Recovering The Louisiana Pearlshell Mussel, A Tiny But Mighty Animal
story by Sherry Morton, LDWF Public Information
The Louisiana pearlshell (Margaritifera hembeli) is a rare species of freshwater mussel that is currently listed by the U.S Fish and Wildlife Service as a threatened species.
The pearlshell mussel is special because it is endemic to Louisiana. This means the species can only be found in Louisiana. In fact, it can only be found in two parishes - Grant and Rapides parishes.
“The pearlshell mussel is only found in small creeks… creeks you can wade around in and almost jump across,” said LDWF biologist Charles “Chuck” Battaglia. “They aren’t found in large creeks or rivers. They need cool spring-fed streams, like those that can be found in Grant and Rapides parishes.”
History of the Louisiana Pearlshell
The Louisiana pearlshell mussel was originally listed as endangered in 1988, and it was reclassified as threatened in 1993. The decreasing pearlshell mussel population can mainly be attributed to loss of habit and deteriorating water quality.
“Louisiana pearlshells need to breathe, like all animals,” said Amity Bass, Biologist Director for LDWF’s Coastal and Nongame Resources Division. “And in order to breathe, they need clean, clear water.”
To survive, mussels gather food and oxygen from the water. They do this by drawing water in, moving the water over their gills, and then passing the water out. The gills absorb oxygen while simultaneously extracting ingestible particles (a.k.a. food) from the water. The food is carried to the mussel’s mouth by tiny hair-like cilia located on their gills. Everything else is then swept out of the mussel’s body. This is called filter feeding.
“If there is too much sedimentation, or the water temperature gets too high, it makes it hard for the mussel to breathe the oxygen in the water, so this creates major problems,” Battaglia said.
Sedimentation also makes it harder for the pearlshell mussel to utilize the creek water - especially the juveniles.
“For adult pearlshells, approximately half of the shell is under the substrate and half of it is above,” Battaglia said. “For the juveniles, they are actually inside the substrate. They will burrow down and use the interstitial space, the space in between the sediment. Large substrate, like gravel, has a lot of water that flows through it; this provides juvenile mussels with plenty of food and oxygen. If the substrate is composed of fine silty material, then the mussels can’t breathe and they suffocate.”
Numerous factors have caused increased sedimentation in their habitat.
One example: If cattle are present near the creeks where the pearlshell mussels live, they often increase the amount of sedimentation just by trudging through the creeks. Furthermore, cattle could directly harm the animals by crushing the mussels since their shells are not that hard.
Sedimentation can also be caused by human activity in the water, such as driving ATVs over the creek, or dumping waste into the creek.
“If you have people that aren’t utilizing the stream in a sustainable manner, this greatly impacts the mussels’ habitat,” Battaglia said.
In addition to increased sedimentation, other habitat changes can affect their survival as well.
“Grant and Rapides parishes have a lot of upland pine, so you have a lot of timber harvesting in those areas,” Bass said. “Clear cutting without following best management practices for forestry, such as maintaining protective riparian areas alongside streams, can cause major changes to the mussels’ habitat. They need shady, cool streams to survive, so if you cut all the trees, it increases the water temperature and therefore changes the habitat.”
Additionally, any structure that dams up the water could have a number of impacts on the mussel by altering water flow, water quality, impeding the movement of the host fish, and creating problems with water levels leading to stream drying or areas that become too deep.
“And if you have a species that already has a limited range to begin with, then any kind of stressor negatively impacts them,” she said.
Importance of the Louisiana Pearlshell
Growing to be all of about 4 inches long as an adult, this little mollusk may not look like much on the surface, but these living creatures provide a lot of benefits to our ecosystem.
“It is hard to get people excited about mussels because they aren’t very charismatic or cute,” Bass said. “But this little innocuous organism helps to improve the water quality, which is important for human health.”
Mussels feed in a way that is very effective at clearing the water of microorganisms, she said. Their filter feeding removes contaminants such as bacteria, algae, and pollutants (such as herbicides, pharmaceuticals and flame retardants), thereby making the water safer to drink.
“This provides a big value to humans, and to wildlife as well. Notably, mussels can clear water of disease-causing microorganisms,” Battaglia said.
“Mussels are important for keeping the environment clean, and here is an example of one that is special to Louisiana,” Bass said.
In addition, the pearlshell mussel is a food source for many animals. This plays a critical role in the food web because the mussel’s method of filter feeding converts an otherwise inaccessible energy source into food for animals that prey on them - fish, crawfish, amphibians, reptiles, birds and mammals. Mussel shells also provide living space for insects and plants, and empty shells serve as nesting sites for small fish.
Another benefit is that mussels are good indicators of water quality.
“If you are in a stream, and you don’t see a lot of mussels, then you know there is something going on with that stream,” said Nicole Lorenz, Program Manager for LDWF’s Wildlife Diversity Program.
LDWF has been working on conservation efforts for the Louisiana pearlshell mussel for decades - ever since it first became a listed species. LDWF receives funding from the federal government in order to survey for mussels on private lands. The department also coordinates with other agencies to maximize the conservation efforts in the state.
“There are many good conservation efforts underway for the pearlshell mussel,” Bass noted. “In fact, both LDWF and the U.S. Fish and Wildlife Service focus efforts and resources on recovering the species to remove it from the federal Endangered Species List.”
The U.S. Fish and Wildlife Service has a number of ongoing recovery efforts underway to help conserve the species. Six projects are being coordinated by the agency’s “Partners for Fish and Wildlife Program,” which works with private landowners and other partners to implement conservation efforts for threatened and endangered species and their habitats. At one creek occupied by the mussel, current projects underway for the Louisiana pearlshell include upgrading and improving fish passage at a parish road crossing on the creek, installing a fence to keep cows out of the creek, replanting and restoring riparian buffers, and replacing a log-and-dirt creek crossing with a small bridge on private property.
These types of projects improve the water quality and species’ habitat by, among other things, helping to keep cattle out of the waterways, and ensuring that creek crossings won’t dam up the water. This helps make sure the water is flowing as naturally as possible to prevent destabilization of the stream and allow fish and mussel migration throughout the stream.
“We also work with the U.S. Forest Service,” Bass said. “Part of the species’ range is found on Kisatchie National Forest, so the forest service handles the pearlshell monitoring on their land and the department monitors pearlshell on private lands. We can monitor the species as a whole through our partnerships.
“This is a good example of collaborative conservation with state and federal agencies and private landowners,” Bass said. “Everyone pulls together to work on collective recovery of the species. As the species’ entire range is in Louisiana, it’s completely up to the state and our partners to recover the pearlshell mussel.”
In order to measure the effectiveness of ongoing conservation efforts, LDWF and the U.S. Forest Service regularly survey Louisiana pearlshell mussel in order to determine trends regarding the mussel population across its entire range.
“One of the big issues with surveying for this mussel is that they form large aggregations we call beds,” Battaglia said. “This makes it difficult to get an accurate count of mussels; however, this also helps because it is easy to find the locations of these beds and know where most of the mussels are concentrated. The LDWF and our federal partners have developed a standardized monitoring protocol that helps us accurately estimate the number of mussels in beds that are too large for visual counts.”
“This helps reduce the deviation in bed estimates, and therefore, allows us to more effectively track population trends. We use these trend lines to make management decisions, so we need them to be as accurate as possible.”
“We are looking forward to the day that our collective efforts will result in the recovery of the Louisiana pearlshell and removal from the federal endangered species list,” said Bass.
Mussels reproduce in complex and remarkable ways. Male mussels release genetic material into the water column that is taken in by females to make larvae, known as glochidia.
Some mussels make intricate lures that mimic a small prey fish to attract specific fish. (You could even say they are fishing!) When a fish bites the lure, the mussel spits all its glochidia into the face of the fish. The glochidia then attach to the fish’s gills as parasites.
While Louisiana pearlshell mussels don’t make a lure, they do something similar by releasing a purplish packet that includes its glochidia into the water column. The “host” fish, believed to be Grass Pickerel (Esox americanus), is attracted to this packet and tries to ingest it. As a result, the fish becomes infected with the glochidia.
Once the glochidia attach to the fish’s gills, they get blood and other nutrients from the fish. When they are large enough, they drop off as juvenile mussels and embed themselves into the sediment, where they spend the rest of their lives -
20 to 50 years!
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