California’s freshwater streams are home to a surprising variety of shellfish. Clusters of bivalves nestle themselves among the small pebbles and overhangs of riverbeds, sometimes almost indiscernible from their surroundings. Though these organisms may not be in the spotlight, each one of the species plays an important role in riparian ecosystems. None are more important than the all-star of filter-feeding: the freshwater mussel. But as native freshwater mussel numbers dwindle, this essential organism could be in big trouble.
Mussels range far beyond the salty shores of the coast—in fact, a handful of species cling to freshwater banks and streams, including Davis’ very own Putah Creek. These seemingly innocuous molluscs trade the glamor of a turbulent tide pool for the secluded, murky bottom of ponds and tributaries. Most people wouldn’t pay much attention to a marine mussel, but their freshwater cousins can be even more unassuming. The freshwater mussels’ murky anonymity is a major issue—despite their critical significance to freshwater ecosystems, research on these species has been severely limited.
In California there are four species of native freshwater mussel: the Western Ridged Mussel (Gonidea angulata), the California Floater (Anodonta californiensis), Western Pearlshell (Margaritifera falcata), and the potentially the Oregon Floater (Anodonta oregonensis) that are in need of more research to support conservation efforts. But what’s so special about these often overlooked organisms, is it so important to lend them a hand—or, in this case, a foot?
Mussels are biofilters—organisms who remove suspended debris from water—and play a key role in filtering and cycling nutrients in freshwater streams. This cleanup work keeps water healthy and clear, something that is essential to stream health and benefits all the organisms in the ecosystem. It’s almost like magic: put some mussels in a tank of murky water, and come back later to see it significantly clearer!
Alongside filtration, mussels are very effective at denitrification, a very important process that converts organic nitrogen to molecular nitrogen which can be moved back into the atmosphere. By moving the nitrogen-filled organic material through sediment during the filtration process—a process that, yes, involves mussel poop—nitrogen is carried to the riverbed where it gets digested by microbes [1] . The buildup of excess nitrogen contributes to algal blooms, which, if uncontrolled, can deplete oxygen in the waterway and result in the death of animals and plants alike [2]. Once thought to feed primarily on phytoplankton, it was recently discovered that mussels are omnivorous, consuming suspended algae as well [3]. Through their incredible filtration system, they are able to tackle two major problems in one swoop.
Mussels are also ecosystem engineers. By anchoring themselves to a stream or lake bed, mussels create stable habitat for organisms to use to carry out their life history, such as nurseries for larval critters, and substrate for sessile organisms [4].
Mussels are extremely sensitive to environmental change—as an indicator species, they have been described as “canaries in a coal mine,” usually being the first to disappear as water quality declines [5]. If a freshwater mussel population is observed to be decreasing, it is a pretty good sign that other animals might follow suit.
Native mussels also use some species of fish to aid in their reproduction. Most juvenile freshwater mussels are obligate parasites, meaning that they need to use fish as hosts in order to complete their life cycle. Glochidia, or the baby mussels that utilize this technique, have evolved lures on their mantle that attract host fish.When a fish bites or agitates the lure, the glochidia are launched and stick themselves onto the fish’s gills and fins. Some species of mussel are highly specific and have coevolved with a particular species of fish, while others are generalists, meaning that they will use a range of species as hosts. By hitching a free ride on the fish, glochidia use them as a means of dispersal and transportation [6].
The loss of mussel biodiversity is a global problem, but California is being hit especially hard. In Southern California alone, 13 out of 14 streams have lost their native freshwater mussel populations [5]. As waterways become more and more contaminated from pollution, the task of filtration becomes overwhelming to the sensitive mussels. Toxin exposure coupled with the loss of glochidia-dispersing fish populations makes it abundantly obvious that freshwater mussel conservation needs to be prioritized. Their loss is devastating not only to the intrinsic value of biodiversity, but also because of the effects it has on other organisms in riparian ecosystems. Mussels play an extremely pivotal ecological role—whether as a source of food, habitat, or overall stream health—their disappearance might not be initially seen, but will be felt everywhere.
However, hope is not lost. It is possible for freshwater mussels to recover: the first step is raising awareness and allocating more resources towards their conservation. The most critical aspect to focus on is restoring and maintaining freshwater habitats—factors like dam construction have altered stream flows and disrupted ecosystems, posing a major threat to mussel survival. By limiting habitat degradation from human interference and pollution, habitat quality can return to levels that allow native freshwater mussels to thrive. Many researchers are even considering the use of hatcheries to reintroduce large numbers of mussels into areas that need them most. The most effective conservation strategies are those that are built on a solid foundation of accurate information; by advocating for research funding and habitat restoration projects, even one person can play a role in helping conserve these essential freshwater filter-feeders [7,8].
To learn more about mussels, check out the California WaterBlog.
Written by: Lila Magbilang is a second year Biological Sciences major and Professional Writing minor. She is one of the undergraduate researchers in the Genomic Variation Laboratory, working under Alana Luzzio on her project involving the genetic analysis of Lahontan Cutthroat Trout to aid in conservation efforts. She is interested in evolution, conservation, genetics, and science communication.
Alana Luzzio is a third year PhD student in the Graduate Group in Ecology and is a member of the Genomic Variation Lab studying conservation genetics of the Lahontan cutthroat trout. She is passionate about conservation of threatened and endangered species, especially species that are often overlooked– like native freshwater mussels! Before graduate school, Alana studied the population genetics of intertidal clams and snails along the coast of the Gulf of Maine.
References:
[3]:https://link.springer.com/article/10.1007/s10750-017-3139-x
[7]:https://link.springer.com/article/10.1007/s10750-013-1524-7
[8]:https://www.xerces.org/endangered-species/freshwater-mussels/conservation-efforts
[Edited by Jacob Johnson and Alice Michel]