QUANTIFYING SPERM PRODUCTION AND MORPHOLOGY FOR TWO SPECIES OF UNIONID MUSSELS
Mussel diversity is declining, as more species are becoming extirpated or extinct at an alarming rate. While little can be done to bring back lost species, we must work on securing the fate of others that are threatened or endangered. This can be facilitated through gene banking and assisted reproduction to propagate these imperiled species. Unfortunately, limited information is available on freshwater mussel reproduction, especially as it relates to paternal impacts. Sperm quality follows a seasonal spawning period where cell characteristics change overtime. Thus, it is imperative to find the window of optimal cell health to further improve freshwater mussel reproduction. Our objectives were to quantify sperm production and morphology for two unionid mussels, Ligumia subrostrata and Lampsilis straminea, throughout the natural spawning season. Both species were held separately in two ponds in fifteen ~1' diameter mesh cages where temperature at the sediment-water interface was recorded with HOBO temperature loggers. Starting 29 Aug. 2018, male mussels (n = 5 for each species) were sampled every two weeks. Using a small syringe, sperm were collected directly from the gonad (Fig. 1A). Sperm density was determined for each male using a hemocytometer and a Nanodrop at 350, 600, and 700 nm. Sperm were fixed in 2.5% glutaraldehyde and examined using scanning electron microscopy (Fig. 1BC).
Sperm density for L. subrostrata ranged from 1.1×109 to 19.60×109 cells/mL with highest production from 26 Sept to 7 Nov (Fig. 1D). L. straminea sperm density peaked (20.02×109 cells/mL) on 13 Sept and declined thereafter (Fig. 1E). There was a positive relationship between hemocytometer counts and Nanodrop absorbance for both species (R2 > 0.94). Sperm were uniflagellated and preliminary results for L. subrostrata and L. straminea showed mean head length and width were 3.28 ± 0.16 μm and 1.64 ± 0.11 μm and 3.41 ± 0.13 μm and 1.59 ± 0.13 μm, respectively. These seasonal cell densities and morphology parameters provide a standard for developing biotechnology for sperm collection and cryopreservation in freshwater mussels.
