Fat pocketbook (Potamilus capax)

Fat pocketbook shell
IUCN Red List species status – Vulnerable VULNERABLE

Top facts

  • The fat pocketbook was once relatively widespread, but it is thought to have declined as a result of habitat loss due to dredging and impoundments.
  • A freshwater mussel, the fat pocketbook is typically found in large rivers with slow- to moderate-moving water.
  • The fat pocketbook has a thin, rounded, tan or light brown shell, which may be darker brown in older individuals.
  • As in other mussels, the fat pocketbook relies on a fish host to complete its life cycle.
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Fat pocketbook fact file

Fat pocketbook description

GenusPotamilus (1)

The fat pocketbook (Potamilus capax) is a large freshwater mussel (1) (3) with a rounded, greatly inflated shell that has an S-shaped hinge line (2) (4) (5). The shell is thin (2) (4) (5) to moderately thick (4), smooth (2) (3) (4) (5), shiny and is typically tan or light brown in colour (2) (3) (4). Older individuals of this species may be dark brown (2). The inside layer of the shell, known as the ‘nacre,’ is white, sometimes with a pink or salmon tinge (2).

As in other mussel species, the fat pocketbook has what are known as ‘pseudocardinal’ and ‘lateral’ teeth. Pseudocardinal teeth are structures found near the anterior-dorsal edge of the mussel (6), and in the fat pocketbook these are thin and compressed, with two located in each valve (2). There are two thin, greatly curved lateral teeth in the left valve, and one in the right (2).

Also known as
Fat pocketbook pearly mussel, grandmaw, pocketbook.
Proptera capax.
Shell length: up to 12.7 cm (2)

Fat pocketbook biology

Like other freshwater mussels, the fat pocketbook is a filter feeder, filtering out food particles (3) as water is continuously pumped through specialised siphons (4) (9). During this process, the adult mussel removes unwanted toxins from the water and gathers necessary nutrients (9) from items such as plankton and detritus (3) (4). Fat pocketbook larvae feed on the bodily fluids of fish (3).

The fat pocketbook has been reported to require a stable, undisturbed habitat with a sufficient population of suitable fish hosts to complete its life cycle (8). To reproduce, the male discharges sperm into the water current and the female positions itself downstream. The female then uses its siphons to intake the sperm and fertilise its eggs (3) (8). Spawning in the fat pocketbook occurs from late August through September (9).

Fertilised eggs are stored in the gill pouches of the female until the larvae (3), known as ‘glochidia’, are fully developed, at which point the glochidia are expelled by the female (8). In the fat pocketbook, glochidia release typically takes place in June and July (9). Once released into the water, the glochidia must find a suitable host fish within a couple of days to enable them to develop into adult mussels (3). Tiny, clasping valves are used to clamp onto the fins or gills of a host fish (8) (9), which in the fat pocketbook is the freshwater drum (Aplodinotus grunniens) (3) (9) (10). After a period of between two and four weeks (9), the juvenile mussels detach from the host fish and settle into the streambed to grow and reproduce (8) (9).


Fat pocketbook range

The fat pocketbook is endemic to the United States, where it was previously known from Arkansas, Illinois, Indiana, Kentucky, Louisiana, Mississippi, Missouri (1) (3) (7), Nebraska, New York and Minnesota (1). Once widely distributed in the Mississippi River drainage (3), this species now inhabits a smaller range, known to exist in approximately 320 kilometres of the St Francis River system as well as a few other areas, including the lower Wabash River and the Ohio River (1) (3)


Fat pocketbook habitat

A freshwater species (1), the fat pocketbook is found in large rivers (2) (3) (4) (8) (9) with slow-flowing water (1) (2) or a moderate current (4). Interestingly, this species is also known from man-made ditches, bayous and sloughs (1). Members of the Potamilus genus typically occur in soft sediments (10), and the fat pocketbook is no exception. This species is often found in gravel, sand or mud substrate (1) (2) (4) (8), although it has been reported from a broad range of substrate types (9) including shifting sand and hard clay (3). The fat pocketbook is usually found in water depths of between a few centimetres and a couple of metres, where it buries itself in the substrate with just the edge of its shell and its feeding siphons exposed (8).


Fat pocketbook status

The fat pocketbook is classified as Vulnerable (VU) on the IUCN Red List (1).

IUCN Red List species status – Vulnerable


Fat pocketbook threats

Historically, loss of habitat as a result of siltation, impoundments and dredging for navigation, irrigation and flood control has been considered the principal threat to this species and the primary cause of its decline (1) (3) (4) (8) (11). However, the core population of the fat pocketbook in the St Francis River system in Arkansas and the lower Wabash in Indiana have been shown to be healthy and tolerant of deteriorating habitat conditions as a result of dredging and sedimentation (1). Interestingly, the fat pocketbook has been found in areas known to be unfavourable to other mussel species (1), yet has disappeared from other regions where other mussels continue to exist (3).

Pollution of streams and rivers through agricultural and industrial runoff may pose an additional threat to the fat pocketbook (1) (4) (8). Chemicals and toxins from pollutants become concentrated in the body tissues of filter feeders, which can be fatal (8).


Fat pocketbook conservation

The fat pocketbook is classified as a Federally endangered species, and is also classified as endangered in Missouri (9) and Illinois (2), with State law protecting it from take or commerce (11). A recovery plan was developed for this species, with objectives that included protecting the existing population in the St Francis Floodway and its tributaries from habitat modification, and establishing new viable populations within the species’ historic range (1).

Aside from a population found within Patoka River National Wildlife Refuge (7), the fat pocketbook is not known to occur in any protected areas (1). Commercial activity has been stopped in the lower Wabash River, removing any immediate harvest threats, and a population has been reintroduced to the Mississippi River in Missouri (1) (3).

Future conservation measures proposed to ensure the future survival of the fat pocketbook include conducting life history studies (1), monitoring habitat and known populations, and working with Federal and State agencies and local communities (11) to promote beneficial management practices (4).


Find out more

Find out more about the fat pocketbook:



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Situated at or near the front.
Litter formed from fragments of dead material.
Relating to the back or top side of an animal.
A species or taxonomic group that is only found in one particular country or geographic area.
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
A category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
Immature stage in an animal’s lifecycle, after it hatches from an egg and before it changes into the adult form. Larvae are typically very different in appearance to adults; they are able to feed and move around but are usually unable to reproduce.
Aquatic organisms, usually tiny, that drift passively with water movements; includes phytoplankton (plants), zooplankton (animals), or other organisms such as bacteria.
In molluscs, a tube-like structure through which water passes into or out of the mantle cavity.


  1. IUCN Red List (May, 2014)
  2. Illinois Natural History Survey - Fat pocketbook:
  3. U.S. Fish & Wildlife Service - Life histories: Fat pocketbook (May, 2014)
  4. Louisiana Wildlife and Fisheries - Rare Animals of Louisiana: fat pocketbook (May, 2014)
  5. Thorp, J.H. and Covich, A.P. (2010) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Waltham, Massachusetts.
  6. Illinois State Museum - Mussel Glossary (May, 2014)
  7. U.S. Fish & Wildlife Service - Species Profiles: Fat pocketbook (May, 2014)
  8. U.S. Fish & Wildlife Service - Endangered Species: Fat pocketbook (May, 2014)
  9. Missouri Department of Conservation - Best Management Practices: Fat pocketbook (May, 2014)
  10. Haag, W.R. (2012) North American Freshwater Mussels: Natural History, Ecology, and Conservation. Cambridge University Press, Cambridge.
  11. U.S. Fish & Wildlife Service - Fat Pocketbook Mussel Recovery Action Plan (May, 2014)

Image credit

Fat pocketbook shell  
Fat pocketbook shell

© Collection of the Illinois State Museum

Illinois State Museum


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