Marine Mussels. Elizabeth Gosling

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Figure 3.4 A‘robomussel’ (right) molded from polyester resin and containing a TidbiT logger thermally matched to the characteristics of a living mussel. On the left is an unmodified Onset TidbiT logger.

      Source: From Fitzhenry et al. (2004). Reproduced with permission from Springer Nature.

In the first study in which robomussels were deployed, Helmuth & Hofmann (2001) continuously monitored temperatures at a location in central California for a period of two years, using loggers designed to mimic the thermal characteristics of M. californianus. Model mussel temperatures were recorded on both a horizontal (Figure 3.5) and a vertical, north‐facing microsite and in an adjacent tidepool. Mussels at each microsite were periodically measured for levels of heat shock proteins (Hsp 70), a measure of thermal stress (Fields et al. 2012). The role of Hsps in the stress response is to assist misfolded proteins to attain or regain their native state and prevent heat‐damaged protein accumulation in cells, thereby preventing the formation of cytotoxic aggregates. Other factors that have a bearing on an individual’s response to thermal stress include: size, shape, mass and colour (Helmuth 2002), vertical position in the sediment (Jost & Helmuth 2007), geographic location (Fields et al. 2012), timing of low tide (Mislan et al. 2009) and interaction with stressors such as anoxia (Veldhuizen‐Tsoerkan et al. 1991), salinity fluctuations (Podlipaeva & Berger 2012), hypoxic conditions (Anestis et al. 2010), reduced food supply (Schneider et al. 2010), pollutants (Mahmood et al. 2014; Banni et al. 2017) and pathogens (Cellura et al. 2007). In Helmuth & Hofmann (2001), mussel temperatures were consistently higher on the horizontal surface than on the vertical one, and differences between these surfaces were reflected in the amount of Hsp70 produced. Temperatures recorded in the tidepool were not as high as those on the exposed horizontal microsite but in general exceeded those recorded on the aerially exposed vertical face. One interesting finding of the study was that seasonal peaks in extreme (acute) high temperatures did not always coincide with peaks in average daily maxima (chronic high temperatures). A subsequent study showed that thermal stress may depend not only on exposure to temperature extremes but also on the thermal history of the temperature signal (Helmuth 2002). For example, on days when daily maximum temperatures increase gradually, mussels may be able to acclimate better to temperature extremes compared to days when temperature maxima are preceded by cool days. Helmuth (2002) also found through deployment of loggers at multiple sites that each site had a unique thermal signature due to interactions of terrestrial climate, tidal cycle and wave exposure, but also significant within‐site differences due to tidal height and substratum angle.