Marine Mussels. Elizabeth Gosling
mangroves and estuaries from Baja California, Mexico to Santa Catarina, Brazil (de Oliveira et al. 2005).
Factors Affecting Geographic Distribution
Temperature and salinity not only set limits on the spatial distribution of bivalves but also affect every aspect of biology, including feeding, reproduction, growth, respiration, osmoregulation and parasite‐disease interactions (see details in Chapters 4, 5, 6, 7, 11 and 12). When it comes to distribution on a large geographic scale, it is generally recognised that temperature plays a more important role than salinity. The synergistic effect of temperature and salinity, acting in concert with other environmental variables such as water depth, substrate type, wave action, food availability, water turbidity and the occurrence of competitors, predators and disease, can have more profound consequences than either factor acting alone. Geographical distribution is also governed by hydrographic barriers to larval dispersal, such as oceanic currents, confluences, gyres and surface water stratification.
Temperature, Salinity and Hydrographic Factors
Most marine bivalves live within a temperature range from −3 to 44°C (Vernberg & Vernberg 1972). Within this range, the degree of temperature tolerance is species‐specific, and within individual species early embryos and larva have a narrower temperature tolerance than adults (see Chapter 5). In addition, the temperature required for spawning is invariably higher than the minimum temperature required for growth. All of these factors set limits on the natural distribution of individual species on both regional and local scales. A few relevant examples from marine mussels will elucidate this.
As described earlier, the distribution of M. edulis on the Atlantic coast of North America extends from the Canadian Maritimes southward to Cape Hatteras in North Carolina, the historical southern limit for this species. It is believed that the northward‐moving warm water Gulf Stream meets the southward‐moving cold Labrador Current in the region of Cape Hatteras (35.25 °N), thus providing a temperature barrier for the distribution and survival of mussel larvae south of this point. Field experimental data, combined with a modelling approach, indicate that the southern limit of M. edulis is mediated by intolerance to high summer temperatures (Jones et al. 2009). Since 1960, seasonal air and water temperatures have increased along the eastern US seaboard, and south of Lewes, Delaware (38.8 °N) summer sea surface temperature (SST) increases have exceeded the upper lethal limits (32 °C) of M. edulis (Jones et al. 2010), resulting in geographic contraction of the southern, equatorward range edge of M. edulis, shifting it approximately 350 km north of the previous limit at Cape Hatteras. At the southern part of the range, high water and air temperatures cause mass mortality events, while along the more northerly portion mortality is caused by high temperatures during aerial exposure. Ultimately, water temperatures in excess of thermal tolerances have caused contraction of the mussel’s biogeographic range.
Two Mytilus conspecifics, M. trossulus and M. galloprovincialis, co‐occur on the West Coast of the United States, and both have exhibited changes in biogeographic ranges over several decades. The range of M. trossulus once extended from Baja California north to Alaska. However, when the Mediterranean mussel, M. galloprovincialis, was introduced into southern California in the first half of the 20th century, the invasive mussel was able to completely displace the native M. trossulus throughout the southern portion of its range. Fields et al. (2006) investigated whether biochemical adaptation to temperature might potentially play a role in invasion success. An examination of cytosolic malate dehydrogenase (cMDH), an enzyme known to exhibit distinct patterns of temperature adaptation in kinetic properties, showed that a minor change in structure permits M. galloprovincialis cMDH to function at warmer temperatures and may be a part of a broad suite of molecular adaptations that has allowed this species to displace its congener throughout the warmer part of M. trossulus’ original range in North America, and also in Japan. In a subsequent study, Lockwood & Somero (2011a) analysed a variety of physiological, biochemical and molecular systems, including cardiac function, enzymatic activity and gene expression, in the same two species. In all comparisons, M. galloprovincialis was more warm‐adapted than M. trossulus. Higher activities of enzymes involved in ATP generation show that the native M. trossulus is better adapted to colder conditions than M. galloprovincialis. The higher thermal tolerance of the invasive species is likely due in part to its enhanced ability to induce changes in the expression of particular genes and proteins in response to acute heat stress. Taken together, these data predict that M. galloprovincialis will continue to be the dominant blue mussel species along the warmer range of the California Current,2 yet its northward spread is limited. In support of this, Hilbish et al. (2010) reported that from 1995 to 1999 the poleward movement of M. galloprovincialis showed a reversal concomitant with a cooling phase of the Pacific Decadal Oscillation (PDO),3 an important driver of climate. M. galloprovincialis has declined in abundance over the northern third of its geographic range (~540 km), and has become rare or absent across the northern 200 km of the range it previously colonised during its initial invasion. The distribution of the native species M. trossulus has, however, remained unchanged over the same time period. The difference in SST between warm and cold phases of the PDO is small (~1 °C), but Hilbish et al. (2010) deduced that even this minor decrease during the cold phase of the PDO might be enough to retard larval development in M. galloprovincialis, such that recruitment is handicapped in northern waters.
The Asian green mussel, P. viridis, has been introduced into coastal waters of Florida; it was also reported in coastal Georgia in 2003 and has spread as far north as South Carolina (Benson 2019). Its current distribution is assumed to be limited by low temperatures during winter. To test this, Urian et al. (2011) analysed lethal and sublethal effects of cold water and air temperatures in two size classes of the mussel from Florida in an effort to determine the effects of current and forecasted temperatures on the potential for range expansion. Mussels were exposed to water temperatures of 14, 10, 7 and 3 °C for up to 30 days, or to air temperatures of 14, 7, 0 and −10 °C for two hours. Mortality was significantly increased at all water and air temperatures ≤14 °C. No differences in mortality rates were observed between small (15–45 mm) and large (75–105 mm) size classes except after exposure to 7 °C air, in which small mussels had higher mortality. The temperature threshold for survival in this population appears to be between 10 and 14 °C, suggesting that under present conditions P. viridis may already be at the northern edge of its potential range in the United States, unless with global climate change northerly flowing currents permit range expansion beyond the coastal waters of South Carolina.
Beside rising air and water temperatures, global climate change may also entail increases in precipitation, and estuarine species in particular may be exposed to increasing hypoosmotic stress due to decreasing salinities impacting on species’ distribution ranges (Somero 2012). Indeed, increased precipitation leading to reduced salinity in estuarine habitats may in some cases be more important in governing local distributions than changes in temperatures (Somero 2012; Braby & Somero 2006a,b). The open ocean has surface salinities between 33 and 37 psu, with an average of 35 psu. In contrast, estuaries and bays are subject to pronounced salinity fluctuations because of evaporation, rainfall and inflow from rivers. Many mussels, in particular Mytilus spp., are euryhaline (i.e. they can tolerate an extremely wide range of salinity, 4–40 psu, in their natural environment). In the northern Baltic, M. trossulus is living at the margin of its salinity tolerance (4.5 psu), and although dwarfed by the low‐salinity conditions,