Lesson: Learn About Texas Mussels
Texas Mussels: Introduction
What is a mussel?
Mussels are bivalves (invertebrates with 2 shells) that feed by filtering algae, phytoplankton, and other small particles from the water. As adults they are sessile, meaning they are fixed in one place. Freshwater mussels inhabit rivers, streams, and lakes around the world, with more than 300 species in North America alone.
Above: Texas Hornshell freshwater mussels. Photo courtesy of Texas A&M NRI.
They are also one of the most vulnerable groups of animals. Of our Texas species, 15 are threatened at the state level, 11 are being considered for listing under the Endangered Species Act, and one (Texas Hornshell) has already been listed.
To learn more about the challenges freshwater mussels face, look for this icon throughout the lesson:
Freshwater mussels have had significant cultural impacts. Native Americans harvested mussels for thousands of years, using their meat as a source of food and their shells as tools or a strengthening agent in clay pottery. The pearls these mussels sometimes produce were also used in jewelry.
Above: Pearls like the one pictured here have been used in jewelry for thousands of years. Photo from Wikimedia Commons.
Others have also recognized the aesthetic value of mussels. The largest mussel-based industry was the production of "pearl buttons," which were made not from pearls but from mussel shells. The name refers to the shiny, pearlescent inner layer of the shells.
Above: This shell has had several pearl buttons punched out. Photo courtesy of Indiana DNR.
This practice began in the late 1800s and grew to the point that it threatened wild populations, at least until plastics provided a suitable replacement in the 1940s.
Above: Mussels were big business in the 1800s. Operations like this one gathered and processed the mussels, then the shells were sent to button factories. Photo from Wikimedia Commons.
Mussels also fill several important roles in freshwater ecosystems. These include:
- Nutrient Cycling & Water Filtration
- Substrate Stabilization
- Habitat Diversity
- A Link in the Food Chain
To learn more about these ecological roles, look for this icon throughout the lesson:
Where do mussels live?
Name: Texas Fawnsfoot (Truncilla macrodon)
River Basin: Brazos and Colorado
Habitat Type: Riffles, Banks, and Backwater
Drought Susceptibility: Moderate
Listing Status: Candidate
Above: Texas Fawnsfoot. Photo courtesy of Garrett P. Weiberg via iNaturalist.
Name: Texas Hornshell (Popenaias popeii )
River Basin: Rio Grande (Rio Grande, Pecos, and Devils Rivers)
Stream Order: Tributaries and Mainstream
Habitat Type: Rocky
Drought Susceptibility: High
Conservation Status: Endangered
Above: Texas hornshell mussel shells. Photo courtesy of dbarclay via iNaturalist.
Name: Texas Heelsplitter (Potamilus amphichaenus)
River Basin: Neches, Sabine, and Trinity
Habitat Type: Mud and Fine Gravel
Drought Susceptibility: High
Listing Status: Currently Under Review
Above: A Texas heelsplitter shell. Photo courtesy of dbarclay via iNaturalist.
Name: Texas Fatmucket (Lampsilis bracteata)
River Basin: Colorado and Guadalupe
Habitat Type: Pool and Backwater
Drought Susceptibility: Moderate
Listing Status: Candidate
Above: A Texas fatmucket mussel. Photo courtesy of Ben Hutchins via iNaturalist.
A closer look at some of the challenges mussels face:
Take a look at the Pecos River:
Above: A Google Earth image of the Pecos River, with two "challenges" indicated by icons. These challenges are described below.
Challenge (#1): Fracking
Fracking is the process of accessing oil and natural gas by injecting fluid into the ground at high pressure. The wastewater fluid used in fracking contains high concentrations of salt and other potentially harmful materials, and the removal of vegetation from injection sites can increase soil erosion to nearby water bodies. Precautions must be taken in fracking operations to reduce environmental harm.
Above: Fracking uses pressurized water to access pockets of fossil fuels. Diagram courtesy of the US EPA.
Challenge (#2): Salinity
One of the most serious implications of fracking is salt pollution in freshwater bodies. Contamination by fracking wastewater is one way that salt pollution may occur; another is the concentration of naturally occurring salts by removal of freshwater, as the water would normally serve as a diluting agent.
Above: Map showing water use for fracking (AKA hydraulic fracturing), courtesy of USGS.
Mussels, like other freshwater organisms, cannot tolerate high concentrations of salt in their environment and are therefore unable to live in rivers that have been polluted by excess salt. This process renders the water unfit for human consumption as well. Increased salinity can also promote dangerous blooms of golden algae--keep an eye out for this mussel threat later in the lesson!
Take a look at the Brazos River:
Above: A Google Earth image of the Brazos River, with 3 "challenges" indicated by icons. These challenges are described below.
Challenge (#1): Thermal Pollution and Stratification
There are several problems that occur when water flow is restricted to create an artificial lake. Deep, still water tends to be significantly colder than shallower water, so when it is released from dams it can abruptly change the temperature of the stream on the other side. Temperature changes that have negative impacts on water quality are referred to as thermal pollution.
Above: In a stratified water body, dissolved oxygen is often lowest in the deepest layers, and mussels need dissolved oxygen to survive. Photo from Wikimedia Commons.
Temperature plays several important roles in habitat suitability for mussels. It affects their rates of growth and reproduction, the dissolved oxygen content of the water, and their feeding behavior. The other problem is stratification, which occurs when water bodies develop layers that vary widely in terms of dissolved oxygen and other factors. Often the deepest layers—unfortunately for mussels—are rendered uninhabitable.
Challenge (#2): Development
Development within the watershed can have major impacts on mussels, whether it's agricultural, residential, or commercial. These activities may remove riparian vegetation, replace permeable surfaces with impermeable ones, and generate pollutants which—due to the first two effects—are more likely to wash into mussel habitat as runoff.
Above: Impermeable surfaces like roads and sidewalks increase runoff to water bodies. Photo from Wikimedia Commons.
Challenge (#3): Dams
Dams impact habitat both upstream and downstream by reducing water flow, restricting fish movement, fragmenting rivers, and altering substrates. Any of these changes can make an area inhospitable to mussels, and because they are sessile, they cannot relocate to more suitable habitat.
Above: An image of a dam. Photo courtesy of the Army Corps of Engineers.
Above: A bankside view with icons indicating 2 mussel challenges and 1 ecosystem role, which are described below.
Challenge (#1): Erosion and Sediment Pollution
Land use changes that remove vegetation from a watershed can increase soil and bank erosion and add excess sediment to water bodies. Too much sediment can clog a mussel's gills, making it difficult for it to extract oxygen from the water.
Above: Excess sediment can cloud the water. Photo courtesy of USDA NRCS Montana.
Challenge (#2): Contaminants
Pesticides, fertilizers, metals, and even excess nutrients are all potential contaminants of mussel habitat. These pollutants are often generated by human activity (mining, agriculture, manufacturing, etc.) within the watershed and can run off the land into freshwater ecosystems.
Above: Floatables (trash pollution) are one of the more obvious types of contaminants, but others include industrial chemicals, agricultural waste, and mine drainage. Photo from Wikimedia Commons.
Ecosystem Role (#3): A Link in the Food Chain
Mussels are an important source of food for predators like birds, fish, otters, muskrats, raccoons, and even humans.
Above: Birds like this American Coot feed on mussels. Photo from iStock.
Above: An underwater image showing various challenges/ecosystem roles for mussels that are indicated by icons.
Ecosystem Role (#1): Nutrient Cycling and Water Filtration
As filter-feeders, mussels remove bacteria, sediment and particulates from the water column, cleaning it and converting the energy and nutrients in that material to forms that are available to other species.
See mussels in action in the video clip below!
Challenge (#2): Reproduction
Reproduction is a challenging but crucial endeavor for any species. For freshwater mussels, it's made more complicated by the fact that they require a host fish. They have a unique life cycle in which eggs hatch into parasitic larvae called glochidia. These larvae then attach to a fish's fins or gills in order to develop. They are often species specific, meaning they will only survive if they find the correct host.
Above: Fish gills with parasitic glochidia larvae, 25x magnification. Photo courtesy of Michael Hart, TAMU NRI.
Ecosystem Role (#3): Habitat Diversity
Mussel shells create new microhabitats for algae and macroinvertebrates, which in turn support other organisms.
Above: Mussels provide attachment points for algae and other organisms. Photo from YouTube courtesy of Ecofact.
Challenge (#4): Golden Algae
Golden algae (Prymnesium parvum) is a single-celled organism that occurs in water bodies all over the world. It's usually a benign part of the environment, but the presence of excess nutrients (such as those found in fertilizer) can trigger a "bloom"—a state of rapid growth and overproduction. The algae bloom can clog the gills of aquatic animals, lower the oxygen content of the water, and produce toxins that kill fish—all bad news for mussels.
Above: Golden algae viewed under magnification. Photo from Wikimedia Commons.
Golden algae also responds well to increased salinity, which is one reason it tends to be more prevalent (and mussels less so) in water bodies that have been subjected to salt pollution. The Pecos River provides a striking example of this phenomenon.
Above: A diagram with labels indicating the steps of the mussel reproductive cycle. These are explained below.
Freshwater mussels reproduce sexually. Sperm is released into the water column by the male and taken in by the female to fertilize her eggs.
Fertilized eggs develop into parasitic glochidia larvae. The glochidia grow in the female mussel's gills where they have an ample supple of oxygen-rich water.
When a potential host fish gets close enough, the glochidia are released and attach themselves to the fish's gills or fins.
Once the glochidia are embedded, cysts form around them and they remain there for 2-5 weeks, growing at a temperature-dependent rate.
When the mussels are ready, they break free from the host fish to begin an independent life.
Ecosystem Role (#6)
When mussels anchor themselves in sediment in large numbers, erosion is reduced and the substrate is made more stable.
Above: When mussels anchor themselves in sediment, they help stabilize the stream bed. Photo courtesy of USFWS.
How Can Private Land Stewards Help Mussels?
One of the easiest and most effective ways to reduce soil erosion is to let vegetation do the work for you. Plants help hold soil together with their roots and slow overland water flow, so leaving riparian vegetation intact, practicing careful grazing management, and planting "cover crops" are all excellent practices. Mulch is another alternative in areas where living vegetation is difficult to establish. Earth dikes, sediment traps or basins can also be constructed to manipulate water flow for erosion prevention.
Above: Vegetation is an effective tool for preventing soil erosion. Photo courtesy of NRCS South Dakota.
Use Efficient Irrigation
Irrigation is essential for agriculture, but its effects on freshwater habitat can be mitigated by making it as efficient as possible. Watering in the cooler parts of the day (morning, especially) can minimize evaporative water loss, and considering the specific moisture needs of different crop types will prevent over-watering. Some types of infrastructure, such as drip irrigation, can vastly increase efficiency. Careful monitoring and maintenance of all irrigation equipment will also help prevent leaks and wasted water.
Above: Drip irrigation requires some initial investment but can be a huge water saver long-term. Photo from Wikimedia Commons.
We may not be able to eliminate waste completely, but we can prevent harm to mussels by keeping it out of their habitat as much as possible. Being conscious of the different forms of waste and how it can end up in freshwater habitat is the first step. Garbage and industrial chemicals may be obvious contaminants, but fertilizers and pesticides are also harmful. It's important to use only as much of these substances as we need and to dispose of byproducts safely. Tips for reducing erosion can help prevent contamination as well, since these materials often end up in streams via erosion.
Above: Dispose of waste properly to prevent contamination of freshwater bodies. Photo courtesy of US EPA.
Demand Safer Fracking
Fracking can have detrimental impacts on mussel habitat, but certain practices and precautions can minimize harm. These include:
- Using existing roads and corridors as much as possible to reduce devegetation and erosion
- When reseeding areas, use locally adapted, native seed to prevent introduction of invasive species
- Ensuring that abandoned wells are closed and plugged properly
- Disposing of wastewater in accordance with existing regulations to prevent contamination
- Monitoring surface and subsurface water quality
In all cases, it's crucial that a plan is established BEFORE fracking activity begins, so that problems can be prevented before they arise. Your local natural resource professionals can help.
Above: Infrastructure used in fracking. Photo from Wikimedia Commons.