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Recovery Strategy for the Hickorynut and the Kidneyshell [Final Version]

Species information – Kidneyshell

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Figure 4: Two kidneyshell specimens from the Sydenham River. Note the characteristic squarish spots. Photo credit: T. Morris, Fisheries and Oceans Canada.

COSEWIC Assessment Summary - May 2003

Common Name: Kidneyshell

Scientific Name: Ptychobranchus fasciolaris

COSEWIC Status: Endangered

COSEWIC Reason for designation: This species has been lost from about 70% of its historical range in Canada due to impacts of the zebra mussel and land use practices. It is now restricted to the East Sydenham and Ausable rivers. Although both populations appear to be reproducing, there is evidence that abundance has declined in the East Sydenham River. Agricultural impacts, including siltation, have eliminated populations in the Grand and Thames rivers, and threaten the continued existence of this species in Canada.

Occurrence: Ontario COSEWIC

Status history: Designated Endangered in 2003.

The Kidneyshell (Figure 4) is one of 5 members of the genus Ptychobranchus that occur in North America, however, it is the only member of the genus with a distribution that extends into Canada.  The species is considered globally secure (G4) and is listed by the American Fisheries Society as being stable within the United States although, in Canada, the Kidneyshell was designated as endangered by COSEWIC in 2003. The Kidneyshell has always had a Canadian distribution limited to southwestern Ontario where it was once found in lakes St. Clair and Erie, as well as the Detroit, Sydenham, Thames, Ausable, Grand, Welland and Niagara rivers.  Recent surveys have shown that this distribution has been reduced and the Kidneyshell is now limited to the Sydenham and Ausable rivers with a few scattered specimens in the Lake St. Clair delta.

The Kidneyshell is a medium to large freshwater mussel that is readily distinguished by its elongate, elliptical shell and yellowish-brown periostracum with wide, interrupted green rays that look like squarish spots (Figure 4).  The type locality is the Muskingham River, Ohio.  The following description of the species, reported in COSEWIC (2003b), was adapted from Clarke (1981), Strayer and Jirka (1997) and Parmalee and Bogan (1998).  The shell is solid, heavy and compressed, and may have a humped shape in old individuals.  The anterior end is rounded and the posterior end is bluntly pointed.  Beak sculpture is poorly developed, consisting of several fine, indistinct wavy ridges.  The surface of the shell (periostracum) ranges in colour from yellowish to yellowish-green, yellowish-brown, or medium brown, with generally distributed broad, interrupted green rays; the shells of old specimens may be a dark chestnut brown and rayless.  The periostracum is unsculptured except for coarse growth rests and a roughened posterior slope.  The nacre is generally white or bluish white, but may be pinkish in young specimens.  The hinge teeth are heavy.  The left valve has two low, thick, serrated triangular pseudocardinal teeth and two lateral teeth that are short, nearly straight, and usually widely separated.  The right valve has one somewhat compressed and pyramidal elevated tooth and one wide, elongated and serrated lateral tooth. The lateral teeth are almost pendulous distally, which is a good distinguishing feature.  The interdentum is wide and the beak cavity is shallow.  Females have a conspicuous groove on the inside of the shell that runs diagonally from the beak cavity towards the posterioventral end; this groove corresponds to the marsupium (COSEWIC 2003b). 


GlobalRange: In the United States, the Kidneyshell is currently found in Ohio, Tennessee, Kentucky, Michigan, New York, Pennsylvania, West Virginia, Virginia, Alabama, Mississippi and Illinois.

CanadianRange: In Canada, the Kidneyshell is found only in southwestern Ontario.  Since 1997, live specimens have only been reported from the Ausable River, Sydenham River, and Lake St. Clair. 

Percent of GlobalRange in Canada: Less than 5% of the global range of this species occurs in Canada.

Distribution Trend: Since the invasion of the Great Lakes by dreissenid mussels the Canadian geographical distribution for this species has been reduced by 70%.

Population Abundance

GlobalRange: In the United States the Kidneyshell is seldom a significant component of the mussel community but may be locally abundant.  It usually represents on average 2.5% (0.2-8.0%) of the mussel community in rivers but at individual sites where it is found the Kidneyshell may account for more than 10% of the community

CanadianRange: The largest Canadian population of the Kidneyshell occurs in the Ausable River where it comprises 1.5% of the overall mussel community.  In the Sydenham River it occurs in an average estimated density of 0.12/m2 at sites where it was found alive.   In the Lake St. Clair delta Kidneyshells comprised only 0.3% of the overall mussel community (COSEWIC 2003b). 

Percent of Global Abundance in Canada: Less than 5% of the global abundance of this species occurs in Canada.

Population Trend: It is estimated that the population of Kidneyshell in Canada has declined by 70% since the invasion of the Great Lakes by dreissenid mussels.  This estimate is based on the number of historical records that occur in waters that now contain dreissenid mussels.



Figure 5 : Global distribution of the Kidneyshell (modified from Parmalee and Bogan 1998)


Figure 6 : Distribution of the Kidneyshell in Canada. Current distribution reflects surveys since 1997

Biological Limiting Factors

Reproductive Attributes: The reproductive biology of the Kidneyshell follows the general reproductive biology of most unionid mussels. During spawning, male mussels release sperm into the water column and females filter it out of the water with their gills. Fertilization is then able to occur in specialized regions of the gills known as marsupia. Immature juveniles, known as glochidia, develop in the gill marsupia and are released by the female into the water column to undergo a period of parasitism on a suitable host fish species. Further development to the juvenile stage can not continue without a period of encystment on the host. Members of the genus Ptychobranchus have evolved a specialized method of delivering glochidia designed to increase the likelihood of encountering a suitable host.  The glochidia are released in mucous encased packages termed conglutinates which have been shown to resemble fish fry complete with eye spots, or benthic invertebrates such as chironomids. These two forms represent prey items of the host species and stimulate the feeding instincts of the host resulting in an active uptake into the mouth where the conglutinates rupture, releasing glochidia in close proximity to the gills of the host.  The hookless glochidia become encysted on the gills of the host and are encapsulated in a fluid filled sac where they are nourished by the host until they metamorphose and break free, settling to the substrate to begin life as free-living juveniles. Three glochidial host fishes have been identified for the Kidneyshell in Canada: blackside darter (Percina maculata); fantail darter (Etheostoma flabellare); johnny darter (E. nigrum) (McNichols and Mackie 2004).  In Canada, Kidneyshells are known to be gravid between September and November and encystment has been shown to last up to 60 days resulting in the potential for encysted glochidia on the host fishes anytime between September and January (McNichols and Mackie 2004).

Dispersal: Like most native freshwater mussels, Kidneyshell adults are essentially sessile with movement limited to only a few meters on the river/lake bottom. Although adult movement can be directed upstream or downstream, studies have found a net downstream movement through time (Balfour and Smock 1995; Villella et al. 2004). The primary means for large scale dispersal, upstream movement, and the invasion of new habitat or evasion of deteriorating habitat, is limited to the encysted glochidial stage on the host fish.