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Round Pigtoe (Pleurobema Sintoxia)

Biology

General

The basic life history of the freshwater mussel is applicable to the Round Pigtoe, and is described briefly as follows (adapted from Kat 1984, Watters 1999, and Nedeau et al. 2000): during spawning, males release sperm into the water and females living downstream filter the sperm out of the water with their gills. Ova are fertilized in a specialized region of the female gills, called marsupia, where they are held until they reach an intermediate larval stage termed the glochidium. The female mussel then releases the glochidia, which must attach to an appropriate host and become encapsulated. The glochidia remain attached and are nourished by the host’s body fluids until they metamorphose into juveniles. The juveniles then break free of the capsule and fall to the substrate to begin life as a free-living mussel. The proportion of glochidia surviving to the juvenile stage is estimated to be as low as 0.000001%. Mussels overcome the extremely high mortality associated with this life cycle by producing large numbers of glochidia – often more than a million.


Reproduction

Pleurobema sintoxia is believed to be sexually dioecious, but is not sexually dimorphic. The lifespan of P. sintoxia has not yet been determined, but other members of the Subfamily Ambleminae are known to live for more than 30 years (Stansbery 1967). Age to maturity for this species is not known, but the juvenile stage for most unionids lasts 2-5 years. The Round Pigtoe is a short-term brooder (tachytictic) with the breeding season lasting from early May to late July in Wisconsin (Parmalee and Bogan 1998). The glochidia are subovate, without hooks, measuring 150 µm in both height and width according to Clarke (1981) and 160 µm according to Hoggarth (1993). The lack of hooks suggests that they are gill parasites. The known host fishes for this mussel are the Bluegill (Lepomis macrochirus), Spotfin Shiner (Cyprinella spiloptera), Bluntnose Minnow (Pimephales notatus), Northern Redbelly Dace (Phoxinus eos), and Southern Redbelly Dace (Phoxinus erythrogaster) (Hove 1995). All of these fishes, except for the Southern Redbelly Dace, occur commonly throughout P. sintoxia’s range in Canada and may therefore serve as glochidial hosts in Canadian waters. 


Movements/Dispersal

In the adult form, freshwater mussels are basically sessile; movement is limited to a few metres of the lake or river bottom. The only time that significant dispersal can take place is during the parasitic phase. Infected host fishes can transport the larval unionids into new habitats, and can replenish depleted populations with new individuals. Dispersal is particularly important for genetic exchange between populations (Nedeau et al. 2000). There is little opportunity for gene flow between Canadian populations of Round Pigtoe and American populations found in tributaries of Lake Erie and Lake St. Clair due to the presence of Zebra Mussels in the lakes and the distance host fish would need to travel. All Canadian riverine populations are geographically isolated from American populations; thus, there is little chance that individuals from American populations could bolster the Canadian populations or repopulate the Canadian range if the Canadian populations should disappear.


Nutrition and Interspecific Interactions

Round Pigtoes, like all species of freshwater mussels, are filter feeders as adults. Their primary food sources are bacteria, algae, particles of organic detritus, and some protozoans (Nedeau et al. 2000). Food availability may be a limiting factor for the Lake St. Clair population due to the presence of high densities of Zebra Mussels, which are also filter-feeders. During the parasitic larval stage, glochidia feed on the body fluids of the host.