Reproductive life history of 2 imperiled and 1 widely- distributed freshwater mussel species from the southwestern United States

Authors: Jack F. Dudding, Michael Hart, Jennifer M. Khan, Clinton R. Robertson, Roel Lopez and Charles R. Randklev

Information on mussel reproductive life history, age, and growth is important for understanding evolutionary and ecological relationships and predicting how species will respond to conservation and management strategies intended to mitigate threats. In Texas, located within the southwestern United States, 11 species are pending review for listing under the Endangered Species Act, and information on mussel reproductive life history, age, and growth is lacking for most of these species. To address this knowledge gap, we examined life-history traits for 2 im-periled mussel species (Cyclonaias necki, Guadalupe Orb, and Fusconaia mitchelli, False Spike) and 1 common, widely-distributed species (C. pustulosa, Pimpleback) from a site in the lower Guadalupe River, located in Central Texas. The resulting information was then compared with existing life-history information for mussels. We observed peak sperm production between late January to early March and peak mean egg diameter from late winter to early summer in all 3 species. Brooding was observed in all species, usually between March and June, and brooding behavior and glochidia morphology were similar to those of congeners studied in other locations. Accumulated degree days was important in regulating the timing of gametogenesis and potentially the duration of brooding for all 3 species. Fecundity estimates for C. necki and F. mitchelli were much lower than the values reported for congeners in other locations. Fecundity was associated with both mussel age and shell length, although length was a better predictor than age. Trematode infestation rates were high (∼30%) in C. necki and C. pustulosa, and sex ratios were skewed toward males, which could mean that females are disproportionately affected. The age distribution and individual growth rate for C. necki and F. mitchelli closely mirror those of related congeners, although the maximum observed age for C. necki did not meet theoretical expectations based on the estimated growth rate for this species. It is unknown why fecundity is reduced for C. necki and F. mitchelli or why C. necki may have reduced longevity, but these differences could be the result of environmental change.

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