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Cold Temperature Effects on Byssal Thread
Production by the Native Mussel Geukensia demissa
versus the Non-Native Mussel Mytella charruana

By: Sasha Brodsky | Mentors: Dr. Linda Walters, Dr. Kimberly Schneider, and Dr. Eric Hoffman

Discussion

We found that both M. charruana and G. demissa produced fewer byssal threads at colder temperatures. Mytella charruana at 23° C produced ten times more byssal threads than at lower temperatures (13 and 10° C); this difference was significant. At 13° C, survival of M. charruana was 100%, and some mussels (25%) produced new byssal threads, although the number of threads was ten times lower than the control (23° C; Figure 1A). Byssal thread production significantly varied on the second day of the control (23° C) in M. charruana, but we have no reason to believe this had any biological significance. We expect M. charruana to have continued survival at 13° C, even though the amount of byssal threads produced was lower than controls, and we base this conclusion on two lines of evidence. The first was that M. charruana created at least two byssal threads at 13° C, which has been documented to be the smallest number of threads needed for attachment in a similar size mussel; two such threads can be formed in as little as one hour (Lesin et al. 2006). It has also been documented (in other mussels) for individual byssal threads to retain their integrity and remain attached to a surface for 4-6 weeks (Carrington 2002). Based on this data, we suggest that the actual thermal minimum of M. charruana is at or very close to 13° C. Our data suggest that M. charruana may experience difficulty surviving for extended periods of time in natural habitats at 10° C. In addition to only 66% survival in our laboratory trial, no byssal threads were produced at 10° C (Figure 1A). Our results suggest that if M. charruana survived and was dislodged at this temperature, they would not be able to re-attach. This would especially pose a problem for M. charruana living on vertical substrates, since attaching near the surface of the water is often necessary for mussels to gain sufficient food and avoid benthic predators, such as crabs (Johnsen and Jakobsen 1987). Therefore, M. charruana would not be able to withstand 10° C for long periods of time. These results may also explain why populations of M. charruana decrease during months of low temperatures, which includes a severe cold weather event that occurred from December 2009 to February 2010 when water temperatures in Jacksonville, Florida, repeatedly dropped below 13° C for 48 days (A. Godwin, unpublished data).

At 10° C, Geukensia demissa produced fewer byssal threads than other tested temperatures, with an overall mean of 2.9 ± 0.3 threads per day (Figure 2A). Unlike M. charruana, G. demissa had 100% survival at this temperature, which suggests that G. demissa does not have difficulty surviving and can remain attached at this temperature. These results suggest that G. demissa may simply be more cold tolerant than M. charruana. In studies on the invasive freshwater mussel Dreissena polymorpha and the blue mussel M. edulis, byssal thread production was found to increase proportionally with temperature, which coincides with what we have found in M. charruana and G. demissa (Young 1985, Clarke and McMahon 1996).

Although temperature influences the survival of these two species differently, this does not dismiss the idea of competition between them. At 23° C, M. charruana had an overall mean of 10.84 ± 1.60 byssal threads, which was twice as high as the 5.78 ± 0.84 threads produced by G. demissa (Figures 1A, 2A). In warmer months, growth rates of these two species are similar, with an average increase in shell length of 5 mm per month for M. charruana and 4.4 mm for G. demissa (Brousseau 1984, A. Godwin unpublished data). Problems may arise for native species such as G. demissa since mussels compete for limited space, which can intensify during times of high growth and settlement (Bayne 1976). A study by Boudreaux et al. (2006) also found that M. charruana attaches to substrates quicker than G. demissa, which could increase survival of M. charruana. Future competition studies between these species will be useful in assessing the extent to which M. charruana might become a threat to other native species, such as the economically and environmentally important eastern oyster Crassostrea virginica (Boudreaux and Walters 2006).

Expansion of M. charruana's invasive range will depend on its ability to produce byssal threads at varying temperatures. In previous laboratory studies, 21% of M. charruana were able to survive at 9° C, and its thermal minimum was predicted to be between 6 and 9° C (Brodsky et al. 2009). However, based on the results from our current study, the thermal minimum is predicted to be at least 3 degrees higher, at or near 13° C. Temperature would thus create a barrier to where M. charruana could establish, and it is likely that it would not be able to survive in habitats where water temperatures are lower than 13° C for extended periods of time.

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