Investigating the Life History, Demography, and Thermal Tolerances of Rare and Endangered Freshwater Mussels (Bivalvia: Unionidae) in the Upper Tennessee River Basin

Abstract

The native freshwater mussel species Lasmigona holstonia is imperiled throughout its entire range and represents a significant management concern for Alabama, Georgia, North Carolina, Tennessee, and Virginia. Lasmigona holstonia remains very understudied, with key aspects of its life-history, biology, and thermal tolerances still unknown. In this study, I document mussel burrowing behavior, estimate population age structure, establish a thermal regime and tolerances, and estimate population density of L. holstonia in Tennessee and Virginia. Results indicate that L. holstonia has an approximate 20-year life-span and ages vary between populations. Mussels experience headwater stream temperatures frequently oscillating 5–7°C each day, and reaching a maximum temperature of 28.5°C. Laboratory exposure of juvenile L. holstonia to environmentally-relevant temperatures suggest that some individuals may be able to withstand water temperatures up to 32°C for at least 14 days, though mussels displayed reduced heart rates (beats per minute) with longer thermal exposure, regardless of temperature. Mussel growth was significantly affected by both time and temperature, with mussels in the coldest test temperature (18°C) growing just 3% in length over 42 days, compared to a 30% increase in length for mussels exposed to 22°C and 28°C water for 42 days. Adult L. holstonia were able to bury up to 30 cm into the sediments, a depth that is beyond detectability during the surface surveys traditionally used to locate and monitor L. holstonia. This burrowing behavior may be important for mussels seeking thermal refuge, as a sediment temperature analysis indicates that sediment temperatures were often less variable compared to in-stream surface water temperatures. Taken together, L. holstonia appears to be uniquely suited to persisting in a hydrologically variable headwater environment, and populations of this mussel may be prone to a boom-and-bust demographic cycle following changes to habitat. The data and information provided in this study will help inform resource managers about the needs for managing populations of this species and for determining conservation actions and potential listing needs in the future. In addition, populations of two mussel species residing in the mainstem of the Clinch and Powell Rivers (Tennessee and Virginia) were surveyed for fifteen years, from 2003–2014 and 2017–2021. Medionidus conradicus is proposed endangered for federal listing and Ptychobranchus subtentus has been listed as endangered since 2013. Densities of both of these mussel species have varied over time, with M. conradicus populations declining since 2017 and P. subtentus populations declining since 2010; however, population densities for both species was quite variable over time. Age data for surveyed mussels showed that the populations of P. subtentus in the Clinch River are mainly composed of large, older individuals and recruitment of juveniles is low, indicating an aging population. Juvenile mussel recruitment for both species may be affected by April and July stream discharge, while juvenile M. conradicus recruitment may be affected by July and September river temperatures. Long-term monitoring data will help managers understand the demographic oscillations of these two mussel species.