Reproductive ecology of the threatened and endemic freshwater mussel Lampsilis bracteata

Abstract

The Texas fatmucket, Lampsilis bracteata, is a unionid mussel endemic to the Colorado and upper Guadalupe River basins of Central Texas and a candidate for federal listing under the Endangered Species Act. There is increased interest in propagation and population restoration of threatened mussels in Texas as a potential conservation method, but still little is known about their life histories and how local populations may differ in aspects of their reproductive ecology, e.g. timing of brooding and potential local adaptations to host fish. The purpose of this study was to compare host fish compatibilities and seasonality of reproduction between two populations: (a) by cross‐infesting fish from the San Saba and Llano Rivers in the Colorado River basin with sympatric and allopatric mussel larvae in the laboratory (hatchery‐produced Guadalupe bass and largemouth bass were also infested); and (b) by assessing gamete concentration, brooding period, viability of larvae and sex ratios using monthly sampling events between February 2017 and February 2018. Reproduction varied with season and between populations. The proportion of females brooding tended to be lower in the summer and the autumn, and higher during winter and spring months before peak water temperatures were reached. The sex ratio in both populations did not significantly differ from 1:1. Fecundity and larvae viability were higher in the Llano River population compared with the San Saba population. Trematode flatworms were found in several female gonad samples from the San Saba population and in a few samples from the Llano population. The highest metamorphosis success occurred on wild green sunfish and largemouth bass, and hatchery‐produced largemouth and Guadalupe bass. The average metamorphosis success tended to be higher for some mussel–fish pairings originating from the same tributary, suggesting that mussels may be locally adapted to host fish, which should be considered in conservation efforts.

Seasonality of Gamete Production of Cyclonaias Species in Central Texas

“…Researchers have shown that gametogenesis can occur year round in many tachytictic species (Holland-Bartels and Kammer 1989;Jirka and Neves 1992;Garner et al 1999;Tsakiris et al 2016), which was observed in C. petrina in the Llano River, but not in the San Saba River. Similar to findings from another study investigating Lampsilis bracteata in central Texas (Seagroves et al 2019), reproduction appeared to be more limited at sites in the San Saba River compared to the Llano River as suggested by a larger number of samples without gametes found in the San Saba River compared to the Llano River (43% vs. 11% when comparing months in which both rivers were sampled). Detrimental environmental conditions in the San Saba River may play a role, either by causing reproductive senescence in mussels as exhibited in populations of other taxa (Nussey et al 2013) or by making mussels more susceptible to diseases and parasites (see below).…”

“…Parasitic infection rates in mussels are usually low (,6%; Haag and Stanton 2003), but some studies have shown relatively high (20-36%) infection rates in multiple species (Zale and Neves 1982;Tsakiris et al 2016;Dudding et al 2020). In the San Saba River, during congruent sampling trips, bucephalid trematodes (Bucephalidae) and unidentified parasites were found in gamete samples of Lampsilis bracteata (6% infection rate for all individuals) consistently and in low abundance from February 2017 to February 2018 but were not found in April and September 2017 (Seagroves et al 2019). Conversely, parasites in our study were found mostly during the summer and fall months (June-November) in both Cyclonaias species.…”

“…We observed gamete densities increasing in the fall (October-November) and peaking in winter and early spring (December-March). However, seasonal variation in gamete densities represents variation in gamete production of mussels only if it is assumed that gamete fluid volume does not vary seasonally (Seagroves et al 2019). Garner et al (1999) suggested that mussels may benefit from starting gamete production in the fall to utilize increased nutrients within the river.…”

“…Digenetic trematodes have been known to feed on gonadal tissue, lower reproductive output, negatively impact physiological condition, and limit growth of freshwater mussels (e.g., Taskinen 1998;Gangloff et al 2008;Müller et al 2015). Prevalence of parasitic trematodes in freshwater mussels has been shown to be related to season, mussel body size, age, and sex, with higher infection rates being found in larger and older females during warmer months (Huehner 1984;Taskinen et al 1994;Taskinen and Valtonen 1995;Müller et al 2015;Seagroves et al 2019). Our study found increased prevalence of trematodes during the summer and fall months (June-November), but no relationship with mussel body size.…”

“…In order to better understand and inform management strategies, it is imperative to examine basic life-history traits of these species. Only three studies have investigated gamete production of mussels within Texas (Tsakiris et al 2016;Seagroves et al 2019;Dudding et al 2020). Hence, the objective of this study was to evaluate the reproductive timing of two congeneric species of mussels from central Texas.…”