Longevity and Growth Rate of Ancient and Modern Dreissenid Mussels in An Environmental Context

The longevity, growth rate, and adult size of an organism are fundamental components to its life history. Understanding the evolution of these traits is central to understanding the evolutionary process. Dreissenid bivalves provide an excellent tool for studying the evolution of life history traits: modern dreissenids are numerous, widespread, and prevalent in a wide variety of fresh and brackish-water habitats. This enables observations of the interactions between dreissenids and their environments because modern environmental conditions can be directly measured. We can then apply the lessons learned to the rich record of fossil dreissenids in ancient Lake Pannon, a Central European Mio-Pliocene lake. This wealth of modern and fossil dreissenids enables us to study the evolution of life-history characters in both environmental and temporal contexts and to test two primary hypotheses: 1) longevity increases in more stable environments, and 2) longevity increases as growth rate decreases.

Numerous methods have been used to investigate longevity in modern dreissenids, including size frequency distributions, growth annuli counts, mark-recapture studies, and growth under experimental conditions. Each of these methods includes complications that can yield biased longevities. Although underutilized in dreissenid studies, stable isotopic (δ¹⁸O and δ¹³C) profiles have been effectively used to determine longevity and growth rates in numerous molluscan taxa.

We are developing isotopic profiles for modern dreissenids from a variety of waterways, two lineages of Pannonian Congeria exhibiting significant morphological change through time, and two morphologically stable species of Pannonian dreissenid. Preliminary results support our hypotheses: individuals from the smallest waterbodies (e.g. Kovada Lake, Turkey) endured the broadest range of environmental conditions (represented by a total δ¹⁸O range of ~5‰ compared to ~3‰ in other waterways) and exhibit both more rapid growth and shorter lifespans (~1-1.5 yrs), whereas those from more stable habitats exhibited slower growth and longer lifespans (1.5-2.5 yrs).