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A veterinarian pens a lyrical love letter to the imperiled freshwater mussel

Immersion: The Science and Mystery of Freshwater Mussels

Abbie Gascho Landis
Island Press
261 pp.
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A veterinarian married to a biologist, Abbie Gascho Landis became fascinated by the seemingly uncharismatic bivalves known as freshwater mussels after tagging along on a research trip in 2009. “I became a freshwater mussel groupie,” she writes in her new book, Immersion. “I fawned over their photographs. … I pored over their bios. Posters of mussels hung in our bedroom.”

With names like “oval pigtoe,” “little spectaclecase,” “fatmucket,” “elephant ear,” and “pistolgrip,” they certainly exceed the naming creativity of marine mussels, which bear such perfunctory titles as “blue” and “ribbed.” There are other differences, too; whereas marine mussels attach to hard substrates with tough threads, freshwater mussels live partially buried in creeks and streambeds and are difficult to find without a trained eye. One must look carefully for the two siphon openings in partially opened shells.

Freshwater mussels are important because their filter feeding can purify the water, benefiting other organisms in the environment. By filtering suspended material, mussels transfer energy and nutrients from the water to the sediment, depositing organic matter. Unfortunately, problems with both water quality and water quantity have reduced their populations to such a degree that many mussel species are endangered.

Joel Sartore, National Geographic Photo Ark

A female Higgins eye pearly mussel displays a fishlike lure to attract host fish that will incubate its eggs.

After covering some basic anatomy and physiology, Landis launches into a detailed description of freshwater mussel reproduction. Unlike marine mussels, which release eggs and sperm into the water column and have planktonic larvae, the sperm fertilize the eggs inside the female freshwater mussel, and the larvae, called glochidia, are incubated in fish gills. This means that the right kind of fish, sometimes a specific species, must be nearby in order for the mussel to reproduce. To improve their odds of success, female freshwater mussels have evolved lures that resemble either a small fish or worm to attract the right type of host. But if the fish disappear, the mussels will disappear as well.

Fortunately for these mussels, they do not taste good to people, but in years past, they were collected for pearls and harvested by the button industry and shell collectors. Any effect collectors may have had on their population does not compare to those wrought by more recent habitat changes, however.

Like their marine relatives, freshwater mussels are filter feeders, eating plankton in the water. As such, they depend on adequate water flow to provide enough food for growth and survival. Droughts, which are becoming more common, can be deadly for a mussel; however, some species can “clam up” and survive several weeks in dried mud.

Excessive stream flow can also be deadly, dislodging mussels and sweeping them downstream into unsuitable habitat. But the main reason so many species of freshwater mussels have become endangered is the presence of dams, which turn rivers into lakes with silty bottoms and no flow.
Connecting rivers together can also change mussels’ habitats, as was seen in the wake of the construction of the Tennessee-Tombigbee Waterway in 1984. Their depletion following such projects has been clearly documented. Among the myriad ways that human-made waterways can be detrimental to the freshwater mussel, the infamously invasive zebra mussel represents a major threat. Zebra mussels, Landis informs us, attach to shells of native mussels and smother them.

In chapter 8, Landis discusses the effects of sedimentation and chemical pollutants on mussels, describing the lethal effects that occur at high concentrations and the sublethal effects, such as altered physiology and disrupted reproduction, that can occur at lower levels of contamination. Here, she documents how scientists and community groups can help local waterways recover. Stream restoration projects, environmental protection, and culturing and restocking the mussels have helped many areas recover.

Disappointingly, Landis repeatedly misuses the term “toxin” in this section. Toxins are poisons made by living things such as rattlesnakes, jellyfish, and bees. It is a shame that no editor or reviewer caught this.

In the book’s closing chapters, Landis returns to water quantity, describing how streams are increasingly being deprived of water because of excess human usage. However, she also shows how habitat conservation plans—agreements about how much water industry and cities can take and how much needs to be left for the native ecosystem—can help mussels survive. The final chapter ends on a happy note, focusing on successes with mussel propagation and restoration.

I did wish that there were more line drawings (especially in the anatomy chapter), as well as photographs to allow readers to better appreciate the beauty and diversity of these interesting, imperiled animals. However, Landis’s writing is engaging and personal, demonstrating her bond with the natural world.

About the author

The reviewer is professor emerita in the Department of Biological Sciences at Rutgers University, Newark, NJ 07102, USA.