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Recovery Strategy For Northern Riffleshell, Snuffbox, Round Pigtoe, Mudpuppy Mussel and Rayed Bean in Canada [Proposed]

1. Species information Northern Riffleshell

Common Name: Northern Riffleshell

Scientific Name: Epioblasma torulosa rangiana

Assessment Summary

Status: Endangered

Reason for designation1: The Northern Riffleshell has suffered a range reduction of more than 95% over the past century.  In Canada, it occurs only in the Ausable River and a 50-km reach of the Sydenham River, with the latter population one of only three known reproducing populations in North America.

Occurrence: Ontario

Status history: designated endangered in 1999

1Reproduction has been confirmed for the Ausable River population since the time of listing.

The Northern Riffleshell is small to medium-sized and extremely sexually dimorphic. The males are irregularly ovate, with a wide, shallow sulcus anterior to the posterior ridge. Females are obovate, greatly expanded post-ventrally with the expansion very broadly rounded and transversely swollen after about the third year of growth. The beaks are elevated above the hinge line and moderately excavated. The pseudocardinal teeth are small, and the lateral teeth are fairly short and moderately thick.

Historically, this species was known from Alabama, Illinois, Indiana, Kentucky, Michigan, Ohio, Pennsylvania, Tennessee, West Virginia and Ontario. It was found throughout the Ohio drainage, the Great Lakes drainage including the western Lake Erie basin, Lake St. Clair and the Detroit River, the Sydenham River and recently was discovered in the Ausable River (Metcalfe-Smith et al. 1999).

Figure 1 : The Northern Riffleshell Epioblasma toruosa rangiana. Photo : S. Staton, Environnement Canada

The Northern Riffleshell has seen dramatic declines in the United States and Canada. In the United States, populations are thought to exist only in French Creek and the Allegheny River in Pennsylvania, Big Darby Creek in Ohio, and Elk and Oak rivers in West Virginia. It may also occur in Kentucky, Ohio and West Virginia. It has been listed as Endangered under the U.S. Endangered Species Act since 1993 and a recovery plan for this species in US waters was published in 1994 (USFWS 1994). In Canada, it is assumed to be eradicated in the Detroit River (Schloesser et al. 2006) Lake Erie (Schloesser and Nalepa 1994) and the offshore waters of Lake St. Clair (Nalepa et al. 1996).  After several surveys in the Sydenham  River between 1973 and 1991 no live Northern Riffleshells were located (Clarke 1981; Mackie and Topping 1988) and the subspecies was assigned a conservation status of SH (no verified occurrences in the past 20 years) in Ontario by the Natural Heritage Information Centre (NHIC 1997). In 1998-1999 Metcalfe-Smith surveyed 66 sites in the Ausable, Grand, Maitland, Sydenham and Thames rivers.  From the results of these surveys, the range of the Northern Riffleshell has been found to extend over a 50-km reach of the Sydenham River between Alvinston and Dawn Mills (Metcalfe-Smithet al.1999). Due to these findings, the subspecies was downlisted to S1 (extremely rare). More recently, a single live individual was found in a wetland area of Lake St. Clair in 2000 (Zanatta 2002) and the presence of a reproducing population in the Ausable River was confirmed in 2006 (pers. comm. S. Staton, Fisheries and Oceans Canada).

The Northern Riffleshell occurs in the most heavily populated and intensively farmed region of Canada, notably southwestern Ontario. Agricultural, urban, and industrial impacts have likely resulted in a loss of habitat forthis species in the Ausable and Sydenham rivers.  Urban impacts on the East Sydenham River are less than in other southwestern Ontario rivers, and water quality may have improved in recent years due to an improvement in sewage treatment.  Agricultural activities have increased, however, and run-off of silt and agricultural chemicals may continue to limit the distribution of the Northern Riffleshell in this system.

Three distinct subspecies ofEpioblasma torulosa are recognized: E. t. torulosa, E. t. rangiana and E. t. gubernaculum. Neither E. t. torulosanor E. t. gubernaculumshave ever been found in Canada, and both are presumed extinct (Williams et al.1993).


Global Range: In the United States, the Northern Riffleshell currently occurs in Illinois, Indiana, Kentucky, Michigan, Ohio, Pennsylvania and West Virginia.  In Canada, the Northern Riffleshell occurs in southwestern Ontario.

Canadian Range: The Canadian distribution of the Northern Riffleshell is limited mainly to a 50-km reach of the Sydenham River. A reproducing population was recently confirmed in the Ausable River although the full extent of its distribution is still being investigated.   A single live individual was found in a wetland area of Lake St. Clair in 2000 (Dextraseet al.2003).

Percent of Global Range in Canada:  Approximately 5% of the Northern Riffleshell’s global distribution is currently found in Canada (the remnant population in Lake St. Clair contributes negligibly to the global distribution).


Figure 2.  Global distribution of the Northern Riffleshell.


 Figure 3 : Current distribution of the Northern Riffleshell in Canada

Distribution Trend:  The range of the Northern Riffleshell has been greatly reduced as it no longer occurs in Illinois or Indiana, and its range has been drastically reduced in all other areas. The current North American distribution represents a range reduction of more than 95%. In Canada, its range once included western Lake Erie, Lake St. Clair and the Detroit and Sydenham rivers in Ontario. It is now limited to a 50 km reach of the Sydenham River, a 55 km reach of the Ausable River with a remnant population possibly occurring in Lake St. Clair.

Population Abundance

Global Range: The Northern Riffleshell is a rare subspecies. Although occasionally abundant, it is usually a minor component of the unionid community (Strayer and Jirka 1997). The Allegheny River and French Creek in Pennsylvania support the largest remaining populations in the United States.

Canadian Range: A few live specimens of the Northern Riffleshell occur over a 55-km reach of the Ausable River between Rock Glen and Brinsley and it occurs at low densities over a 50-km reach of the Sydenham River (Staton et al.2000b). Twenty years ago, the Sydenham River population was described as the healthiest extant population of Northern Riffleshell in North America.

Percent of Global Abundance in Canada: Approximately 25% of the global        population abundance of the Northern Riffleshell occurs in Canada.

Population Trend:  The current Canadian distribution of the Northern Riffleshell is restricted to three populations. The population remaining in the St. Clair delta is known from one live specimen observed in 2000, despite surveys in this region in 2003 and 2004 (D. McGoldrick, NWRI, pers. comm.). A previously unknown population of Northern Riffleshell was discovered in the Ausable River in 1998 and judging from the large number of dead shells collected this population was once larger than that in the Sydenham River. The population in the East Sydenham River is the largest remaining reproducing population in Canada. A survey of the Northern Riffleshell in the Sydenham River, found 228 live animals combined over the 2001 – 2003 field seasons.  

Biological Limiting Factors

Reproductive Attributes: The reproductive biology of the Northern Riffleshell follows the general reproductive biology of most mussels. During spawning, male mussels release sperm into the water and females living downstream filter it out of the water with their gills. Female mussels brood their young from the egg to the larval stage in specialized regions of their 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. Females of the genus Epioblasma, including the Northern Riffleshell, have developed complex behaviours involving luring mechanisms and the physical capturing of potential hosts to increase the likelihood of successful encystment.  Further development to the juvenile stage can not continue without a period of encystment on the host.

The glochidia are semi-circular and have a straight hinge line without hooks. This morphology is typical of glochidia that parasitize fish gills, rather than the fins of their host fish. The juveniles remain attached to the gills for 27 to 33 days, after which they fall to the substrate and complete their development into free-living adults.

To determine fish hosts for the Northern Riffleshell, fourteen host species underwent infestation experiments in the laboratory at the University of Guelph from 2002 – 2005. The Northern Riffleshell successfully transformed on 7 of these: the logperch (P. caprodes), blackside darter (Percina maculata), Iowa darter (Etheostoma exile), fantail darter (E. flabellare), johnny darter (E. nigrum), mottled sculpin (Cottus bairdi), and rainbow darter (E. caeruleum) (McNichols and Mackie 2003; McNichols et al.2004).

Dispersal: Like most freshwater mussels, the Northern Riffleshell has very limited dispersal abilities. The Northern Riffleshell 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.