Читать книгу Marine Mussels - Elizabeth Gosling - Страница 46

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).