Perseverance paying off for La. pearlshell
The Margaritifera hembeli is not a new adult frozen beverage consumed to waste away summer happy hours, as the name might imply; it is an animal once commonly found in small waterways that is struggling with a one in 100 million shot at survival each time a female Louisiana pearlshell mussel releases its young into a stream.
More than 30 years ago, scientists recognized the tenuous hold on life for the rare freshwater mussel. Since that time, biologists with the U.S. Forest Service (USFS), U.S. Fish and Wildlife Service (USFWS) and the Louisiana Wildlife Diversity Program (originally Louisiana Natural Heritage Program) of the Louisiana Department of Wildlife and Fisheries have worked diligently to understand the life cycle of this creature and how they could help it from disappearing off the face of the earth.
After years of studying the secrets of the mussel, about 900 young mussels raised at the Natchitoches National Fish Hatchery were placed into streams on the Kisatchie National Forest in July. It was a monumental feat to reintroduce the mussel.
The Louisiana pearlshell was first placed on the Endangered Species List in 1988 and was believed to be found in only a few streams in Rapides Parish in the Bayou Boeuf watershed. However, new populations were discovered in the 1990s in Red River drainage watershed streams in Grant Parish and the species was downlisted to threatened in 1993. Many of the streams where the Louisiana pearlshell were found are on the Kisatchie National Forest, so the USFS began issuing management initiatives to help protect this rare species.
No longer were beavers allowed to dam up creeks and streams the mussel was known to inhabit. Off-road vehicle traffic was also restricted as well as the establishment of streamside zones to minimize sedimentation during logging operations.
Biologists from federal and state agencies, as well as various university students, began in 1985 to conduct surveys every few years, painstakingly counting (by hand) the number of pearlshell mussels on federal and private lands.
Freshwater mussels are bivalve (two calcium carbonate shells attached at a hinge) mollusks. You may be familiar with their close cousins — clams, oysters and scallops.
Like their cousins, mussels are filter feeders, wedged into the bottom of streams and rivers, filtering food particles out of the water. Dirty water — from pollutants or increased sedimentation and silt — can harm mussels. However, loss and degradation of habitat from a variety of land uses are the main reasons why the Louisiana pearlshell is struggling to survive.
Mussels do not have legs (although they have one foot — that’s a great riddle!) so they can’t pick up and move when things get rough.
Adult pearlshells live in gravelly, sandy areas of small, clear-flowing, undisturbed streams which cannot be too shallow or they will not survive (they may dry out and would be vulnerable to predators).
The male mussel discharges sperm into the rippling stream to be carried to females nearby (but they have to be downstream). The female mussel will filter in the sperm and fertilize her eggs stored in her gills.
Her gills serve as brood pouches (marsupia — think kangaroo and other Australian critters) and the fertilized eggs develop into glochidia (tiny round larvae).
Eventually the glochidia are released into the stream by their mother, must find (or more accurately, be found by) a host fish in the stream and attach to the fish’s gills or fins to develop into the juvenile stage. If the glochidia cannot attach to a fish, it sinks to the bottom of the stream and dies.
If the glochidia attaches to the wrong species of fish, the fish’s immune system attacks the glochidia and kills it, (hence the one in 100 million survival rate). It is a good thing mom releases on average about thousands of glochidia at a time.
Once the glochidia attaches itself to the gills of a host fish, the fish’s tissue will encapsulate the glochidia, forming a cyst. After about three months in the cyst, the glochidia turns into a juvenile mussel. The juvenile breaks from the cyst and drops from the host “hoping” to land in the gravel bed of the stream. Here the juvenile will develop for up to 12 years, completing its internal development, creating its adult shell and eventually becoming sexually mature.
For years, the host fish species for the Louisiana pearlshell was unknown, making raising the mussel in captivity impossible. In April 2016, Dr. Tony Brady from the Natchitoches National Fish Hatchery determined that the primary host fish species is the grass pickerel.
How did he do it? Luck? Serendipity? Yes, as well as through laborious fish trials and the process of elimination. Biologists knew which fish species lived in the streams with mussels, so it was a matter of watching to see which fish the glochidia successfully parasitized (don’t worry, it almost never kills the fish).
With that discovery, the fish hatchery and the Alabama Aquatic Biodiversity Center worked to develop a way to produce and rear captive pearlshell mussels. Biologists found pregnant (gravid) female pearlshell mussels and took them to the hatchery.
The gravid females were allowed to release their multitude of glochidia in individual rearing chambers. One lucky grass pickerel was placed into an “infestation bath” of several thousand Louisiana pearlshell glochidia looking for a host. After about five minutes in its special bath, the inoculated fish was removed and placed into its own tank.
This process was repeated to infest six or seven fish. The infested fish were maintained in special tanks for about three months, while the encysted glochidia transformed into juvenile mussels and eventually dropped off the fish.
The juveniles were put into special chambers and given a “head start” and allowed to develop for two years. The fish hatchery will continue to tweak and streamline the process of rearing the mussels in captivity. Brett Hortman, USFWS project leader at the hatchery, estimates the facility will eventually be able to rear up to 2,000 pearlshell mussels a year.
In mid-July, a team of biologists from the USFS and USFWS trekked into the hardwood bottoms of three streams on the Kisatchie National Forest and released about 900 Louisiana pearlshell mussels created in 2018.
Prior to this date, Forest Service biologists Steve Shively and Tedmund Soileau researched which streams in Rapides Parish would be ideal candidates for the Louisiana pearlshell mussel release. These streams either did not have any known Louisiana pearlshell mussels or mussels in the stream had previously disappeared resulting in a “blank slate” for scientists studying the genetics of these released mussels.
David Byrd, Forest Service Ecosystem, Conservation & Planning Team leader for the Kisatchie National Forest, is thrilled to see the years of partnership develop into a successful venture.
“This is a wonderful conservation success story that all involved can be very proud of,” David said. “I have waited many years to see the pearlshell mussel be reintroduced into Louisiana streams. All those hours of wading in streams and counting mussels is coming to fruition.”
Each mussel was oriented in the proper direction and placed into the gravel substrate facing into the current of the stream, where it began to filter the water for food particles.
The juvenile head-start mussels were outfitted with pit tags and number labels. The pit tag is adhered to the mussel with a marine epoxy that does not harm the animal. A pit tag reader allows biologists to locate the mussel in the stream. Data will be collected and recorded for these new mussel populations and will help with future reintroductions and releases.
“This day is probably the highlight of my career,” said Aaron Frater, USFWS biologist, as he released the juveniles into the Kisatchie streams. “Many years of research have gone into this moment and it is very rewarding to finally be able to put the Louisiana pearlshell mussels back into the wild.”
Scientists hope these youngsters will dig in to their new home and survive to maturity where they will start the life cycle all over again and one day be delisted as a threatened species.
(Stacy Blomquist is a public affairs specialist with the U.S. Forest Service.)