texas’ major river systems.
In this Garden of Eden for mussels, is it tolerable to accept having a few resilient species and let the rest go the way of the dodo bird? Preserving our natural heritage for its own sake might be reason enough, but simply having biodiversity serves an important function. “Species diversity tends to increase stability in the ecosystem,” explains Prentice. “A stable ecosystem is better able to withstand various and varying stresses.” Healthy ecosystems also provide benefits to people. “Although some of us think of ourselves as above the ecosystem, we are not,” Prentice notes. “Our health and well being is connected to the health and well-being of the ecosystem.”
Dams affect mussel habitats downstream as well, causing an unstable zone of underwater erosion (scour) caused by the rapid flow immediately below the dam. At the Elephant Butte Dam on the Rio Grande, scientists documented more than 4 meters of accumulated sediment downstream, which destroyed mussel habitat and reduced fish diversity and abundance. Changes to the fish fauna affect mussels because mussels require a fish host to disperse their young. Mussels have an extraordinary life cycle. It might look like a rocky lump, but inside that mussel shell lies a mighty surprise.
Mussels extend a part of their body outside their shell as a lure to attract fish. The lure, formed from a fleshy part of the mussel called the mantle, can be incredibly realistic. Some look like worms that waver in the flowing water, while others resemble small minnows. Since mussels can’t move far, they’ve solved the evolutionary puzzle of how to disperse by having their larvae, called glochidia, piggyback on the gills of fish. When a fish takes the bait, it gets a mouthful of glochidia instead of a meal.
Scientists used to think the mussel larvae parasitized the fish host, causing them harm, but recent studies have found at least some fish species benefit from the glochidia, making it more of a symbiotic relationship. When the glochidia attach to a fishes’ gills, it initiates an immune reaction, which apparently guards against other infections. Many mussels require a specific fish species for their larval stage, and if that fish disappears from a river drainage, mussel species that depend on it will eventually disappear too.
Rushing, the kids and I bank our kayaks at a gravelly sandbar to look for mussel shells. As the kids explore the sandbar, Rushing shows me the common shells scattered among the gravel, sand and clay. The abundant threeridge grows to nearly 9 inches across and like its name, has three distinct ridges across its dark shell. The slightly less common washboard looks similar but has more ridges, like an old-fashioned washboard. The fragile papershell breaks easily and has a smooth, sandy yellow shell. The mapleleaf has a pimply shell, while the smooth pimpleback has none. Non-native Asian clams are everywhere.
Who came up with these names? I think to myself. Pistolgrip. Deertoe. Pimpleback. Little Spectaclecase. Heelsplitter. I can just picture Billy Bob and George in overalls mucking through streams. “Hey George, look at this ’un. Looks like a rock pocketbook!” as he slips the mussel in his back pocket in jest.
“Most common names you’ll see used come from the shellers that were collecting shells for the pearl button industry,” explains Arthur Bogan, a research associate with the American Museum of Natural History. My mental image of mussel-naming may not be far off base.
Until the plastics industry replaced shell buttons in the mid-1900s, shellers harvested mussels by the thousands. The buttons were punched out of the mother-of-pearl from the inside of mussel shells, which varies from pure white, once favored for buttons, to pearly pink, purple, peach, or silver-gray, depending on species. This same material, also called nacre, creates pearls.
Freshwater mussels have a long history of importance to humans. Large middens, or shell mounds, can be found in archaeological sites in the southeast U.S. where Native Americans lived. Not only did Native Americans eat mussels, but they used pearls for ornamentation and for trade.
Pearls develop more frequently in certain mussels than others, such as the pearly mussel family (Margaritiferidae) but also species in the Unionidae family such as the washboard and Tampico pearlymussel. Natural pearls only form in a small percentage of individuals, and often they stay attached to the shell. The pearl industry overcame these difficulties by developing pearl culturing, in which a round object, or nucleus, is inserted into a living mussel. Today, commercial musselers harvest washboard, threeridge and mapleleaf shells to create bead nuclei for pearl culturing. An AMNH traveling pearls exhibit on marine and freshwater pearls recently visited the Houston Museum of Natural Science.
In Texas, an industry grew up around so-called Concho River pearls in San Angelo. First noticed by Native Americans and later by Spaniards, the pearls still provide an economic boon to the region. These unique pink and purple pearls form in the Tampico pearlymussel, a species found only in Texas and northern Mexico. Some believe that the Spanish crown jewels include Concho river pearls. Unlike many other species, Tampico pearlymussels are relatively common and not currently a species of concern.
At the sandbar I notice an exposed group of about a hundred large mussels, mostly dead, and I ask Rushing if it’s unusual. “Mussels are indicator species. They’re like the canary in the coal mine,” Rushing explains to me as well as the students. Since mussels feed by siphoning algae out of water, they are vulnerable to pollutants such as agricultural runoff.
The students lay out orange squares in a line, each 10 steps apart, and count half-shells and whole shells and identify the species present. This standard sampling method provides a way to compare from site to site, or from year to year. As the kids count shells, Rushing mentions that the water dropped a full foot from yesterday, when he took a separate group of students on the river. Rapid changes to the water level can kill mussels because of their low mobility. Although mussels do have a foot, they can’t move far, and when the water level drops they become easy prey for raccoons or other scavengers. Most likely the rapid water-level drop killed the mussels.
As we head back out onto the river, I think about the myriad challenges mussels face from so many directions, and I wonder, what it would take to reverse the trend?
“One is the need to improve riparian buffers, control erosion and improve land management practices in construction, farming, and silviculture,” says Bogan. Riparian buffers are forested strips along riverbanks, and they protect streams from excessive runoff, cool the water and provide organic matter via falling limbs and debris, which helps drive the aquatic ecosystem. “Clean water is critical to the survival of freshwater mussels. Recovery of the waters will assist with the recovery of the freshwater mussels and fishes, as long as they have refuge populations they can expand outward from.”
Besides destroying mussel habitat, dams prevent the migration of anadromous host fish, those that spawn in the ocean but live in rivers, and this affects any mussel species whose larvae depend on them. “If you put in a dam, that migration can no longer occur,” Bogan continues, “It’s only a matter of time before a mussel species depending on such fish disappear. Removal of dams would facilitate the movement of fishes and the recolonization of areas where the mussels have been extirpated.”
Though lack of information should not be an excuse for not conserving, research provides an integral component of conservation. “There’s a great need for further research on the basic biology of freshwater mussels, their physiology, ecology, and role in nutrient cycling, since they are very long-lived invertebrates. There is the need to better understand and determine fish hosts for native species,” urges Bogan.
TPWD just instituted a grant with two universities to help expand knowledge of mussel populations, but budgets are not infinite. Reestablishing species requires money and know-how. “The mussels we know least about are the rare or endangered species,” says Prentice, “and they are the most difficult to study because of their limited numbers.”
For species on the brink of extinction, scientists in other states are developing captive propagation methods, so they can reintroduce mussels where they once lived. In Texas, captive rearing could become a tool for working with mussels-in-decline in the future, says Prentice. “The habitat needs to be there to support any mussels we might someday want to reintroduce, so we work at land- and water-use practices that can enhance river/stream flow and natural setting, stream bank stabilization, river rehabilitation, and water allocation.”
Paddling slowly down the Navasota River in the still quiet of the morning, you can almost forget that a world exists beyond the banks. The river’s world is a quiet one, punctuated with an occasional cicada song or the plunk of a snake dropping from an overhead tree. I think about the great loss we endure when species as interesting as mussels disappear — the colorful pearls, the unusual shells, the hilarious names, the history. Mussels have so many adversities to overcome — exotic species, pollution, sedimentation, dams and other river modifications — it seems understandable that species diversity has plummeted, and of the lineages that remain, many struggle to keep their place in the tree of life.
It’s easy to understand why people don’t know much about mussels. They live hidden lives buried under the riverbank. They look like bumpy rocks, and not many people care when they silently disappear, species by species, from a river.
For more information about the Texas Mussel Watch program, visit: www.tpwd.state.tx.us/nature/education/tracker/mussels/.