by Andrew L. Rypel

Note: this is a re-post from August 2020.

Throughout my career I’ve spent some time studying the fascinating ecology and conservation issues of freshwater mussels (Fig. 1). For me, learning about mussels has fortified a recurring theme of the natural world – that everything is connected and that small changes in one part of a system can yield unexpected changes elsewhere, often many years later. More importantly, freshwater mussels are essentially threatened everywhere. And because we don’t often hear about them, it is hard to save them, because public will is so critical to generating change.

What is a freshwater mussel?

Some basics. Freshwater mussels, along with freshwater snails (another taxonomic group in major trouble) are varieties of molluscs. Most molluscs on Earth live in saltwater, however several families live in freshwater. Freshwater bivalves (order Unionidae) are one of the major clades of these species. This group’s fossil record extends back to the upper Devonian (416-365 million years ago or “mya”). In North America, first fossils appear to have emerged in the Triassic (250-200 mya). So these animals are not quite as old as fish (>400 mya), but certainly old.

Reproduction

Now here’s where it gets really interesting. [Caution…your mind may explode like mine once did!] With some exceptions, most juvenile freshwater mussels are obligate parasites on fishes. This means they cannot complete their life cycle without using fish as hosts. Baby mussels (glochidia) attach to the gills and fins of fish hosts. In some cases the host fish for a mussel is highly specific, e.g., mussel species x must use fish species y. In other cases, mussels are generalists which can use a range of fish species. By studying these obscure dynamics, a web of interactions begins to emerge between mussels and fishes that is ecologically vital.

Perhaps more interesting is how mussels transmit the glochidia to the fish. Because mussels are mostly sedentary, they have evolved lures to attract their fish hosts. The lures are often part of the mussel’s mantle and are loaded with magazines of baby glochidia. Once a fish bites the lure, the glochidia are ejected onto the host fish. Some lures are basic while others can be exceedingly ornate. Lures resemble all types of potential fish prey including bluegill, minnows, darters, sculpins, salmon, and aquatic insect larvae. Elaborate superconglutinate lures even emerge from the mussel as though on a mucus string, with the same goal of attracting a host fish to bite. In these cases, the mussel literally appears as though it is “going fishing”. Check out the following video of a mantle lure in action!

Ecosystem services

Freshwater mussels perform a variety of important functions in aquatic ecosystems, well-reviewed by Vaughn 2018. For example, mussels regulate water quality through biofiltration. They also process and store excess nutrients in ecosystems. An interesting thought experiment is to consider the role oysters (a similar group of animals) have in regulating water quality in Chesapeake Bay. According to the Chesapeake Bay Foundation, oysters once filtered the entire ecosystem (estimated ~19 trillion gallons of water) every week. However, with declining oyster abundance, it now takes oysters > 1 year to filter the Bay just once. To what extent do mussels similarly promote water quality in freshwater ecosystems? And what might their losses mean?

Freshwater mussels were important to Native American cultures. Mussels seemed to be highly central to mound-building societies of the midwestern and southern USA. However, shell mounds (or “middens”) were also common on the West Coast, notably in San Francisco – driving down “Shellmound Street” might be one major clue! Shell mounds were created by tribes as sacred ceremonial places and as burial sites. Most mussel shells in archaeological sites are “unworked”, meaning mussels were used as food. But, their low caloric value means mussels were probably supplementary food and used more frequently during times of food scarcity. In the Columbia River, mussels were harvested by tribes well into the 20th century and prepared for consumption mainly by steaming. Shells were also for pottery-making, utensils, and jewelry. Because shells are relatively well-preserved, middens can also reveal how mussels and aquatic ecosystems responded to human activities. For example, one genus of mussels in southeastern USA (Epioblasma spp.) showed relatively constant abundance at archaeological sites over a 5000 year period; but around 1000 years ago, mussel abundances declined rapidly with the rise of maize and bean cultivation. In the last 100 years, Epioblasma spp. have declined substantially further due to ecosystem alterations such that species are now listed under the US Endangered Species Act.

In addition to filtering, mussels support fish populations by engineering benthic habitats to be more suitable for fishes like darters, minnows, dace and sculpins that require firm and complex structures. Mussels also are useful in monitoring water quality because they are among the first species that disappear as water quality declines (Augspurger et al. 2003). Because mussel organs are exposed constantly to ambient environmental conditions, they are figurative “canaries in the coal mine” for aquatic environmental change. Mussels support aquatic and terrestrial foodwebs by providing energy to diverse species including fishes, otters, muskrats, raccoons, waterfowl, crayfish, turtles, frogs, salamanders and people. 

The market for mother-of-pearl buttons was a leading factor in initial mussels declines and extirpations in North America (Fig. 2). Another factor was pearl hunting, even though pearls from native freshwater mussels are mostly rare and imperfect. Gossip in Rome during the 1st century speculated that one reason Julius Caesar invaded England was to search for pearls in mussels from local streams, and that he returned with a piece of armor decorated in pearls (Haag 2012). In the USA, there were two “pearl rushes” – one in the mid-late 19th century, and a second round in the 1950s related to a new process of culturing pearls in Japan. These rushes had horrible consequences for mussel populations, particularly long-lived species.

Diversity, California & Decline

While Class Bivalvia contains ~20,000 living species worldwide, only ~1000 species are known to live in freshwater, and 850 of these species are in the Order Unionida. Estimates suggest there are ~300 species of freshwater mussels in North America. Like many North American taxa, a diversity hotspot for freshwater mussels is the southeastern USA where a lack of glaciation and a correspondingly old evolutionary landscape promote local evolution and high diversity. 

While freshwater mussel diversity is low in California (3-4 native species), these organisms can be locally abundant. Mussels therefore play underappreciated roles in our ecosystems, and in many locations are apparently in significant decline. Native California mussels include the Western Pearlshell (Margaritifera falcata), the Western Ridged Mussel (Gonidea angulata), the California Floater (Anodonta californiensis), and possibly the Oregon Floater (Anodonta oregonensis) (Fig. 3). The Western Pearlshell is notable for its long life, often exceeding 60 years. These mussels are so old, they can be used to reconstruct past climates, parallel to the ways tree-rings can be used (Black et al. 2015). Ecologically, their host fishes are likely trouts and/or salmon. As recently as 1942, Western Pearlshells were a dominant species in the Truckee River with at least 20,000 individuals surveyed; however only 120 mussels were found recently (Murphy 1942, Howard 2008). Today, the only known populations of Western Pearlshell Mussel in the Tahoe basin are in the Upper Truckee River, Trout Creek, and the Truckee River. Western Ridged Mussels are also native to California and are the dominant native mussel species in the Rocky Mountains and to the West. This species is also in decline across the western USA and sculpins might be a host fish. Finally, the California Floater is common in large rivers and pools of streams. Floaters of the genus Anodonta are frequent specialists on floodplain lakes and pools. We frequently encounter floater shells on banks in the Yolo Bypass and in farm ponds. Speckled dace and sculpin are known host fishes for the California floater (Main et al. 2016), but it is likely that this mussel is a generalist species.

There is great variation in the life-histories of different freshwater mussel species (Haag and Rypel 2011). Some species are exceptionally long-lived. The freshwater pearl mussel (Margaritifera margaritifera) regularly lives >100 years of age. One specimen from Finland had an estimated age of 162 years – this individual was born 37 years before California became a US State. If you go marine, some bivalves live even longer; a specimen of Arctica islandica from the coast of Iceland in 2006 lived 507 years!

In a classic examination of the conservation status of freshwater mussels in North America, Williams et al. 1993 noted 72% (213 of 297) of species are endangered, threatened or of special concern 21, and 7.1% of species had probably gone extinct. These are the highest rates of imperilment of any other known group of freshwater taxa. According to the Xerces Society, crayfishes, freshwater fishes, amphibians, dragonflies have imperilment rates of 51%, 37%, 36%, and 18% respectively. As another contrast, imperilment rates for flowering plants, mammals and birds are 33%, 16% and 14%, respectively. Mussels are…uniquely endangered.

California problem – California solution?

While mussels are declining globally, the mussel problem is also a California problem. Studies in the Truckee River showed a 99.4% decline in abundances of a long-lived mussel (Murphy 1942, Howard 2008). Howard et al. 2015 re-surveyed 450 historical records from 116 sites throughout California,  showing freshwater mussels re-occured at only 47% of these sites. Mussel losses were especially acute in southern California, with 13 of 14 streams having lost their mussels. Yet while loss of native mussels is occurring in many ecosystems, the addition of invasive mussels is simultaneously wreaking havoc on others. Invasion of the Sacramento-San Joaquin Delta by Asiatic clams (Corbicula spp.) is a commonly-studied mechanism for changing zooplankton and fish communities. The ecology of invasive mussels in California and the West will be a good topic for a future blog post.

Like many natural resource issues, we have inherited and contributed to the freshwater mussel problem, which will require hard and focused work to correct. The mechanisms for losses remain elusive. Commonly cited factors include loss of fish hosts, poaching and overharvest, pollution, invasive species and climate change (Haag 2012). Substantially more basic information is needed on the status and distribution of native mussels in California. Where are our mussels? What are their host fishes? Is there cryptic diversity? What are trends in diversity and abundance? What are the impacts of water and riparian land management and invasive species? How have native fish declines impacted mussels? 

A “mussel-building program” in California could take many shapes. I suggest a draft 4-point plan that might be useful to consider:

1. A statewide inventory: A statewide inventory of freshwater mussel abundance and diversity is long overdue. Currently no native California mussel is listed at the state or federal level as threatened or endangered, even though some of these species are already identified as in jeopardy or extirpated from other Western states. A statewide inventory could occur with a series of targeted grants or directed actions, or using a special research unit within one of the relevant state agencies. These data are needed to assess if some of our native mussel species, populations, and ecosystems should be protected. New environmental DNA (eDNA) tools hold promise for such an endeavor.

2. Sentinel sites: Long-term data shows trends over time. Without these data, scientists have little hope of adequately investigating hypotheses of species declines or developing models for testing potential conservation solutions. A series of sentinel sites (e.g., 10-15) strategically located around the state could monitor freshwater mussel populations over time. Sentinel sites/data would give an immediate benefit to scientists struggling to understand mussel dynamics across our large and complex landscape.

3. Conservation hatcheries?: Some eastern US states are developing mussel hatcheries to recover rare species after habitat issues are corrected. In many cases, older decommissioned hatcheries and aquaculture facilities are being repurposed for conservation aquaculture. Are mussel hatcheries a possibility in California? If mussel species were to become listed at some point, hatcheries may be needed to achieve goals. While hatcheries are never intended to replace natural reproduction, they can be an emergency back-up plan for populations and species.

4. Look for win-wins: Strategic prioritization is needed to locate restoration opportunities that would benefit fishes and freshwater mussels. Because mussels rely on suitable fish hosts, loss of fish hosts from habitats will eventually crash mussel populations. Yet because mussels live long lives, mature adults can subsist for decades without host fishes following landscape fragmentation by dams and other structures. This pattern is often referenced in the ecological literature as a “extinction debt” (Timan et al. 1994). Restarting natural mussel recruitment necessitates recolonization by native fishes. Ecosystems with a high potential for boosting native mussel populations could then be prioritized for restoration. Rehabilitation might include large dam removals, but also could be as simple as adjusting the myriad in-stream culverts that also block fish passage to headwater streams, some of which contain mussels. Strategic prioritization along these lines also ties in with existing mechanisms for restoration funding and climate resilience (e.g., through Propositions 1 and 68). In short, the future of mussels is closely tied to the future of fishes – we need to save them together.

Further Reading

America’s freshwater mussels are going extinct — Here’s why that sucks https://blogs.scientificamerican.com/extinction-countdown/americas-freshwater-mussels-are-going-extinct-heres-why-that-sucks/

A freshwater mussel apocalypse is underway – and no one knows why https://www.nationalgeographic.com/animals/2019/12/freshwater-mussels-die-off-united-states/

California floater mussel take fish for an epic joyride https://video.kqed.org/video/california-floater-mussels-take-fish-for-an-epic-joyride-iqmps1/

Nature’s britta filter is dying and nobody knows why https://www.npr.org/2019/12/06/784422726/natures-brita-filter-is-dying-and-nobody-knows-why

Oyster fact sheet https://www.cbf.org/about-the-bay/more-than-just-the-bay/chesapeake-wildlife/eastern-oysters/oyster-fact-sheet.html

The tiny clams that ate the Bay-Delta https://www.kcet.org/redefine/the-tiny-clams-that-ate-the-bay-delta

There Were Once More Than 425 Shellmounds in the Bay Area. Where Did They Go? https://www.kqed.org/news/11704679/there-were-once-more-than-425-shellmounds-in-the-bay-area-where-did-they-go

Scientists find 507-year-old clam. Are older ones out there? https://www.latimes.com/science/sciencenow/la-xpm-2013-nov-18-la-sci-sn-ming-507-year-old-clam-20131118-story.html

Black, B.A. J.B. Dunham, B.W. Blundon, M.F. Raggon, and D. Zima. Spatial variability in growth-increment chronologies of long-lived freshwater mussels: Implications for climate impacts and reconstructions. Ecoscience 17: 240-250.

Haag, W. R., 2012. North American freshwater mussels: Natural history, ecology, and conservation. Cambridge University Press, Cambridge UK.

Helama, A., and I. Valovirta. The oldest recorded animal in Finland: ontogenetic age and growth in Margaritifera margaritifera (L. 1758) based on internal shell increments. Memoranda Soc. Fauna Fennica 84: 20-30.

Augspurger, T., A.E. Keller, M.C. Black, W.G. Cope, and F.J. Dwyer. Water quality guidance for protection of freshwater mussels (Unionidae) from ammonia exposure. Environmental Toxicology and Chemistry 22: 2569-2575.

Howard, J. K. 2008. Strategic inventory of freshwater mussels in the northern Sierra Nevada Province. Final Report by Western Mollusk Sciences, San Francisco, CA to US Forest Service PSW Regional Office, Vallejo, CA. 65 pp.

Howard, J.K., J.L. Furnish, J.B. Box, and S. Jepsen. 2015. The decline of native freshwater mussels (Bivalvia: Unionidae) in California as determined from historical and current surveys. California FIsh and Game 101: 8-23.

Haag, W.R., and A.L. Rypel. Growth and longevity in freshwater mussels: evolutionary and conservation implications. Biological Reviews 86: 225-247.

Maine, A., C. Arango, and C. O’Brien. Host Fish Associations of the California Floater (Anodonta californiensis) in the Yakima River Basin, Washington. Northwest Science 90: 290-300.

Murphy, G. 1942. Relationship of the fresh-water mussel to trout in the Truckee River. California Fish and Game 28: 89-102.

Timan, D., R.M. May, C.L. Lehman, and M.A. Nowak. Habitat destruction and the extinction debt. Nature 371: 65-66.

Vaughn, C.C. 2018. Ecosystem services provided by freshwater mussels. Hydrobiologia 810: 15-27. 

Williams, J.D. M.L. Warren, K.S. Cummings, J.L. Harris, and R.J. Neves. Conservation Status of Freshwater Mussels of the United States and Canada. Fisheries 18: 6-22.