Kidneyshell (Ptychobranchus fasciolaris)

IUCN Red List species status – Least Concern LEAST

Top facts

  • Freshwater mussels, such as the invertebrates_freshwater, are one of the most endangered groups of animals in Canada.
  • The juvenile form of the snuffbox is a larva that must undergo a period of parasitism on a host fish to develop into an adult.
  • The kidneyshell may live for up to ten years.
  • The kidneyshell can be locally common in the United States, but is rare in Canada.
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Kidneyshell fact file

Kidneyshell description

GenusPtychobranchus (1)

The invertebrates_freshwater (Ptychobranchus fasciolaris) is medium- to large-sized freshwater mussel with a thick kidney-shaped shell (2) (3) (4). The shell is smooth and solid, and its colour ranges from yellowish to brown and is patterned with wide green squarish patches (3) (4). The inside of the shell, known as the ‘nacre’, is white or bluish-white, but can be pinkish in young individuals (2). The shells of older kidneyshells may be a dark chestnut brown colour and lack the green patches seen in younger specimens (4).

Maximum length: 12 cm (2)

Kidneyshell biology

Like all species of freshwater mussel, the invertebrates_freshwater filters its food from the water column with its gills (2) (4) and bacteria and algae are its primary food sources (2).

Similarly to many other mussel species, spawning involves the male releasing sperm into the watercourse, where a female can collect it by filtering it through its gills, allowing fertilisation to take place (2). The kidneyshell, as a member of the Order Unionidae has a somewhat unusual life cycle. The juvenile form of this mussel is an obligate parasite so it must spend this life stage attached to the gills of a host fish (2) (4). The juvenile form is called a ‘glochidum’, and the female kidneyshell mussel releases many glochidia together in packets. This increases the chances of ingestion by a host fish, due to their similarity to usual prey items. Once the packet is inside of the mouth of the fish the glochidia are released and flow through the gills (2) (5), where each glochidium may attach and become encysted. After a period of time the larval glochidium transforms into a juvenile mussel and becomes detached from the gills, before dropping onto the substrate where it can begin life as a free-living mussel (2) (5). Five host fishes have been identified for the kidneyshell, the blackside darter (Percina maculata), fantail darter (Etheostoma flabellare), johnny darter (Etheostoma nigrum), Iowa darter (Etheostoma exile), and brook stickleback (Culaea inconstans) (5).

The kidneyshell, along with other North American freshwater mussels, is prey for muskrats (Ondatra zibethicus), racoons (Procyon lotor), American mink (Neovison vison), North American river otters (Lontra canadensis), and some birds (5).


Kidneyshell range

In the United States, the invertebrates_freshwater is currently found from Alabama in the south to Michigan in the north, and from Virginia in the east to Illinois in the west (5). It is believed to be extirpated in Georgia and North Carolina (2) (4).  In Canada, the invertebrates_freshwater is found only in south-western Ontario (2), where it is now limited to a 100 kilometre reach of the Sydenham River, a 25  kilometre reach of the Ausable River, and there are very small populations in the St. Clair River delta and the Thames River (2).


Kidneyshell habitat

The kidneyshell inhabits small- to medium-sized rivers (3) and streams and can be found in riffle areas, with clear, swift-flowing water (2). This mussel can usually be found buried in river beds composed of coarse sand or gravel (2).


Kidneyshell status

The kidneyshell is classified as Least Concern (LC) on the IUCN Red List (1).

IUCN Red List species status – Least Concern


Kidneyshell threats

One of the greatest threats to the invertebrates_freshwater, and other freshwater mussels in North America, is the growing presence of the invasive zebra mussel (Dreissena polymorpha) (2) (4). Hundreds to thousands of these minute mussels can attach to the shell of the kidneyshell, which eventually kills the individual by interfering with its ability to feed, respire, excrete, and move (2) (4). Approximately 60 percent of the historical range of the kidneyshell has now been colonised by zebra mussels (4).

Remaining kidneyshell populations in Ontario are threatened by intensive agriculture and increasing urbanisation, particularly heavy loadings of silt, sediments, nutrients, and pollution that can destroy mussel habitat, reduce gill function, and reduce water quality (2) (4).

One of the most important limitations to kidneyshell populations is the availability of suitable host fish for the larval stage of this species’ lifecycle (4) The invasive round goby (Neogobius melanostomus) may be reducing the population size of various host fish species by increasing competition for resources, thereby indirectly diminishing kidneyshell numbers (2).


Kidneyshell conservation

The invertebrates_freshwater is considered critically imperilled or imperilled in Alabama, Illinois, Indiana, Mississippi, and New York, and nationally endangered in Canada (4). This mussel may be locally extinct in Georgia and Illinois (5). In areas where it is listed as imperilled, the kidneyshell is afforded some protection, but this does not guarantee continued population health due to the negative effects of invasive species and pollution (4).

In Canada, a recovery strategy and action plan has been developed to prevent further declines of the kidneyshell population and research, land and water stewardship, monitoring and awareness activities are underway. Provincial legislation regulating agricultural nutrients and protecting clean water in Ontario have been enacted and critical habitat for the kidneyshell has been identified under the Species at Risk Act (2).

Plans for the artificial propagation of freshwater mussels are underway, however, only endangered mussels of the genera Epioblasma and Lampsilis have been released currently, and the long-term survival of these individuals is completely unknown (4).


Find out more

Find out more about freshwater mussels in the Order Unionoida:

Read more about mussel conservation in North America at:



This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:



Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
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’ scientific species name; the second part is the specific name.
Describes species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, outcompete natives and take over the new environments.
Of or relating to the 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.
A category used in taxonomy, which is below ‘class’ and above ‘family’. All members of an order have characteristics in common.
An organism that derives its food from, and lives in or on, another living organism at the host’s expense.
Light rapids where water flows across a shallow section of river.
The production or depositing of eggs in water.


  1. IUCN Red List (June, 2014)
  2. Government of Canada - Aquatic Species at Risk - The Kidneyshell (May, 2014)
  3. Illinois Natural History Survey - Ptychobranchus fasciolaris (May, 2014)
  4. Committee on the Status of Endangered Wildlife in Canada Assessment and Status Report on the Kidneyshell Ptychobranchus fasciolaris in Canada (May, 2014)
  5. Government of Canada, Recovery Strategy for the Round Hickorynut (Obovaria subrotunda) and Kidneyshell (Ptychobranchus fasciolaris) in Canada (May, 2014)

Image credit


© J. Michael Butler

Jason Butler


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