Effects of environmental temperature and flow on brooding phenology of the Texas heelsplitter <i>Potamilus amphichaenus</i> and the Brazos heelsplitter <i>Potamilus streckersoni</i>

ABSTRACT Objective Unionid mussels are among the most vulnerable and threatened aquatic organisms. The reproductive phenology of mussel is important for understanding its population dynamics and to inform management actions that are focused on captive propagation. Texas has approximately 52 species of unionid mussels of which 15 are state and/or federally listed, and relatively little is known about the reproductive phenology of many of these species. Methods To address this knowledge gap, we examined the brooding phenology and stages of glochidia development (egg, immature glochidia, developed glochida, and fully developed glochidia) of two rare species in Texas, the Texas heelsplitter Potamilus amphichaenus in the Neches River and the Brazos heelsplitter P. streckersoni in the Brazos River. We then used general additive models to relate environmental parameters such as temperature, accumulated degree-days, and flow to peak brooding times using data that we collected from the sampling sites. The models were ranked for parsimony using the Akaike information criterion corrected for small sample size (AICc), and models with a ΔAICc ≤2 were considered plausible. We then calculated AICc weights (w) to identify the model with the best support. We also compared shell dimensions between gravid and nongravid individuals to check for potential size differences to evaluate glochidia release/host fish infestation strategies. Results Brazos heelsplitter and Texas heelsplitter were observed brooding beginning in late August and carrying brood in various stages of development into the following June. Accumulation of heat (accumulated degree-days) and maximum water temperature were correlated with the timing of glochidia release. Gravid mussels were significantly smaller on average than nongravid individuals. However, we did not determine the cause for the size difference between gravid and nongravid mussels. Conclusions Environmental temperature and heat accumulation are important factors that are related to the brooding phenology of Brazos heelsplitter and Texas heelsplitter. Understanding how environmental cues like temperature and accumulated degree-days shape mussel reproduction is important for evaluating population persistence and for informing conservation activities such as captive propagation.