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

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 extraordinary variation in life history traits (Table 3).

## **3.1 Reproduction**

140 Biodiversity Loss in a Changing Planet

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

> Arcidae Mytilidae Etheriidae Hyriidae Iridinidae Margaritiferidae Mycetopodidae Unionidae Cardiidae Corbiculidae Sphaeriidae Dreissenidae Solenidae Donacidae Navaculidae Corbulidae Erodonidae Teridinidae Lyonsiidae

Total 209 1026

Table 2. Classification of freshwater mussels (6 orders and 19 families), including number of

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

Family Genera Species

**2.2 Freshwater mussel distribution** 

knowledge that currently exist.

*Order* Arcoida *Order* Mytiloida *Order* Unioniformes

*Order* Veneroida

*Order* Myoida

*Order* Anomalodesmata

**2.3 Endemism and conservation** 

genera and species for each family (after Bogan, 2008).

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

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 143

In many mussel species, the gill, mantle margin, or other tissue has evolved into a lure that very realistically mimics a small minnow or invertebrate prey item used to attract a host fish. When a host fish nips at the lure, the glochidia are released into the vicinity of the fish's mouth, thus greatly increasing the odds of the glochidia attaching onto the fish's gills (Haag and Warren, 1999). Other species release large packages of glochidia called conglutinates, which often mimic prey items themselves, that rupture and release glochidia upon being bitten by potential hosts (Grabarkiewicz and Davis, 2008). These unique reproductive strategies have important implications for unionid conservation that will be discussed later

As adults, freshwater mussels live on the surface or in the top layers of sediment; filter feeding suspended phytoplankton, bacteria, detritus, and other organic matter out of the water (Strayer et al., 2004). Juveniles often bury themselves in sediment below the surface, filtering interstitial water (Grabarkiewicz and Davis, 2008) or feeding pedally by scooping food into their mouths with their foot (Yeager et al., 1994). Unionids are highly sedentary, moving only short vertical and horizontal distances to reproduce or in response to seasonal or environmental cues (Amyot and Downing, 1998; Balfour and Smock, 1995). They are found in a wide range of habitats, from soft sediment bottoms in lakes and ponds to cobble and rock substrates in fast-moving rivers, although the majority of species are found in clear, highly oxygenated streams and rivers with sand, gravel, or cobble bottoms

Freshwater mussels are often found in large multispecies aggregations known as mussel beds that can have densities of 10-100 individuals per square meter (Strayer et al., 2004). The biomass of freshwater mussels can higher than all other benthic macroinvertebrates by an order of magnitude (Layzer et al., 1993), and as a result of their large size and sheer numbers they can significantly influence both the biotic and abiotic conditions around them. Although the critical factors determining the location of mussel beds are still unclear, most researchers agree that water velocity and substrate, most notably where water velocity is low enough to limit shear stress and allow for substrate stability but high enough to prevent siltation, are strongly influential. Land use, geology, water quality, and availability of food and suitable host fish species are also strongly correlated with mussel presence/absence in other studies (Arbuckle and Downing, 2002; Newton et al., 2008; Strayer, 1983, 1999; Strayer et al., 2004). These habitat requirements result in a "patchy" distribution of mussels in riverine systems in non-continuous beds that may or may not be reproductively connected

As ecosystem engineers that modify their environment, freshwater mussels play many ecological roles where they are found in large numbers. These roles are a function of their life histories and behaviors, and can strongly affect both the biotic and abiotic components of the ecosystems in which they live. Loss of unionid biodiversity can result in loss of these functions and changes to the ecological regimes in those areas where mussels are in decline

**4. The role of freshwater mussels on ecosystem functioning** 

in this chapter.

**3.2 Feeding behavior and habitat preferences** 

(Grabarkiewicz and Davis, 2008).

by host fish (Strayer et al., 2004).

(Vaughn and Hakencamp, 2001).

**3.3 Small-scale spatial distribution** 

they are mature and ready to live freely on the bottom of the stream or lake. Because glochidia are heavy, short-lived, non-motile, and poorly carried in currents, facultative dispersal by fish species is necessary for the spread and maintenance of most Unionid populations (Strayer et al., 2004).


Table 3. Comparison of life history traits of freshwater mussels (Unionoidea and Sphaeriidae) in North America (after McMahon and Bogan, 2001).

Fig. 3. a.) Fish-imitating lure of a gravid female broken-ray mussel, *Lampsilis reeveiana*; b.) Crawfish-imitating lure of the rainbow-shell mussel, *Villosa iris*; c.) Glochidia of the fluted kidneyshell, *Ptychobranchus subtentum.* Each glochidia is approximately 220 micrometers long; d.) Fish-imitating conglutinate of the Ouchita kidneyshell, Ptychobranchus occidentalis; e.) Rainbow darter, *Etheostoma caeruleum, attacking a conglutinate of*  Ptychobranchus occidentalis; f.) Glochidia attached to the gills of a host fish. After attachment, the host fish's gill tissue forms a cyst around the glochidia. All photos are courtesy of Chris Barnhart (http://unionid.missouristate.edu).

In many mussel species, the gill, mantle margin, or other tissue has evolved into a lure that very realistically mimics a small minnow or invertebrate prey item used to attract a host fish. When a host fish nips at the lure, the glochidia are released into the vicinity of the fish's mouth, thus greatly increasing the odds of the glochidia attaching onto the fish's gills (Haag and Warren, 1999). Other species release large packages of glochidia called conglutinates, which often mimic prey items themselves, that rupture and release glochidia upon being bitten by potential hosts (Grabarkiewicz and Davis, 2008). These unique reproductive strategies have important implications for unionid conservation that will be discussed later in this chapter.

#### **3.2 Feeding behavior and habitat preferences**

142 Biodiversity Loss in a Changing Planet

they are mature and ready to live freely on the bottom of the stream or lake. Because glochidia are heavy, short-lived, non-motile, and poorly carried in currents, facultative dispersal by fish species is necessary for the spread and maintenance of most Unionid

Trait *Unionoidea Sphaeriidae* Life span range < 6 to > 100 years < 1 to > 5 years Age at maturity 6 to 12 years >0.17 to <1 year Reproductive mode gonochoristic hermaphroditic Fecundity (young/female/season) 0.2 – 17 million/female 2 – 136/female/season

Juvenile size at release 50 – 450 μm 600 – 4150 μm

Adult survivorship high intermediate

Table 3. Comparison of life history traits of freshwater mussels (Unionoidea and

Fig. 3. a.) Fish-imitating lure of a gravid female broken-ray mussel, *Lampsilis reeveiana*; b.) Crawfish-imitating lure of the rainbow-shell mussel, *Villosa iris*; c.) Glochidia of the fluted kidneyshell, *Ptychobranchus subtentum.* Each glochidia is approximately 220 micrometers

long; d.) Fish-imitating conglutinate of the Ouchita kidneyshell, Ptychobranchus occidentalis; e.) Rainbow darter, *Etheostoma caeruleum, attacking a conglutinate of*  Ptychobranchus occidentalis; f.) Glochidia attached to the gills of a host fish. After attachment, the host fish's gill tissue forms a cyst around the glochidia. All photos are

courtesy of Chris Barnhart (http://unionid.missouristate.edu).

Semelparous or iteroparous iteroparous semelparous or iteroparous

Juvenile survivorship extremely low high

Reproductive efforts per year 1 1-3

 (i) growth (%) 85-98 65-96 (ii) reproduction (%) 3-15 4-35

Sphaeriidae) in North America (after McMahon and Bogan, 2001).

per breeding season

populations (Strayer et al., 2004).

Non-respired energy allocated to:

As adults, freshwater mussels live on the surface or in the top layers of sediment; filter feeding suspended phytoplankton, bacteria, detritus, and other organic matter out of the water (Strayer et al., 2004). Juveniles often bury themselves in sediment below the surface, filtering interstitial water (Grabarkiewicz and Davis, 2008) or feeding pedally by scooping food into their mouths with their foot (Yeager et al., 1994). Unionids are highly sedentary, moving only short vertical and horizontal distances to reproduce or in response to seasonal or environmental cues (Amyot and Downing, 1998; Balfour and Smock, 1995). They are found in a wide range of habitats, from soft sediment bottoms in lakes and ponds to cobble and rock substrates in fast-moving rivers, although the majority of species are found in clear, highly oxygenated streams and rivers with sand, gravel, or cobble bottoms (Grabarkiewicz and Davis, 2008).

### **3.3 Small-scale spatial distribution**

Freshwater mussels are often found in large multispecies aggregations known as mussel beds that can have densities of 10-100 individuals per square meter (Strayer et al., 2004). The biomass of freshwater mussels can higher than all other benthic macroinvertebrates by an order of magnitude (Layzer et al., 1993), and as a result of their large size and sheer numbers they can significantly influence both the biotic and abiotic conditions around them. Although the critical factors determining the location of mussel beds are still unclear, most researchers agree that water velocity and substrate, most notably where water velocity is low enough to limit shear stress and allow for substrate stability but high enough to prevent siltation, are strongly influential. Land use, geology, water quality, and availability of food and suitable host fish species are also strongly correlated with mussel presence/absence in other studies (Arbuckle and Downing, 2002; Newton et al., 2008; Strayer, 1983, 1999; Strayer et al., 2004). These habitat requirements result in a "patchy" distribution of mussels in riverine systems in non-continuous beds that may or may not be reproductively connected by host fish (Strayer et al., 2004).
