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Description
The purple bankclimber usually grows to be between 10.16-13.97 cm (4-5.5 in), though they may grow up to 20.32 cm (8 in) long (Florida Fish and Wildlife; Butler). These mussels are gray and black on the outside, and their shells are asymmetric and nearly rhomboidal (Florida Fish and Wildlife). Parts of the shell may have folds and ridges, but these patterns may change throughout development (Butler). On the inside of their shell, the color transitions from white to purple moving outward, and there is some iridescence (Butler). They have two valves: the left valve contains two teeth and the right contains one tooth (Florida Fish and Wildlife). Purple bankclimbers do not show sexual dimorphism–males and females show no physical differences (Butler).

Life History
Very little is known about the exact life history of this mussel species. However, estimates can be made based on what is known about family Unionidae, which includes the bankclimbers.

Unionid bivalves:

Unionid bivalves fertilize in the spring, then incubate as glochidia until the summer or early fall (Butler). Female mussels release larvae into the water, where they attach to the fins or gills of a host fish and form a cyst (Florida Fish and Wildlife). The mussel's parasitic stage generally lasts a few weeks, but can be longer since it is temperature dependent (Butler). The larvae metamorphose into juveniles and then release from the host fish. After their release, they drift in the current until they find solid material to settle on. This is where they will fully mature into adults.

The purple bankclimber's primary host fish are the eastern mosquitofish (Gambusia holbrooki), the guppy (Poecilia reticulata), and the blackbanded darter (Percina nigrofasciata) (NatureServe Explorer).

The growth rate of freshwater mussles is relatively fast in the first few years, then slows down substantially once they reach sexual maturity (Butler). Out of tens of thousands to several million larvae that are released, only 0.000001 to 0.0001% of larvae survive to this point. (Butler). The age of maturity for Unionid mussels ranges from one year to nine years old. (IUCN). No information on the life span for this specific species is available, but related species of freshwater mussels were found to have individuals up to 24-56 years old. Most sources estimate that the average lifespan of mussels is between 50-70 years, but some species can live up to 100-200 years. (Butler).

Diet
The purple bankclimber is an internal filter feeder, like most mussel species. Water flows in through one of their siphons (incurrent siphon), and any food that is present is caught in their gills. The water is then expelled through a separate siphon (excurrent siphon). Plankton, algae, and small particles of organic matter make up most of their diet (Florida Fish and Wildlife; Newport Bay Conservancy).

Behavior
Males release sperm into the water column where it is taken in by the female through their siphon. The egg is fertilized inside of the female's shell and then the larvae release into the water. (Florida Fish and Wildlife)

Adult mussels live in localized patches of streams, where they burrow into the substrate. After burrowing, they expose the area around their siphons for feeding and reproduction. (Butler).

Habitat
Purple bankclimber mussels are found primarily in inland freshwater and wetland habitats, usually in muddy, sandy, limestone, or graveled substrates (IUCN). In general, they inhabit water deeper than 3 meters and can tolerate slow, moderate, and fast-moving currents. (IUCN).

Range
Their historical range includes parts of Georgia, Alabama, and Florida (IUCN). They are now most commonly found in the Ochlockonee, Apalachicola, Flint, Chipola, and Suwannee Rivers, and were historically found in the Chattahoochee river as well (Butler). Purple bankclimbers are known to be found in St. Mark's National Wildlife Refuge, a US Fish and Wildlife Service Refuge in Florida (Florida Fish and Wildlife).

Historic and Current Population Dynamics
The purple bankclimber was historically found in ten sub-basins across six rivers, and currently is only known to occupy eight sub-basins across five rivers (Recovery Plan Amendment). It has not been observed in the Chattahoochee River since 2001 (Recovery Plan Amendment).

Population Size and Distribution
The purple bankclimber's population range is highly fragmented. Despite occurring over 45,000 km2, they only inhabit approximately 5,000km2 of that area (IUCN). Scientists have recorded a 39% decrease in their range, from 737 river miles to 543 river miles (IUCN).

Major Threats
Major threats to the purple bankclimber include human development and habitat modification, invasive species, and pollution (IUCN).

Human development includes urban and residential development, mining, and the construction of dams. Dams impound waterways and decrease water velocity, which can be very problematic for species living nearby. (Florida Fish and Wildlife). They can also cause sediment to build up, which may harm mussel populations and lead to increased population fragmentation. (Florida Fish and Wildlife). Dredging to remove sediment can also destroy mussel populations (Florida Fish and Wildlife).

Invasive species are a growing problem everywhere, but particularly in waterways. These species can cause resource competition that native species have not adapted to. The invasive Asian clam (Corbicula fluminea) directly competes with the Purple bankclimber throughout its habitat (Florida Fish and Wildlife). Invasive species such as the Flathead catfish (Pylodictis olivaris) eat mussels directly, and also may outcompete the host fish that bankclimbers rely on for reproduction (Georgia Department of Natural Resources).

Pollution from urban and domestic wastewater, military and industrial discharge, and agricultural run-off and discharge often flow directly into waterways. Filter feeders like the bankclimber may intake harmful chemicals as a result of this (Florida Fish and Wildlife).

Overfishing of fish populations leads to fewer opportunities for larvae to attach to host fish. This applies to Gulf sturgeons, the primary hosts for purple bankclimber larvae. As sturgeon populations within the range of the mussel decline, it results in a dependency on secondary hosts, which tend not to be as effective. (U.S. Fish & Wildlife Service)

ESA Listing
The purple bankclimber was listed on 3/16/1998 as threatened wherever found. The major threats for listing were habitat destruction, overutilization, and inadequacy of existing regulatory mechanisms. The bankclimber was found to be abundant in some places, but in recent sampling only large individuals were found, indicating a lack of recruitment. It also mentioned that the distribution of the bankclimber has become so restricted that any significant modification or destruction of their habitat could jeopardize their continued existence. (U.S. Fish & Wildlife Service)

Recover Plan
The goal of the recovery plan is to restore "viable populations" to a significant portion of their historic range (Butler). Recovery of the purple bankclimber population is expected to take fifteen years because of the extent of their decline, isolation of subpopulations, sensitivity to pollutants, and the continued destruction of their habitat (Butler).

The original 2003 Recovery Plan did not include recovery criteria specific to the purple bankclimber (Recovery Plan Amendment). A 2018 amendment to the plan outlined two goals specific to the bankclimber and one goal for all seven mussels (Recovery Plan Amendment):


 * The bankclimber must show stable or increasing population growth in at least seven populations, shown through natural recruitment and the presence of multiple age classes (Recovery Plan Amendment).
 * The Chipola, Flint, Apalachicola, and Ochlockanee Rivers all must contain at least one subpopulation of purple bankclimbers.
 * Threats to the bankclimber must be managed to the point that the population will remain viable in the future.

Achieving these goals could result in the bankclimber being delisted. These goals aim to create resilient populations by increasing their size until they are able to survive extreme or severe events without going extinct (Recovery Plan Amendment). They also aim to create redundancy by making sure there are many resilient populations in a variety of locations (Recovery Plan Amendment). Having these widespread units of resilient populations ensures the species is represented in a way that allows for gene flow and for existence in multiple habitat types (Recovery Plan Amendment).

Designating critical habitat areas for listed species is expensive and time consuming, but is a crucial element of recovery plans (Critical Habitat Assessment). In 2006, the US Fish and Wildlife Service proposed a rule that would designate 11 units, or about 1,864 km (1,158 miles) of rivers and streams as critical habitat for the purple bankclimber and six other threatened and endangered mussel species (Critical Habitat Assessment). This habitat includes drainage zones from Econfina Creek and the Apalachicola, Chattahoochee, Flint, and Suwannee Rivers in Alabama, Florida, and Georgia (Critical Habitat Assessment).

No budget information is available for the purple bankclimber or the seven mussel's recovery.

Five Year Review and Species Status Review
The most recent five-year review of the purple bankclimber's population status was conducted in 2020, and recommended that the species' listing status be maintained as "threatened" (Five Year Review).

There has been insufficient management of the threats the species faces. Water quality is continuously decreasing, natural flow regimes are changing, and population isolation is increasing (Five Year Review). Recruitment remains low, and the species remains dependent on secondary hosts that also face increasing threats (Five Year Review). Purple bankclimber habitat and resiliency are severely negatively impacted by the same threats identified at the time of the bankclimber's listing (Five Year Review).

There are currently five geographic areas with stable purple bankclimber populations: Apalachicola, Chipola, Middle Flint, Lower Flint, and Lower Ochlockonee (Five Year Review). There is still extreme uncertainty about the long-term resilience of these populations, since surrounding habitats (Upper Flint, Upper Ochlockonee, and Chipola sub-basins) are getting more vulnerable and species abundance is decreasing (Five Year Review).

As long as the habitat quality of the purple bankclimber continues to decline, the species has a low potential for recovery and will not be eligible for delisting (Five Year Review). To ensure current populations are maintained, a proper management plan and necessary measures must be taken to deal with threats facing this species.