The inspiration for this MotM post is the recent article by Hopper et al. (2025) that reported on the content of the Freshwater Mussel Host Database served by the Illinois Natural History Survey. That paper described very well the scope and utility of those data. As a person reading the tiny type through my bifocals, I found myself waxing nostalgic about the path to our collective understanding of and concern for freshwater mussel parasitism. I want to focus on three key works in my own intellectual development on the subject: Fuller (1974), Hoggarth (1992), and Watters (1994). But, before we get to those, let’s digress for a paragraph or three to talk about freshwater mussel biology and — perhaps surprisingly — buttons.
I have previously summarized the life histories of freshwater mussels in various contexts, and I will gratuitously direct you, gentle-reader, to those for more details (Graf, 1997, 2013; Graf & Cummings, 2006; Cummings & Graf, 2010). The gist can also be found in the most beautiful life cycle diagram on the subject ever published (drawn by Kyle Luckenbill for Cummings & Graf, 2010).
Lifecycle of a freshwater mussel (Cummings and Graf, 2009).
Freshwater mussels usually have separate sexes. Males release their sperm to the surrounding water, those gametes are entrained in the respiratory current created by the females’ gills, their eggs are fertilized, and the developing embryos are protected within brooding spaces in the females’ gills. Such parental care is typical of freshwater bivalves generally. Those embryos develop into larvae that are parasitic on freshwater fishes. The tiny larvae encyst in the gill or fin tissue of their hosts, and it is within that “cocoon” that they undergo metamorphosis into a juvenile with the organs and body plan of an adult freshwater mussel. The juveniles break out of their cysts, fall to the bottom of the river or lake, and develop into sedentary adults. To keep this paragraph even as short as it is (or not), I have glossed over lots of interesting details and exceptions, but you can check out the articles I cited above for more detail.
Obviously, even that little taste of freshwater mussel reproductive biology is mind-blowingly interesting, but here in America, “interesting” has never paid the bills. This knowledge — in large part worked out by the first quarter of the twentieth century — was also profitable. Freshwater mussels were big business because buttons were cut from the pearly shells of freshwater mussels. Until the functions of little white buttons were taken over by plastic, freshwater mussels were harvested for their nacre (i.e., “mother of pearl”) to keep clothes closed. The propagation of freshwater mussels, with their complex life histories, was the focus of much public and private enterprise (Pritchard, 2005), and our familiarity of which fishes hosted which mussel larvae was based on the need to cultivate buttons.
Here is some more information about mussels and buttons: https://www.molluskconservation.org/MUSSELS/Buttons.html.
Between the collapse of the button industry and President Nixon’s conservation push — which included the Environmental Protection Agency, the Clean Water Act, and the Endangered Species Act — interest in freshwater mussels waned away from academic taxonomists and ecologists. But, the Endangered Species Act (1973) happened, by 1976 there were 23 freshwater mussels on the list of endangered species, and mussels regained an applied interest by their imperiled conservation status (Neves, 1999). There was a renaissance in freshwater mussel propagation, but this time it was for their own sake and that of their ecosystems.
When I first began studying freshwater mussel biology in the early 1990s, the source for data on the host fishes of North American mussels was the synthesis by Samual Liberty Harvey Fuller (1974). Fuller exhaustively reviewed the primary literature and produced a table listing the freshwater mussels for which hosts had been reported, the relevant fish species, and the source citations. According to Watters (1994), “This review article is the single most comprehensive source of literature on the natural history of North American unionoideans to date. … The list of unionoideans and hosts is indispensable.”
Fuller’s (1974) tallies demonstrated the wide variation in freshwater mussel host-specificity. Some mussels, like Potamilus alatus, had only a single host fish listed, Aplodinotus grunniens (freshwater drum), while other species had more. For example, for Mussel of the Month Pustulosa nodulata, Fuller (1974) listed six species in two families (Ictaluridae and Centrarchidae). For others, many more hosts were listed: Amblema plicata (15 species, 7 families), Pyganodon grandis (19, 9). Table 1 in Fuller (1974) was the freshwater mussel host database of its day.
The next major upgrade to the synthesis of mussel-fish relationships came from Michael Hoggarth (1992), published in the Malacology Data Net — once hard-to-get, but now available online from the Freshwater Mollusk Conservation Society. Hoggarth (1992) updated Fuller’s (1974) Table 1 with new observations but also included the nature of the evidence. It turns out Fuller had ignored a key fact of freshwater mussel life history: mussel larvae infect indiscriminately. If a mussel larva encysts on a suitable host, then transformation to a juvenile can take place, as described above. But, if the fish mounts a successful immune response, the larva will be rejected before metamorphosis. It is necessary for a mussel larva to encyst on a fish to transform into a juvenile, but it isn’t sufficient. It needs to be the right fish. Infection without transformation is merely evidence that larva and fish were in the water together. Hoggarth’s (1992) review revealed that transformation had not actually been observed for many of the fishes then reported as hosts.
Looking at the information available for Pustulosa nodulata is illustrative of the whiplash I felt when I first got a hold of Hoggarth (1992). I made the table below by querying the Freshwater Mussel Host Database and supplementing that with the data reported in Fuller (1974), Hoggarth (1992), and Watters (1994). Fuller (1974) listed six hosts by name, and he cited the four original publications from the button days. Literature references are arrayed in top row of the table, fish data are in the left columns. Hoggarth (1992) had the same six species, but he also showed that no one had reported transformation. The inferences of mussel-host relationships were based on observed infections, either in nature or the lab. In 1974, Pustulosa nodulata had 6 “known” host fishes, but in 1992, it had none. Whisky Tango Foxtrot.
Evidence of mussel-host relationships for Pustulosa nodulata.
As reported in the Freshwater Mussel Host Database, Sietman et al. (2010) eventually demonstrated, by observing transformation in the lab, that Pustulosa nodulata has at least five hosts (all in the same family), and only two of those species had been previously implicated. We get closer to reality with time, but, in the case of Pustulosa nodulata, it took a century.
Contemporaneous and sympatric with Hoggarth (1992), G. Thomas Watters (1994) compiled a global database and annotated bibliography of freshwater mussel reproductive biology that spanned works from 1695 to 1993. For any question dealing with mussel reproduction, including host fishes, Watters (1994) was the place to start. Watters built his bibliography using database software, and that work lives on today as the Freshwater Mussel Host Database at the Illinois Natural History Survey.
Even though malacology’s best and brightest have been working and thinking on the problem of freshwater mussel hosts for many decades, our data and perspectives are still growing and changing. It was a sea-change in our collective understanding to make the distinction between transformation/metamorphosis versus infection/encystment. Nowadays, the community is recognizing the distinction between generally compatible hosts versus those that are actually used in nature — i.e., physiological hosts vs. ecological hosts (Robinson et al., 2025; Hopper et al., 2025). Whereas any of the former might be suitable for breeding-up lots of captive juveniles to reintroduce into a restored habitat, it may be only the latter that can sustain the population. There is still lots of work to do and room for new ideas in the study of freshwater mussels!
Visit https://musselproject.substack.com/ to subscribe and get the MUSSEL Project Mussel of the Month in your inbox! |
