**2.1 Classification of freshwater mussels**

Freshwater mussels (order Unioniformes) belong to the subclass Paleoheterodonta, class Bivalvia, and phylum Mollusca. A total of 18 bivalve families have at least one species found in freshwater, although only about 9 have radiated to any degree there (Bogan, 1993). The order Unioniformes contains the largest number and diversity of groups with 180 out of 206 genera and 797 out of 1026 species. Within the Unioniformes, the family Unionidae is the largest, comprising nearly 80% of both the genera and species within the order (Bogan and Roe, 2008). Other important families include Hyriidae (17 genera, 83 species), Mycetopodidae (12 genera, 39 species), Sphaeriidae (8 genera, 196 species) (Bogan and Roe, 2008). As the order Unionidae is the most diverse, and has had the most research dedicated to it, we shall from here on out refer to freshwater mussels simply as Unionids, or unionid mussels.

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 141

extinction due to the fact that limited distribution puts most or all of a population at risk to environmental stresses simultaneously (Gaston, 1998) and also limits the ability of a population to recover through recruitment from other populations, especially in species with low dispersal ability, such as unionid mussels (Burlakova et al., 2010). One recent study showed that endemic species were critical determinants of the uniqueness of unionid communities, and as such, should be made a conservation priority (Burlakova et al., 2010).

Fig. 2. Map showing the distribution of freshwater mussel species by biogeographic region.

Freshwater mussels are long-lived organisms, often living for decades, and some species can survive over 100 years (Bauer 1992). Typically, unionids live buried in fine substrate in unpolluted streams and rivers with benthic, sedentary, suspension-feeding lifestyles. The mussels use their exposed siphons to inhale water and use their gills to filter out fine food particles, such as bacteria, algae, and other small organic particles. Their benthic, sessile lifestyle, their obligatory dependence on fish hosts for reproduction, and their patchy distribution as a result of specific habitat requirements all contribute to their decline in the face of human disturbances. Freshwater mussels have complex life cycles with

Freshwater mussels are broadcast spawners, with males releasing sperm into the water to fertilize the eggs that are retained internally in the females' body (Wachtler et al., 2001). The defining characteristic of Unionids is their specialized larval stage known as glochidia that are released from a gravid female's modified "marsupial" gills where they developed from embryos following fertilization (McMahan and Bogan, 2001). One female mussel can produce up to 4 million or more glochidia and eject them in a sudden and synchronized action (Bauer 1987). If the glochidia are released in the proximity of a suitable host fish, they clamp onto the gills of the host, which then carries the glochidia for weeks or months until

**3. Ecology and life history of freshwater mussels** 

extraordinary variation in life history traits (Table 3).

**3.1 Reproduction** 

#### **2.2 Freshwater mussel distribution**

Freshwater mussels are found on every continent with the exception of Antarctica, but reach their highest level of diversity in the Nearctic geographic region, with one-third of all species (297 recognized taxa) being found there (Bogan, 2008). The Neotropical region has 179 described species, the Oriental has 121, the Palaearctic 92, the Afrotropical 74, and the Australasian region has 29 (Bogan, 2008). Data on the conservation status of freshwater mussels globally is incomplete, with relatively strong data from only a few areas (North America, Europe, and Australia). In other areas (Africa and South America), detailed taxonomic information including the total number of species currently or historically present is lacking, which makes determining changes in species abundance and richness difficult (Bogan, 2008). There has been increased interest in the biodiversity of freshwater mussels worldwide over the last few decades, though, as scientists have realized just how rapidly this group is declining (Graf and Cummings, 2007). Hopefully this increased awareness will lead to more surveys in these understudied areas to fill in the gaps in basic knowledge that currently exist.


Table 2. Classification of freshwater mussels (6 orders and 19 families), including number of genera and species for each family (after Bogan, 2008).

#### **2.3 Endemism and conservation**

One of the major reasons for the high proportion of extinct and endangered freshwater mussels is the high degree of endemism found in this group, which is characteristic of many freshwater organisms. Endemic species have a limited geographical range, often limited to a single drainage basin or lake, and often have unique characteristics suited to that particular locale (Strayer and Dudgeon, 2010). Local rarity also puts a species at a much higher risk of extinction due to the fact that limited distribution puts most or all of a population at risk to environmental stresses simultaneously (Gaston, 1998) and also limits the ability of a population to recover through recruitment from other populations, especially in species with low dispersal ability, such as unionid mussels (Burlakova et al., 2010). One recent study showed that endemic species were critical determinants of the uniqueness of unionid communities, and as such, should be made a conservation priority (Burlakova et al., 2010).

Fig. 2. Map showing the distribution of freshwater mussel species by biogeographic region.
