Seasonality of Gamete Production of Cyclonaias Species in Central Texas

Abstract

Reproductive traits, which can impact population health, are important life-history characteristics for freshwater mussels. Little research has been done on the reproductive ecology of mussels, and crucial information is missing for many threatened and endangered species, especially in Texas. The objective of this study was to examine gamete production, parasitic infection rates, and sex ratios of two freshwater mussel species (Cyclonaias petrina and Cyclonaias pustulosa) in the Llano and San Saba rivers in central Texas. Gamete densities and egg diameters of C. petrina in the Llano River varied seasonally, with peak gamete densities occurring in December and February 2017 and being significantly lower from June through September 2017, while the relative abundance of the largest size classes of eggs was highest in February 2018. Few to no differences were detected in gamete production and egg diameter sizes for C. petrina between rivers. Cyclonaias pustulosa had significantly higher sperm densities and smaller egg diameters compared to C. petrina but exhibited similar egg densities in the San Saba River. There were no significant differences in gamete densities between rivers and no significant correlations between shell length and gamete density. Infection rates of parasitic trematodes varied from <1% to 14%, with the highest infection rates occurring in C. petrina in the San Saba River. Sex ratio of C. petrina was slightly skewed toward females in the Llano River and toward males in the San Saba River, with C. pustulosa having a 1:1 sex ratio in the San Saba River. The high percentage of samples without gametes suggests that the reproductive outputs of Cyclonaias appear to be more limited in the San Saba River due to several potential stressors. Further research will need to investigate the relative importance of the various stressors that affect the reproductive ecology of mussels and their persistence.