Mollusks

What is a mollusk? Simply put, a mollusk is an animal that is a member of the phylum Mollusca. Found in this diverse group are familiar creatures such as snails, slugs, clams, oysters, octopuses, and squid. There is no single defining feature for a mollusk, though all have a specialized foot that allows the mollusk to move, attach, or grab.

Mollusks have not been studied thoroughly at Big Thicket; however, a recent survey of its freshwater mussels provided insight into the current status of these creatures.

Freshwater Mussels

Freshwater mussels inhabit the bottoms of Big Thicket’s rivers, creeks, and bayous. Mussels, along with oysters and clams, are classified as “bivalves” because they have two shells which protect their internal organs. By acting as natural water filters, mussels play a large role in keeping waterways healthy for all kinds of aquatic life.

Mussels are filter feeders, which means they get their nutrients by filtering water. Small openings between their shells siphon water into their gills, which allows them to breathe and extract tiny aquatic organisms for food, including bacteria, phytoplankton, and algae. The cleaned water is then pumped out of the mussel and back into the water column.

As mussels are mostly stationary, they utilize a clever method to reproduce and distribute their offspring. First, male freshwater mussels release sperm into the water. A nearby female mussel—carrying thousands of eggs—inhales the sperm as she filters water through her gills. The sperm fertilizes her eggs, which develop into microscopic larvae called “glochidia”. When the glochidia are ready to spawn, the female mussel waits for a fish to approach, then ejects the glochidia toward the fish. If successful, the glochidia latch onto the fish’s gills as harmless parasites. After several weeks, the juvenile mussels detach from the host fish and sink to the bottom of the waterway, where they will continue to mature and live independently.

Because they filter water, mussels are especially sensitive to pollutants in waterways. Heavy metals, ammonia, and other toxins can accumulate in mussels over time, affecting their health. Declining mussel populations affect water quality and overall ecosystem health, as mussels are part of the food chain for mammals, fish, and birds.

Mussel Research at Big Thicket

In the summer of 2023, park staff conducted a survey of freshwater mussels within Big Thicket National Preserve. The study helped researchers learn which species were present and which locations had the most diversity.

To find mussels, researchers waded into waterways and searched along the banks and bottoms. Big Thicket’s naturally murky water makes it hard to see mussels from the surface, so researchers scanned the area with their hands. Mussels that were found were identified and recorded, then placed back where they came from.

Park staff conducted surveys at 72 sites along the Neches River, Pine Island Bayou, Beech Creek, Big Sandy Creek, Menard Creek, Turkey Creek, and Village Creek. Twenty-four species were identified (listed below), with the greatest diversity on the Neches River and Village Creek. Narrower waterways like Beech Creek and Turkey Creek had fewer species present.

Some mussel species were more dominant in certain waterways:

• Big Sandy Creek: 41 of 46 mussels found were Texas lilliputs.
• Beech Creek: 178 of 200 mussels found were pondhorns.
• Village Creek: 298 Texas pigtoes and 240 western pimplebacks out of 891 total mussels found.
• Neches River (Jack Gore Baygall area): 345 of 479 mussels found were round pearlshells.

Most importantly, researchers found several mussel species that are listed as threatened in the state of Texas, including Louisiana pigtoe, sandbank pocketbook, southern hickorynut, Texas heelsplitter, and Texas pigtoe. In addition to their state-threatened status, two species have been proposed to be listed as endangered (Texas heelsplitter) or threatened (Louisiana pigtoe) under the Endangered Species Act.

Future research may investigate mussels inhabiting certain areas of concern in the preserve. One such area is the Neches River south of the saltwater barrier in Beaumont; research could examine how freshwater mussels and other aquatic organisms there are affected by saltwater intrusion. Another area of focus is bridge expansion along US Highway 69/287 and how mussel populations beneath bridges may be affected by construction.

Mussel Species

Researchers observed 24 species of freshwater mussels during surveys:

• Bankclimber Plectomerus dombeyanus
• Bleufer Potamilus purpuratus
• Fragile papershell Leptodea fragilis
• Giant floater Anodonta grandis
• Gulf mapleleaf Quadrula nobilis
• Louisiana fatmucket Lampsilis hydiana
• Louisiana pigtoe Pleurobema riddellii (state threatened; proposed to be federally threatened)
• Paper pondshell Anodonta imbecillis
• Pistolgrip Tritogonia verrucosa
• Pondhorn Uniomerus tetralasmus
• Rock pocketbook Arcidens confragosus
• Round pearlshell Glebula rotundata
• Sandbank pocketbook Lampsilis satura (state threatened)
• Southern hickorynut Obovaria jacksoniana (state threatened)
• Southern mapleleaf Quadrula apiculata
• Tapered pondhorn Uniomerus declivis
• Texas heelsplitter Potamilus amphichaenus (state threatened; proposed to be federally endangered)
• Texas lilliput Toxolasma texasensis
• Texas pigtoe Fusconaia askewi (state threatened)
• Threehorn wartyback Obliquaria reflexa
• Threeridge Amblema plicata
• Washboard Megalonaias nervosa
• Western pimpleback Quadrula mortoni
• Yellow sandshell Lampsilis teres