Mussel Mystery

Please note the publication date of this article. Statistics and seasonal information were accurate at the time of publication. Check links provided for the most current information.

By Larry D. Hodge

Published in Texas Fish & Game, February 2005

Perhaps the best phrase to characterize mussels is “out of sight, out of mind.”

Freshwater mussels of the family Unionidae have lived in fresh waters around the world for the last 400 million years or so. Nearly 300 species live in North America; more than 50 of those occur in Texas. Because these bivalves live on the bottoms of lakes and streams, they tend to be largely overlooked or ignored.

Mussels deserve better. “Mussels are sensitive barometers of environmental quality,” says Robert Howells, a Texas Parks and Wildlife Department fisheries scientist who’s been leading TPWD’s work with mussels for more than a decade. “Whenever any reduction in environmental quality occurs, mussels are the first organisms to decline and vanish.”
That process seems to be well under way across North America. According to the U.S. Geological Survey, “Poised on the brink of mass extinction, freshwater mussels are the largest group of endangered animals in North America. About 70 percent of the 300 native species are considered endangered, threatened or of special concern. Biologists see the mussels’ plight as a serious warning for our global ecology as a whole—when mussels begin to disappear, it is a sign that other species, and entire ecosystems, may be in peril as well.”

At the risk of being obvious, I’ll point out that mussels live in the one thing all life on earth depends on—water. Anyone who hunts or fishes should care about mussels, for their fate portends what may happen to other, more visible species.
Gilled animals, mussels feed and breathe by pumping water through their bodies—they are nature’s biofilters. They concentrate environmental contaminants like heavy metals in their tissues, improving water quality. Some species attain ages of more than 100 years, typically producing a new layer of shell each year. By studying the annual layers, scientists can determine how environmental conditions changed over a mussel’s life.

Mussels have value beyond their role as environmental indicators. If you have vintage buttons made before the advent of plastics, chances are they were made from mussel shells. If you buy a cultured pearl necklace, there’s about a 95 percent chance each of the pearls has a center of American freshwater mussel shell. Mussels are harvested and pieces of the shell shaped into beads, then inserted into live pearl oysters or even other freshwater mussels. The oyster exudes layers of mineral to surround this foreign object, and a cultured pearl results. Using a round nucleus produces pearls that are rounder and more uniform than natural pearls

The use of mussel shell in the Japanese cultured pearl industry put mussels in the spotlight in Texas in the early 1990s. A price war among shell buyers in 1991 resulted in some species of shell bringing $10 a pound, and large “washboard” mussels could bring as much as $40. Commercial harvesters descended on Texas. TPWD moved quickly to regulate commercial mussel harvest, establishing a collection-by-hand-only rule, size and bag limits and no-harvest sanctuaries. Currently individuals may harvest up to 25 pounds of whole mussels (or 12 pounds of shell) daily for personal use and consumption; a fishing license and freshwater fishing stamp are required.

Mussel harvesting predates recorded history. First Americans probably ate mussels and used their shells and pearls for decoration. Spanish explorers took hundreds of pounds of pearls collected by Indians in East Texas and shipped them home. There’s some evidence that Spanish interest in harvesting pearls from Concho River mussels led to their interest in establishing control over western parts of the state. Much later, there was a “pearl rush” on Caddo Lake and nearby waters in 1909, and for many years, a Mason businessman had a standing offer to buy pearls from the Llano River. Some trade, centered in San Angelo, still exists in Concho River pearls and jewelry made from them. The shell of the mussels producing those pearls, the Tampico pearlymussell, is also used in making jewelry and craft objects.

Despite the interest in pearls and shells, harvest by humans does not constitute the main threat to mussels, though it can be a factor, particularly when populations are at low levels for other reasons. Following the 1991 price war on mussel shells, TPWD began statewide surveys and research into mussel biology. The picture that has emerged since is of a species that is in decline primarily due to environmental degradation. At present, TPWD is funding two university investigations into the current status of mussels; results will not be known for at least a couple of years.

TPWD’s Howells has identified a number of factors responsible for mussel declines in Texas. Too much rain falling too fast on an eroded landscape is the main problem. Most of Texas was badly overgrazed by 1890, and average rainfall has actually been increasing since 1900, with a trend toward long periods of drought punctuated by heavy storms. That scenario leads to low water levels followed by massive quantities of soil being washed into streams and lakes. Slow-moving creatures that depend on stable water conditions for survival, mussels can be left high and dry when water levels fall, and they can be smothered in silt when rains come. Either can be fatal. Massive floods that scour river beds clean down to bedrock can change habitat so much that mussels can no longer live there.

Reservoir building since the 1950s has altered the flow characteristics of every Texas river of any size, sometimes to the detriment of mussels. Reservoirs flood some habitats while dewatering others, and mussels are unable to cope with rapid changes in their environment.

Freshwater mussels are unique in that their larvae, called glochidia, must live for a time as a parasite on a fish. Different species of mussels require different species of host fish. If the right species of host fish is not present, the glochidia will die. To increase the chances their young will find and attach to a fish host, some types of female mussels display tissues that mimic a small prey fish to try to lure fish to swim near them. When she senses a fish of the right species nearby, she releases her young into the water so they can attach to the fish. It’s amazing that such a seemingly simple organism can be so sophisticated.

As with most living creatures, whether mussels survive or fall into the abyss of extinction depends on whether they have the proper habitat. James D. Williams of the National Biological Service offers this sobering assessment: “If the decline of aquatic mollusks continues, we will witness the greatest extinction of these organisms experienced in modern times.”
And we might well ask who or what will be next.

Additional Information:

To join Texas Mussel Watch, contact Texas Nature Trackers at (800) 792-1112, extension 7011.

© Copyright Texas Parks and Wildlife Department. No part of this work may be copied, reproduced, or translated in any form or medium without the prior written consent of Texas Parks Wildlife Department except where specifically noted. If you want to use these articles, see Site Policies.

Nobody Loves Fatmuckets

Freshwater mussels are incredibly important, but most people never see one, and if they do, it’s not cute. Mussels are appearance-challenged. Beauty would serve little purpose for a creature that spends its life partially buried in mud. One of the main methods of searching for mussels is walking along the bottom with bare feet, feeling for mussels with your toes. Many Texans have fond memories of summer days spent doing just that.

For some reason, mussels have been the inspiration for some of the more colorful names given to wild creatures. The appellations deer, or coyote, or squirrel pale when compared to Texas pigtoe, threehorn wartyback, or Rio Grande monkeyface. Some other names of Texas mussels that strike my fancy are white heelsplitter, bankclimber, lilliput, washboard, and fragile papershell. I can figure out where those names probably came from, but others, like Texas fatmucket and rock-pocketbook, remain fascinating mysteries.

Perhaps that’s appropriate for a creature so important, yet so little understood.