Growth and longevity in freshwater mussels: evolutionary and conservation implications

Abstract

The amount of energy allocated to growth versus other functions is a fundamental feature of an organism's life history. Constraints on energy availability result in characteristic trade-offs among life-history traits and reflect strategies by which organisms adapt to their environments. Freshwater mussels are a diverse and imperiled component of aquatic ecosystems but little is known about their growth and longevity. Generalized depictions of freshwater mussels as 'long-lived and slow-growing' may give an unrealistically narrow view of life-history diversity which is incongruent with the taxonomic diversity of the group and can result in development of inappropriate conservation strategies. We investigated relationships among growth, longevity, and size in 57 species and 146 populations of freshwater mussels using original data and literature sources. In contrast to generalized depictions, longevity spanned nearly two orders of magnitude, ranging from 4 to 190 years, and the von Bertalanffy growth constant, K, spanned a similar range (0.02-1.01). Median longevity and K differed among phylogenetic groups but groups overlapped widely in these traits. Longevity, K, and size also varied among populations; in some cases, longevity and K differed between populations by a factor of two or more. Growth differed between sexes in some species and males typically reached larger sizes than females. In addition, a population of Quadrula asperata exhibited two distinctly different growth trajectories. Most individuals in this population had a low-to-moderate value of K (0.15) and intermediate longevity (27 years) but other individuals showed extremely slow growth (K = 0.05) and reached advanced ages (72 years). Overall, longevity was related negatively to the growth rate, K, and K explained a high percentage of variation in longevity. By contrast, size and relative shell mass (g mm⁻¹ shell length) explained little variation in longevity. These patterns remained when data were corrected for phylogenetic relationships among species. Path analysis supported the conclusion that K was the most important factor influencing longevity both directly and indirectly through its effect on shell mass. The great variability in age and growth among and within species shows that allocation to growth is highly plastic in freshwater mussels. The strong negative relationship between growth and longevity suggests this is an important trade-off describing widely divergent life-history strategies. Although life-history strategies may be constrained somewhat by phylogeny, plasticity in growth among populations indicates that growth characteristics cannot be generalized within a species and management and conservation efforts should be based on data specific to a population of interest.

Molecular phylogeny of the Nearctic and Mesoamerican freshwater mussel genus Megalonaias

“…Reduced rates of molecular evolution are common in taxa with large body size, long generation time, and slow metabolic rate (Martin & Palumbi, 1993), a relationship that has been recently documented in several other freshwater mussel genera (e.g., Margaritifera and Potomida) (Araujo et al, 2016a, b;Bolotov et al, 2016). Megalonaias nervosa has the largest body size and greatest estimated generation time of all documented Ambleminae, and one of the slowest growth rates (Haag & Rypel, 2011;Haag, 2012). Furthermore, the shallow genetic divergences within Megalonaias do not appear to be a product of a recent radiation: fossil material dates M. nervosa back at least 1 MY (Bogan & Portell, 1995), and the genus as far back as the Miocene (MacNeil, 1935;Watters, 2001).…”

“…Understanding the evolution and ecology of the Ambleminae and its tribes is an important focus of freshwater biodiversity research in North America and has positively influenced many aspects of applied freshwater science (e.g., Serb et al, 2003;Zanatta & Murphy, 2006;Barnhart et al, 2008;Haag & Rypel, 2011;Campbell & Lydeard, 2012;Haag, 2012); however, the current theory of amblemine phylogeny, biogeography, and ecology is strongly geographically biased due to the near complete exclusion of Mesoamerican taxa from recent research. Comprehensive understanding of the Ambleminae, its tribes, and several of its genera, necessitates inclusion of Mesoamerican representatives, and their consideration is likely to have major implications in various disciplines of freshwater mussel research (e.g., evolutionary biology, ecology, and conservation).…”

What can we infer from the shell dimensions of the thick-shelled river mussel Unio crassus?

“…This pattern is explained by the theory of allocation of energy to growth and reproduction (Kozłowski, 1992;Haag & Rypel, 2011), according to which after reaching some critical size, it pays to allocate more and more energy to reproduction, at the expense of growth. The question of the factors governing allocation of energy is basic to many ecological and conservation biology studies of mussels (Aldridge, 1999;Haag & Staton, 2003): predictions of age, growth pattern, size at maturity, and the growth asymptote are crucial to understanding the ecology of mussels (e.g., Jokela & Mutikainen, 1995;Czarnołęski et al, 2003;Ren & Ross, 2005) and population processes (e.g., Hastie et al, 2000).…”

Mortality as a bivariate function of age and size in indeterminate growers

“…Similarly, we obtained from Fig. 3 in Haag and Rypel (2011) age and size data for the freshwater mussel, with 108 individual records. We ran the model in Eq.…”