The Soft-Shell Clam Mya arenaria: Biology, Fisheries, and Mariculture

Chapter 7: Population Dynamics of the Soft-Shell Clam Mya arenaria

John A. Commito


It is easy to understand in a general way the population dynamics of benthic marine invertebrates, including the soft-shell clam Mya arenaria. To paraphrase Gunnar Thorson (1950), all it takes for a population to maintain itself is for every adult female–male pair to produce two successful larvae over their entire lifetime. But the devil is in the details. The purpose of this chapter is to present some of those details.

Thorson (1950) emphasized the importance of larval supply and settlement in regulating bottom-dwelling invertebrate populations. Yet mid-20th century benthic ecologists focused more on the roles of postsettlement processes involving juveniles and adults in population regulation (Morgan 2001; Underwood and Keough 2001). What controls the population dynamics of Mya arenaria? As I discuss in this chapter, we cannot really say for sure. Or, more accurately, it depends on the place, weather, and year. Increasingly, it depends on the climate. And it certainly depends on what researchers are looking for and how they attempt to measure it. Related questions arise, too. Why does the soft-shell clam live so long, reproduce so many times, and make so many offspring? Why do population sizes vary so widely both spatially and temporally? Why is recruitment successful in some years, but a total bust in other years? How good are population dynamics models at capturing these phenomena? Answers are difficult to find, especially because soft-shell clam populations experience a wide array of environmental conditions in both intertidal and shallow water habitats, with a geographic distribution across multiple continents (Strasser 1998), from warm mid-latitudes (e.g., Hakata Bay, Japan: Goshima 1982; Chesapeake Bay, USA: Kennedy and Mihursky 1971; Pamlico Sound, North Carolina, USA: Skilleter 1994) to polar regions (e.g., Alaska, USA: Powers et al. 2006; White Sea, Russia: Gerasimova et al. 2015). Note, however, that some controversy exists about confusing M. arenaria with other Mya species in Arctic and northwest Pacific regions (Strasser 1998; Zhang et al. 2018).

Detailed reviews of the biology of Mya arenaria, including topics directly related to population dynamics, are presented in other chapters of this book. In this chapter, I review the important processes that affect the abundances of larvae, recruits, juveniles, and adults. I then discuss researchers’ attempts to integrate demographic patterns and build population models. The insights gained from their work may help deepen our understanding of soft-shell clam population dynamics, with implications for management in a rapidly changing world.