(e.g. the amino acids glycine, alanine and proline), as well as the end products of anaerobic metabolism (e.g. lactate, succinate and strombine (Loomis et al. 1988; Loomis & Zinser 2001) and glucose (Gionet et al. 2009)). Calcium also acts as a cryoprotectant in the mussel G. demissa by binding to cell membranes and reducing cell damage during freezing, either through physical stabilisation of the membrane against mechanical disruption caused by cell shrinkage or by prevention of the denaturation of membrane compounds (Ansart & Vernon 2003). Another mechanism to avoid intracellular ice formation – an invariably lethal process – is the production of ice‐nucleating proteins, which are secreted into the ECF and act to induce and control extracellular ice formation. These proteins reduce undercooling from the range −15 to −20 °C to the range of −5 to −10 °C. G. demissa is a freeze‐tolerant saltmarsh mussel that is regularly exposed to subzero temperatures for extended periods during low tides. The species’ cold tolerance varies seasonally, ranging from a lower lethal temperature of −10 °C in the summer to one of −13 °C in the winter. Although it lacks ice‐nucleating proteins, it utilises at least one strain of ice‐nucleating bacteria, Pseudomonas fulva, from seawater. These bacteria, which are found in the gills of G. demissa, could perform the same function as hemolymph ice‐nucleating proteins by limiting ice formation to extracellular compartments (Loomis & Zinser 2001).
A decrease in temperature usually reduces membrane fluidity, which can lead to membrane dysfunction. Ectotherms typically counteract this temperature effect by remodelling membrane lipids via changes in phospholipid head groups, fatty acid composition and cholesterol content. Lipid dynamics and the physiological responses of cold‐adapted M. edulis and the warmer‐water oyster, Crassostrea virginica, were compared during simulated overwintering and onward to spring conditions (Pernet et al. 2007; see also Fokina et al. 2018). To simulate overwintering, bivalves were held at 0, 4 and 9 °C for three months. They were then gradually brought up to 20 °C and held at that temperature for five weeks, to simulate spring–summer conditions. Major differences were observed in triglyceride (TAG) metabolism during overwintering. TAGs are the main constituents of natural fats and oils. Mussels used digestive gland TAG stores for energy metabolism or reproductive processes during the winter, whereas oysters did not accumulate large TAG stores prior to overwintering. Mussel TAG contained high levels of 20:5n−3 compared to levels in oysters, which may help to counteract the effect of low temperature by reducing the melting point of the TAG, thereby increasing the availability of storage fats at low temperature. The physiological responses of mussels to temperature change and acclimation to high and low temperatures are dealt with in more detail in Chapter 7.
Salinity
On fully marine shores, mussels experience a salinity of about 35 psu most of the time. M. edulis is likely to encounter hyper‐saline conditions in tide pools, crevices and sediments exposed on hot, breezy days, where evaporation can increase salinity to values as high as 42 psu (Tyler‐Walters 2008). In contrast, large amounts of rainfall dilute standing water on the shore, decreasing its salinity. But since many mussels, in particular Mytilus spp., are euryhaline, they can tolerate an extremely wide range of salinity (4–40 psu) in their natural environment (see Chapter 7 for details on salinity tolerances in marine mussels). Mussels in subarctic Norway commonly occur in shallow intertidal pools. By living in pools, they are insulated against low air temperatures but exposed to high salinities beneath overlying ice. They avoid exposing their tissues to salinities as high as 65 psu because a shell valve closure response to low temperature operates at about −1.5 °C, before ice sheets form and bottom water salinities rise (Davenport & Carrion‐Cotrina 1981). The increase in salinity appears to take several hours, but the return to normal seawater salinities occurs in just a few minutes when the ice is melted and the pools are flushed out by the relatively warm (3–5 °C) water of the incoming tide. In estuarine waters, mean salinity decreases and salinity variation increases with distance upstream, and both these factors have deleterious effects on bivalve distribution, with the result that species diversity is significantly less in estuaries than on fully marine shores.
For marine species in general, salinity is one of the most important abiotic factors contributing to distribution and affecting many physiological rates (Dame 2012; see Chapter 7). To understand the invasion potential of Mytella charruana, newly introduced to the southeastern United States, Yuan et al. (2010) examined salinity thresholds for this species, which inhabits colonising rocky substrates in estuaries. Their study addressed the following questions: (1) In what range of salinities can this species survive if it is slowly adjusted to test salinities? (2) In what range of salinities can this species survive when it experiences rapid changes of salinity? and (3) In what range of salinities can this species survive when it experiences temporary, rapid changes of salinity (6 hr duration). They tested survival in salinities ranging from freshwater to hypersaline conditions (0–45 psu) and determined whether mussel size affected experimental results. All experiments examined survivorship of mussels by increasing or decreasing the salinity from the field value under laboratory conditions. Mortality in each tank was recorded daily for 43 days for the gradual adjustment trials and for 12 days for permanent and six‐hour shock trials. Large mussels (20–54 mm) survived best in salinities from 2 to 23 psu, with 100% mortality at 0 psu and 45 psu with gradual adjustment. Small mussels (3–19 mm) survived in a wider range of salinities (2–40 psu) with gradual adjustment to new salinities. However, survival of both large and small mussels was significantly lower in permanent shock trials at salinity extremes. Six‐hour shock trials had no effect on survival at any of the test salinities (0–45 psu) for both large and small M. charruana. Acclimation experiments on Philippine charru mussels (Rice et al. 2016) provided very similar results, except that significant size‐based differences in salinity tolerances were not detected. Both studies indicated that charru mussels have the capacity to invade a wide variety of saline environments with significant freshwater or marine input, particularly when they have an opportunity to acclimate to gradual changes in salinity with significant freshwater or marine input, and possibly surviving wide salinity swings in Philippine estuaries associated with the rapid onset of abundant monsoonal rains. See also the study (described earlier) of Yuan et al. (2016b), who investigated the simultaneous effects of salinity and temperature on two invasive species, M. charruana and Perna viridis, in order to obtain a better understanding of their respective invasion potentials.
Habitat salinity may also determine the outcome of competition between native and invasive mussel species (see Tomanek et al. 2012 and Lockwood & Somero 2011b, described earlier, and Sarà & de Pirro 2011, described in Chapter 7).
Wave Exposure
Wave exposure, through both wave force and changes in immersion patterns, has a powerful influence on patterns of zonation and abundance on rocky shores. Lack of tolerance to high wave forces may limit species composition in this habitat, but because intertidal zonation patterns are driven by emersion time, wave action tends to extend biological zones vertically (upshore) by effectively decreasing the frequency and duration of emersion. High wave action will therefore cause a point on the shore to behave as if it is effectively lower than its still water tidal height. Harley & Helmuth (2003) described a method for measuring effective shore level (ESL), a metric that combines the influence of wave splash and tidal regime on patterns of emersion and immersion. They identified immersion events as sharp drops in temperature recorded by submersible dataloggers and compared the tide height at the time of the temperature drop to the wave height recorded by an offshore buoy. Using this method, Gilman et al. (2006) compared the effects of wave action on immersion patterns within multiple sites on the Pacific coast of North America. They deployed miniature temperature loggers at fixed intertidal heights at each site and recorded temperatures at intervals of 5–15 min for periods of up to five
