Читать книгу Marine Mussels - Elizabeth Gosling - Страница 45

Organisms in the rocky intertidal zone have to cope with being out of water at regular intervals. For those in the high intertidal, emersion times are longest, and consequently these individuals are often subjected to temperature extremes and desiccation. Upper distribution limits for a species are set by its ability to tolerate such extremes via a well‐coordinated set of physiological, behavioural, biochemical and molecular adaptive responses (McQuaid et al. 2015).

Considering the enormous amount known about the ecology and physiology of mussels, the main focus in this section will be on Mytilus species and the impacts of thermal stress on them in the rocky intertidal zone – a habitat that has been described as among the most physically harsh environments on Earth (Tomanek & Helmuth 2002), but which has emerged as a model system for investigating the ecological impacts of global climate change (Zhou et al. 2018; see later). When mussels are submerged during high tide, their body temperature is close to that of the surrounding water. However, during aerial exposure at low tide, body temperatures are driven by the interactions of climatic factors such as solar radiation, cloud cover, wind speed, relative humidity and air and ground temperatures, which drive the flux of heat into and out of the mussels’ bodies (Helmuth & Hofmann 2001; Helmuth 2002 and references therein). Consequently, body temperatures can be considerably higher (≥20 °C) than those experienced during submersion and can vary substantially from surrounding air and substrate temperatures (Helmuth 1999). It is important to note that the rate of heat transfer between an organism and its environment is determined to some extent by the size and morphology of the organism, and can be strongly affected by characteristics such as colour and material properties (Helmuth 2002). Also, changes in shell coloration, presence or absence of the periostracum and algal growth on the shell are all likely to modify a mussel's reflectivity and hence its body temperature (Helmuth 1999). Consequently, organisms exposed to identical environmental conditions can experience quite different body temperatures (Helmuth 2002). For example, barnacles keep a relatively large proportion of their total surface area in contact with the underlying substrate and display body temperatures that are tightly coupled with ground temperature. In contrast, mussels are predicted to have body temperatures that are largely decoupled from the temperature of the underlying substrate, at least while living in beds of conspecifics, and are often several degrees warmer than the surrounding air. Daily variations in temperature may also be extremely different between individuals that are just a few metres apart but located on surfaces facing different directions, or in or out of crevices or other shaded areas (Helmuth et al. 2010).

Until recently, measuring body temperatures in the intertidal zone was inefficient or unfeasible at large spatial or temporal scales, as temperature loggers were large and expensive and required frequent servicing (Lima et al. 2011). In addition, the intertidal zone, with its rapidly varying temperatures, tremendous wave forces and hard substrata, often caused loss of sensors and data loggers. In the early 2000s, however, advances in technology made it possible to measure the body temperatures of intertidal organisms over long time scales, because commercially available temperature loggers were sufficiently small and robust to be deployed for long periods of time. As wtih the organism being monitored, the colour, morphology and mass of a temperature logger can significantly affect the temperature that it records. Therefore, biomimetic sensors/loggers (robomussels) that are specifically matched (modified) to the thermal characteristics of the mussels being measured have been developed. These can provide realistic measurements of body temperature in the field, over a broad variety of temporal and spatial scales (Fitzhenry et al. 2004). Robomussels have the shape, size and colour of actual mussels (Figure 3.4), with miniature built‐in sensors that track temperature inside the mussel beds. The fact that they are self‐contained, robust and inconspicuous makes it unlikely that they will be intentionally destroyed. Robomussels are made from a polyester resin with a black colouring, which is poured into moulds created from Mytilus shells. A TidbiT logger (Figure 3.4) is embedded in the polyester resin, which after hardening is smoothed and shaped to resemble a real mussel. The logger battery lasts for more than five years and the memory stores ~42 000 12‐bit temperature measurements (enough for more than two and a half years) with a sampling frequency of 30 min (can be set from 1 s to 18 hr). The loggers have an accuracy of ±0.20 °C at 0–50 °C (details in Fitzhenry et al. 2004; Lima et al. 2011). In the field, robomussels are usually deployed on hard rock in intact mussel beds using epoxy putty, taking care to ensure that they are completely surrounded by other mussels, since loggers deployed as solitary individuals tend to yield abnormally high readings. Logger programming and data retrieval is done through the instrument’s LEDs, which are exposed on the outside of the robomussel. An optical USB interface allows users to offload data in seconds. Biomimetic loggers have also been used in animals such as limpets, barnacles, snails and seastars. See Helmuth (2002) and Fitzhenry et al. (2004) for the potential pitfalls of logger use, Lima et al. (2011) for different types of biomimetic data loggers and Judge et al. (2018) for recent advances in temperature logging in intertidal systems.

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.

Figure 3.5 Example of fluctuations in temperature experienced over one month (August 1999) at a horizontal microsite. Daily maxima were calculated from temperature data collected every 5–10 min. The highest daily maximum was recorded as the monthly extreme (‘acute’) high temperature at each site. The average of the daily maxima was calculated as a measure of ‘chronic’ high‐temperature exposure. Similarly, average daily minima and monthly minima were calculated.

Source: From Helmuth & Hofmann (2001). Reprinted with permission from The University of Chicago Press.

In a more recent study, Olabarria et al. (2016) investigated the effect of a heatwave on the physiological and behavioural responses in monospecific or mixed aggregations of the invasive mussel Xenostrobus securis, which has successfully colonised the inner part of the Galician Rias Baixas (NW Spain), where it co‐occurs with the commercially important mussel M. galloprovincialis (see Chapter 10). In a mesocosm experiment, mussels were exposed to simulated tidal cycles and similar temperature conditions to those experienced in the field during a heatwave that occurred in the summer of 2013, when field robomussels registered temperatures up to 44.5 °C at low tide. In monospecific aggregations, M. galloprovincialis was more vulnerable than X. securis to heat exposure during emersion. However, in mixed aggregations, the presence of the invader was associated with lower mortality in M. galloprovincialis. The greater sensitivity of M. galloprovincialis to heat exposure was reflected in a higher mortality level, greater induction of Hsp70 protein and higher rates of respiration and gaping activity (opening of shell valves to permit evaporative cooling), which were accompanied by a lower heart rate (bradycardia). It appears that the invader enhanced the physiological performance of M. galloprovincialis, highlighting the importance of species interactions in regulating responses to environmental stress. In a complementary study, Nicastro et al. (2012) found that under conditions of heat stress, aggregations of the gaping mussel P. perna exhibited lower mortality rates than isolated occurences or aggregations of the nongaping mussel M. galloprovincialis, because the gaping behaviour of P. perna ameliorated stressful environmental conditions of mussels through evaporative cooling (see Chapter 7 for details on gaping). The drawback of this response is an increased risk of desiccation. Large mussels have greater amounts of water available in their tissues than small ones (Kennedy 1976; Helmuth 1998), which provides greater protection from desiccation and enables them to use evaporative cooling for longer periods. To test this, laboratory experiments were conducted to determine the sensitivity of mussels (M. trossulus) to the full range of temperatures and desiccation levels experienced in the field (Jenewein & Gosselin 2013). Mussels (1–2 mm shell length) experienced a threshold of heat tolerance at 34 °C and a threshold of desiccation tolerance at vapour pressure deficit levels of 1.01 kPa. Extended periods of temperatures reaching or exceeding lethal levels for newly settled M. trossulus occur relatively rarely in Barkley Sound, British Columbia, Canada, the study mussel collection site, which has a consistently high M. trossulus settlement. Extended periods of temperatures reaching or exceeding lethal levels for newly settled M. trossulus occurred relatively rarely at this site, whereas lethal levels of desiccation occurred often during the recruitment season and were usually sustained for several hours, indicating that desiccation appears to be a substantially greater threat to recently settled M. trossulus than heat. Mussels became highly tolerant to desiccation when they reached a size of 2–3 mm shell length. This closely corresponds to the size at which juvenile M. trossulus relocate from protective filamentous algal habitat to adult habitat, suggesting ontogenetic shifts in habitat use by juvenile M. trossulus are a response to changing sensitivity to desiccation. In a scenario of global warming, survival of newly settled mussels, and thus possibly the persistence of mussel populations, will likely depend even more upon the persistence of protective microhabitats created by filamentous and fucoid algae.

Body temperature during aerial exposure is driven by multiple interacting climatic factors, and extremes during low tide far exceed those during submersion (Helmuth 2002). Physiological effects of thermal stress in both low‐ and high‐tide conditions were compared between M. galloprovincialis and M. trossulus under laboratory conditions (Schneider 2008). To simulate the natural range of tidal thermal stress, mussel populations of the two species were established in seawater tanks that mimicked a daily tidal cycle by filling and draining at different times. During high tide, mussels were completely immersed in water, while during low tide, they were exposed to one of three aerial temperature treatments: 20 °C (cool), 25 °C (warm) or 30 °C (hot). A subtidal control group, which remained underwater during the entire tidal cycle, was also established. Separate experiments were carried out in two different water temperatures, 18 and 12 °C, which represented those pertaining where the species co‐occur in California (Braby & Somero 2006a) and found during the summer along the west coast of the United States. In 18 °C water, there was no effect of the aerial treatments on growth or survival in either species. In contrast, in 12 °C water, aerial exposure affected the survival and growth of both species. Growth and survival rates of M. galloprovincialis were higher in all conditions than those of M. trossulus, especially in the 18 °C water experiments and in the aerial exposure treatments of the 12 °C water experiment. M. galloprovincialis seems to be warm‐adapted with regard to both low‐ and high‐tide thermal stress. These results suggest that M. galloprovincialis will spread, at least compared to M. trossulus, as water and air temperatures increase in a global warming scenario.

M. californianus has a large distributional range (Washington to southern California, United States) and experiences a correspondingly wide geographical range in temperature. Therefore, one might ask to what extent its temperature regime can be predicted from latitudinal position, or even from a few temperature measurements taken at a site in air or water over a relatively short period of time. To investigate this, Helmuth et al. (2006) used long‐term (five years) high‐frequency temperature data from biomimetic sensors deployed at 10 locations along the West Coast of the United States (Figure 3.6). Their objective was to test the hypothesis that local modifying factors, such as the timing of low tide in summer, can lead to large‐scale geographic mosaics of body temperature. The results indicated that patterns of body temperature during aerial exposure at low tide varied in physiologically meaningful and often counterintuitive ways, over large sections of the species’ geographic range. An evaluation of spatial correlations among sites to explore how body temperatures change along the latitudinal gradient showed that ‘hot spots’ and ‘cold spots’ exist where temperatures are hotter or colder than expected based on latitude. Four major hot spots and four cold spots were identified along the entire geographic gradient, with at least one hot spot and one cold spot occurring in regions as climatically distinct as Washington/Oregon, Central California and Southern California. This pattern was driven by differences in the timing of low tides among regions. At northern latitudes, low tides in the summer occur at midday and expose intertidal organisms to high aerial temperatures, while at southern sites low tides during summer occur mostly at night and organisms are thus often submerged during hot days. Temporal autocorrelation analysis of year‐to‐year consistency and temporal predictability in the mussel body temperatures revealed that southern mussels experience higher levels of predictability in thermal signals compared to northern animals. Helmuth et al. (2006) also explored the role of wave splash at a subset of sites and found that while average daily temperature extremes varied between sites with different levels of wave splash, yearly extreme temperatures were often similar, as were patterns of predictability. In summary, rather than simple latitudinal gradients, these intertidal mussels experience a complex thermal mosaic, with many potential variables affecting the thermal state of an intertidal organism. The results of this study highlight the importance of quantitatively assessing biogeographic patterns in temperatures and other environmental variables at scales relevant to the organisms themselves and of forecasting the impacts of changes in climate across species ranges.

Figure 3.6 Map of the 10 deployment sites used by Helmuth et al. (2006) along the United States Pacific coast.

Source: From Helmuth et al. (2006). Reproduced with permission from John Wiley and Sons.

Between 1998 and 2016, Helmuth et al. (2016) further deployed biomimetic sensors that approximated the thermal characteristics of intertidal mussels at 71 sites worldwide. Biomimetic loggers (robomussels) were programmed to record at intervals of 10–30 min and were left in the field for periods up to nine months before they were removed for downloading and replaced with another logger. The loggers were used at multiple intertidal elevations to estimate temperatures of the mussels M. californianus (west coast of North America), M. edulis and Geukensia demissa (east coast of North America), M. chilensis (Chile), P. perna (South Africa) and P. canaliculus (New Zealand). Unmodified (‘off‐the‐shelf’) commercial loggers were also deployed on rock surfaces at multiple sites (Australia, Ireland, Mexico, Scotland, United Kingdom, United Staes), which recorded temperatures relevant to barnacles, newly settled mussels and other organisms that are sufficiently small that their temperatures mirror those of the underlying rock. Comparisons against direct measurements of mussel tissue temperature indicated errors of ~2.0–2.5 °C during daily fluctuations that often exceeded 15–20 °C. Geographic patterns in thermal stress, based on biomimetic logger measurements, were generally far more complex than expected, based only on ‘habitat‐level’ measurements of air temperature or SST. This unique data set provides an opportunity to link physiological measurements with spatially‐ and temporally‐explicit field observations of body temperature in intertidal mussels.

In cold conditions, mobile intertidal species can hide in rock crevices or migrate to deeper water to avoid freezing. But sessile bivalves, often exposed to subzero temperatures during winter, do not have such protection. In northeastern Canada, temperatures can drop to −35°C in winter. Mussels (M. trossulus) survive such low temperatures even when their tissue temperatures are as low as −10°C (Williams 1970), with large adults surviving laboratory conditions of −16 °C for 24 hours (Bourget 1983). As much as 60% of their extracellular fluid (ECF) is frozen at this temperature. The unfrozen ECF becomes more concentrated with solutes, and this process draws water by osmosis out of cells, thus lowering the intracellular freezing point. The high osmotic concentration of the ECF places an osmotic stress on the cells that can damage membranes and enzymes. This damage can be minimised through the production of cryoprotectants (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.