Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 6

Biology

Freshwater mussels like the Fawnsfoot are long-lived (Heller (1991) reports lifespans for the Lampsilinae of approximately 20 years), relatively sedentary filter-feeders. They have a complex reproductive cycle involving a period of obligate parasitism on a vertebrate host. Juvenile mussels are believed to burrow completely below the substrate surface where they will spend the first 3-5 years of their life (Balfour and Smock 1995; Schwalb and Pusch 2007). During this time they are likely feeding on a combination of detritus, algae and bacteria obtained from the interstitial pore water or through pedal feeding (Gatenby et al. 1997). Adult mussels are found at the substrate surface during the summer months, but are known to burrow below the surface during the winter months likely in response to dropping water temperatures or changing flow regimes (Schwalb and Pusch 2007). Adults feed by siphoning algae from the water column but may also engage in some pedal feeding (Nichols et al. 2005). The following discussion is based on a survey of the available literature and the personal observations of the report writer.

Life cycle and reproduction

The reproductive biology of the Fawnsfoot is believed to follow 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. Further development to the juvenile stage can not continue without a period of encystment on a vertebrate host. During encystment the immature juvenile will feed from the body fluids of the host and undergo significant differentiation, although virtually no growth occurs during this time (Haag pers. comm. 2007). Natural glochidial mortality is difficult to estimate but assumed to be extremely high. The duration of encystment is unknown for the Fawnsfoot. After releasing from the host, the juveniles settle to the river bottom and begin life as free-living mussels. Juvenile mussels remain burrowed in the sediment for several years until sexual maturity is reached at which point they migrate to the substrate surface and begin the cycle again (Watters et al. 2001). Age at maturity is unknown for the Fawnsfoot, but the average age of maturity for unionids is 6-12 years (McMahon 1991).

The Fawnsfoot is dioecious (i.e., has separate sexes) but males and females do not appear different externally. The spawning season for the Fawnsfoot is not well known but Parmalee and Bogan (1998) reported that this species is probably bradytictic (i.e., glochidia overwinter on the female). Clarke (1981) reported that females are gravid (i.e., brooding their young) from spring through summer, but no studies have been conducted on Canadian populations. Glochidia are very small at approximately 60 µm in length and height and lack hooks (Utterback 1915-16), suggesting they are gill parasites.

Clarke (1981) states that the most likely host for the Fawnsfoot in Canada is freshwater drum (Aplodinotus grunniens) although sauger (Sander canadensis) has also been reported as a host (Surber 1913; Wilson 1916). Neither species has yet been examined as a potential host for Canadian populations. Although the Great Lakes distributions for both fish species overlay well with the current and historical Fawnsfoot distribution (Figure 5), freshwater drum is a more abundant species in Ontario. Within the area bounded by the distribution of the Fawnsfoot, Fisheries and Oceans Canada’s fish distribution database contains 533 records for freshwater drum and only 29 records for sauger, supporting Clarke’s assertion that freshwater drum is the more likely host for Canadian populations. Freshwater drum and/or sauger have been collected from all areas in southern Ontario where the Fawnsfoot has also been collected with the exception of Muskrat Creek in the Saugeen River watershed (Figure 5). Although Fisheries and Oceans Canada’s fish database contains 4681 fish records from this area, there are no records for either freshwater drum or sauger from the entire Saugeen watershed.

Many species of freshwater mussels have evolved complex host attraction strategies to increase the probability of encountering a suitable host (Zanatta and Murphy 2006). These strategies range from the formation and release of conglutinates (bundles of glochidia bound in mucous) and the development of complex lures to extreme cases of host capture in the genus Epioblasma. Little is known of the reproductive behaviours of the Fawnsfoot; however, the closely related Deertoe (Truncilla truncata) is reported to use a valve-gaping display in response to physical manipulation (Zanatta and Murphy 2006). When stimulated, the female mussel gapes and exposes the bright white swollen marsupia containing the glochidia. Adult freshwater drum, particularly those that inhabit rivers, are known to be molluscivorous (Scott and Crossman 1973). Predation on gravid female mussels may present a unique route to facilitate the host-parasite relationship. During consumption the glochidia will be released into the mouth of the host then passed out through the gills where they may be able to attach and encyst.

Predation

Predation by terrestrial mammals such as muskrats (Ondatra zibethicus) and raccoons (Procyon lotor) has been shown to be an important limiting factor for some populations of freshwater mussels (Neves and Odom 1989). Muskrats are both size and species-specific predators and although they will consume the Fawnsfoot, Tyrrell and Hornbach (1998) report that it was selected against in the St. Croix River likely because of its small size and deep burrowing behaviour. Metcalfe-Smith and McGoldrick (2003) reported observing raccoon predation on mussels in Ontario waters. Human-related activities, such as the adoption of conservation tillage practices, have resulted in surges in predator populations which may increase the importance of predation-related threats in the future (Metcalfe-Smith and McGoldrick 2003). This anecdotal observation needs verification in order to quantify the effects of human-related activities on predator populations.

As discussed in the previous section (Life cycle and reproduction) predation by molluscivorous fishes such as the freshwater drum may facilitate the complex life cycle.

Physiology

No specific studies on the physiology of the Fawnsfoot could be located. In general, freshwater mussels of the family Unionidae are good indicators of overall ecosystem health and are particularly sensitive to heavy metals (Keller and Zam 1990), ammonia (Goudreau et al. 1993; Mummert et al. 2003), acidity (Huebner and Pynnonen 1992) and salinity (Liquori and Insler 1985, as cited in USFWS 1994).

Dispersal/migration

There are no specific studies on movement of the Fawnsfoot. In general, adult mussels have very limited dispersal abilities. 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). Small-scale movements on the order of centimetres/week have been reported by Amyot and Downing (1998) for Elliptio complanata; however, the primary means for large-scale dispersal, upstream movement, and the movement into novel habitats is limited to the encysted glochidial stage on the host fish. Both suggested hosts for the Fawnsfoot, freshwater drum and sauger, are capable of large-scale dispersal. Funk (1957) reported annual movement of freshwater drum in Missouri of up to 300 km although most fish dispersed less than 20 km. Pegg et al. (1997) reported that radio-tagged sauger in the Lower Tennessee River moved 67 km on average, while some travelled up to 276 km over a period of approximately 6 months.

Interspecific interactions

Immature Fawnsfoot are obligate parasites on vertebrate hosts. These hosts are believed to be freshwater drum and sauger. Freshwater mussels of the Great Lakes region have been severely impacted by negative interactions with the introduced dreissenid mussels D. polymorpha and D. bugensis (see Fluctuations and trends and Limiting factors and threats).

Adaptability 

The Fawnsfoot is reported to have fairly broad habitat tolerances (see Habitat requirements) both in terms of flow and substrate preferences suggesting they may be able to tolerate some environmental changes. However, the sedentary nature of adult freshwater mussels, general sensitivity to water quality (see Limiting factors and threats) and host dependency may offset these broader habitat tolerances. At this time there have been no studies to specifically address the adaptability of the Fawnsfoot and no known attempts to artificially rear this species (Brady pers. comm.2007).

Page details

Date modified: