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First published online June 15, 2006
Journal of Experimental Biology 209, 2554-2566 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02259
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Following the heart: temperature and salinity effects on heart rate in native and invasive species of blue mussels (genus Mytilus)

Caren E. Braby* and George N. Somero{dagger}

Hopkins Marine Station, Department of Biological Sciences, Stanford University, Oceanview Boulevard, Pacific Grove, CA 93950, USA


Figure 1
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  		Fig. 1. Heart rate response to temperature and methods for impedance pneumography. Heart activity patterns from a single individual over a range of temperatures from 14.5–31°C are shown. Each heart rate trace spans 3 min. Inset shows the schematic of the heart rate monitoring apparatus, and the placement of electrodes. Mussel drawing adapted from (Brusca and Brusca, 1990Go).

 

Figure 2
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  		Fig. 2. Environmental exposure and heart rate response to stress in M. trossulus. (A) Heat stress; as temperature increases (broken line; beginning at 01:00 h, experimental time), heart rate increases until the animal reaches a temperature at which heart rate falls rapidly (Hcrit). With continued heat stress, heart rate remains depressed, but as soon as temperature drops, heart rate returns to a normal level. (B) Cold stress; as temperature decreases from acclimation level, heart rate decreases to zero. (C) Low salinity stress; as salinity decreases from acclimation level, heart rate decreases very gradually until the animal reaches a salinity at which there is a rapid fall in heart rate (Scrit). Heart rate remains depressed during continued low salinity treatment but recovers immediately when salinity begins to increase.

 

Figure 3
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  		Fig. 3. Heart rate response to temperature and salinity in differently acclimated mussels. Species are referred to by the first letter of the species' name: M. trossulus (t), M. galloprovincialis (g), M. edulis (e), and M. trossulus/M. galloprovincialis hybrids (h). Symbols connected by solid lines are 14°C-acclimated; symbols connected by dotted lines are 21°C-acclimated. Values are means ± s.e.m. For N and P values, see Table 2. (A) Resting heart rate measured at the acclimation temperatures (14°C or 21°C). (B) Response to heat stress (Hcrit); all species show an increase in Hcrit with increased acclimation temperature. While there are some differences in Hcrit among salinity treatments, salinity acclimation does not contribute statistically to the observed pattern. (C) Response to cold stress (lowest heart rate at 0°C); at comparable acclimations, M. trossulus has an equal or higher heart rate than M. galloprovincialis. Note that to make overlapping symbols visible, we offset the g-14°C data by +0.2°C along the x-axis. (D) Response to low salinity stress (Scrit); there is a clear increase in Scrit with increasing salinity acclimation in all species.

 

Figure 4
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  		Fig. 4. Temperature compensation of resting heart rate (RHR; beats min–1). t, M. trossulus; g, M. galloprovincialis; e, M. edulis. Symbols connected by solid lines are observed RHR values averaged across salinity treatments. Symbols connected by broken lines are observed RHRs at 14°C and expected RHRs at 21°C, using the species-specific Q10 estimate and assuming no physiological compensation. If there was complete temperature compensation, there would be no difference between the observed values for 14°C- and 21°C-acclimated animals (symbols connected by a horizontal line; not shown). However, the observed RHR at 21°C is lower than expected for all species, indicating partial compensation with acclimation temperature.

 

Figure 5
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  		Fig. 5. Mortality in temperature and salinity acclimation treatment groups. There was nearly equal mortality of M. edulis (e) and M. trossulus (t) in 2003, regardless of acclimation treatment, and both species had greater mortality at the warmer acclimation temperature. In the following year, M. trossulus had equivalent mortality at the 14°C acclimation and decreased mortality under 7°C acclimation, reinforcing the positive relationship between acclimation temperature and mortality. Only three M. galloprovincialis (g) individuals died in both acclimation years (out of 348 total), and all three were from the 21°C acclimation in 2003 (M. galloprovincialis 2004 is not included in the graph because there was no observed mortality).

 




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