Ferreira-Rodríguez, N. et al. (2019). Research priorities for freshwater mussel conservation assessment. In Biological Conservation. https://doi.org/10.1016/j.biocon.2019.01.002
Family Sphaeriidae, Sphaerium corneum. Source: Wikipedia.com
The secret life of shellfish
The threehorned wartyback, purple bankclimber, fawnsfoot, and finelined pocketbook: the common names of freshwater mussels sound like creatures out of a fantasy novel.
Unlike their more famous marine counterparts, such as oysters and clams, freshwater mussels are typically not edible. They are buried in soft sediments under flowing water, so even in areas where they are abundant, it is not too hard to miss them. One scientist studying these elusive creatures described them as “the opposite of charismatic”.
Despite their tendency to fade into the background, freshwater mussels are integral to the functioning of healthy freshwater ecosystems. They filter dirt and contaminants from flowing water, keeping it clean and clear. Their shells provide habitat for invertebrates and algae, even after they have died and their soft bodies have decomposed away. Humans have used them to harvest pearls and made buttons from their shiny shells.
Aside from their ecological and economic value, freshwater mussels are simply…cool! Many have built-in lures to trick and attract fish. Why would they do such a thing? When the fish gets close enough, mussels will release their larvae, called glochidia, into the fish’s mouth (you can watch a video of this process here). The glochidia latch onto the gills and get a free ride until they are released as juveniles. Glochidia can’t just hitch a ride on any old fish; rather, each mussel species has a specific fish species with which it may associate, creating a complex web of mussel-fish relationships.
Mussels in trouble
Unfortunately, these filter-feeding, hitch-hiking, habitat-creating animals are in deep trouble. Globally, 6% have recently become extinct. In North America, 30 species have gone extinct in the last century, and 65% of remaining species are threatened, endangered, or vulnerable.
We don’t know why for certain, but there are several potential contributing factors. Dams create deep, cold, still-water, and oxygen-poor environments, while mussels prefer warmer, shallower, and faster-flowing water. Pollution is also a potential contributor. Because mussels filter feed from flowing water, they consume every contaminant that passes by.
With so many potential contributors to the decline of mussels, and a vast array of mussel species intwined in tight and complex webs with fish and other aquatic species, the research community is understandably overwhelmed. Together, an international team of over 40 scientists worked together to compile the available knowledge on mussels and their conservation, identify priorities for freshwater mussel research, in order to guide the scientific community. The team also provided advise to practitioners on how to utilize this research to create conservation and restoration plans.
The team identified research areas under two main categories: 1) intrinsic (factors related to the freshwater mussel species themselves) and 2) extrinsic (factors related to the environment surrounding and interacting with the mussels). Extrinsic factors may lead individuals or populations to adapt through changes in their intrinsic characteristics.
In order to conserve species, we must know their names and how many there are! Some freshwater mussels have been over-described, meaning that there are too many names for the same species. Others have been under-described: molecular analyses sometimes show that species that look the same on the outside may differ on a genetic level.
We must also study populations, comparing current ones with those that existed in the past. Population information includes their size, the number of male versus female and young versus old individuals, and whether populations are connected via reproduction, immigration, and emigration.
Extrinsic factors affecting mussels include fish hosts, habitats, land and water use, climate change, and non-native species.
Because mussels are dependent on fish hosts to keep their larvae alive and distribute them to various locations, we need to study fish to understand the health and habits of mussels. How many fish typically become infected by mussel larvae, how far do they travel, and which fish species are hosts for which mussels?
Human activities may influence the habitats where mussels are able to settle. Deforestation, agriculture, and mining can lead to erosion and pollution, and climate change can lead to droughts and floods. Erosion can clog gills and fill small spaces at the stream bottom that could have otherwise been occupied by juvenile mussels. Toxicity tests have been performed to attempt a better understanding of which pollutants can be survived at what amounts, but there is also a need to understand how pollution affects bodily processes and reproduction.
Invasive shellfish, such as zebra mussels, may compete with native mussels for food and habitat, and may adapt more effectively to a changing climate.
What comes first, the habitat or the species?
Some scientists are working to raise mussel juveniles in labs then reintroduce them into the wild. This conservation strategy is relatively unique to mussels. Because mussels are ecosystem engineers (animals which create, change, or destroy habitats), reintroducing them can help restore the habitat because they will begin to filter the water and provide habitat with their shells.
Other proposed strategies target the habitat before the species. Creating strips of vegetation (called riparian buffers) along streams or rivers may be beneficial, as some studies have shown that more shade cover from trees is associated with healthier mussel populations.
Finally, dam removal may be beneficial in some cases. However, this may not be feasible due to human needs. Dam removal may also release pollutants and sediments that could harm mussels, so potential effects would need to be researched on a case-to-case basis.
Where do we go from here? Conservation and recovery plans
Scientific inquiry often leads not to answers alone, but also to more questions! There is clearly a lot of information, from current and past research, and which we expect to glean from future research, that could be used to protect mussels. The first step to solving complex problems is identifying the questions that need to be answered. Since this review was published, just last year, several published studies have cited it, suggesting that it is indeed influencing ongoing freshwater mussel research.
Practitioners and scientists will need to work together to prioritize species and populations for protection. The challenge will be to engage stakeholders, policymakers, authorities, and the public by convincing them that mussels are equally as worthy of protecting as pandas and whales.