Marine Mussels. Elizabeth Gosling

M. trossulus along much of its historic range in southern California (Shinen & Morgan 2009). Other examples are those between M. californianus and the sea palm, Postelsia palmaeformis, on the Pacific coast (Dayton 1973; Blanchette 1996). The sea palm grows quickly and may overgrow M. californianus, eventually causing mussels to be torn free in the waves. This creates the space necessary for settlement of spores and recolonisation by Postelsia. Then there is the competition between M. galloprovincialis and the large indigenous limpet, Scutellastra argenvillei, on the west coast of South Africa, where the mussel is capable of forming dense, almost monospecific stands low on the shore. A survey indicated that at wave‐exposed locations, the abundance of M. galloprovincialis changes with exposure (Steffani & Branch 2003). At such locations, the mussel covered up to 90% of the primary substratum, whereas in semi‐exposed situations it was never abundant. As the cover of M. galloprovincialis increased, the abundance and size of S. argenvillei on rock declined, becoming confined to patches within a matrix of mussel beds. Both species were absent from sheltered shores and diminished where wave action was extreme. Comparisons with previous surveys indicated that exposed sites now largely covered by the alien mussel were once dominated by dense populations of the limpet. Therefore, the results of this survey provide circumstantial correlative evidence of a competitive interaction between M. galloprovincialis and S. argenvillei, and suggest that wave action mediates the strength of this interaction. The presence of mussel beds provides a novel settlement and living substratum for recruits and juveniles of S. argenvillei, albeit at much lower densities than in limpet patches. Adult limpets are virtually excluded from the mussel beds owing to their large size, which indicates the unsuitability of this habitat as a replacement substratum after competitive exclusion from primary rock space.

The American slipper limpet, Crepidula fornicate, was unintentionally introduced to Europe in the 1870s with oysters imported for farming purposes. Since the limpet is a filter feeder, trophic competition and associated negative effects when epizootic on bivalves have been assumed (Figure 3.14). Thielges et al. (2005) were the first to experimentally test the effects of C. fornicata on survival and growth of its major basibiont, M. edulis. In two field experiments (each lasting two weeks), epigrowth by C. fornicata resulted in a four‐ to eightfold reduction in survival of mussels, equivalent to a mortality of 28 and 30%, respectively. Shell growth in surviving mussels with attached C. fornicata was three to five times lower compared to unfouled mussels, but similar to that with artificial limpets. As a causative agent, interference competition in the form of changes in small‐scale hydrodynamics due to C. fornicata stacks was suggested. This could result in a high energy expenditure for byssus production of mussels. In general, interference and not exploitation competition is suggested to be the major impact of epizootic C. fornicata on its basibionts (organisms that are host to epibionts) in Europe.

Figure 3.14 Stack of four American slipper limpets, Crepidula fornicata, attached to the mussel Mytilus edulis.

      Source: From Thieltges (2005). Reproduced with permission from Inter‐Research.

      Typically, however, intraspecific competition for space is a more serious problem than interspecific competition, in that heavy spat fall of mussels on to adult beds can cause the underlying mussels to suffocate, thus loosening the entire population from the rock surface (Seed 1976). See Chapter 5 for information on interspecific and intraspecific gamete competition in Mytilus taxa.

Climate Change and Potential and Observed Impacts on Marine Mussels

Climate in a narrow sense is usually defined as the average weather, or more rigorously as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. Climate encompasses patterns of temperature, precipitation, humidity, wind and seasons. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization (https://public.wmo.int/en). Our climate is now changing at a rate faster than any seen in the last 2000 years (www.ecology.wa.gov). This is due to rising levels of carbon dioxide and other heat‐trapping (greenhouse) gases in the atmosphere. The primary greenhouse gases in Earth’s atmosphere are water vapour (H₂O), carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄) and ozone (O₃). These act like a blanket around Earth, causing it to warm – a phenomenon referred to as ‘global warming’. According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by about 0.8 °C since 1880. Two‐thirds of the warming has occurred since 1975, at a rate of roughly 0.15–0.20 °C per decade (Figure 3.15). Wide‐ranging impacts of rising temperatures include rising sea levels, melting snow and ice, more extreme heat events, fires and drought and more extreme storms, rainfall and floods. This rise in global average temperature is attributed to an increase in greenhouse gas emissions. The link between global temperatures and greenhouse gas concentrations, especially of CO₂, has held throughout Earth’s history (Lacis et al. 2010). CO₂ concentrations in the atmosphere are now well over 400 ppm, their highest levels in over 800 000 years (Figure 3.16). Globally, we emit over 36 billion tonnes of CO₂ per year, and this continues to increase. There are large differences – more than 100‐fold – in per capita CO₂ emissions between countries (Richie & Roser 2020). For example, the United States has contributed most to global CO₂ emissions to date, accounting for 25% of cumulative emissions. This is followed by the EU‐28 (22%), China (13%), Russia (6%) and Japan (4%). There are large inequalities in CO₂ emissions: the world’s poorest countries have contributed less than 1% of emissions, but will be the most vulnerable to climate change impacts. Rising levels of atmospheric CO₂ is increasing ocean acidity, which has been predicted to double over the next 100 years in an uncontrolled emission scenario (Fauville et al. 2013). Approximately one‐third of the anthropogenic carbon added to the atmosphere is absorbed by the oceans. Uptake of atmospheric CO₂ results in a decrease in ocean water pH, an effect referred to as ‘ocean acidification’ (OA; see later).

The Intergovernmental Panel on Climate Change (IPCC) is the UN body responsible for assessing the science related to climate change. It was established by the United Nations Environment Programme and the World Meteorological Organization (WMO) in 1988 to provide policymakers with regular scientific assessments concerning climate change, its implications and its potential future risks, as well as to put forward adaptation and mitigation strategies. The IPCC’s reports are comprehensive and balanced assessments of the state of knowledge on topics related to climate change. There are different types of reports, but all go through a rigorous process of scoping, drafting and review to ensure the highest quality. To date, the IPCC has 195 member states. The Paris Agreement is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC) dealing with greenhouse gas emissions mitigation, adaptation and finance. As of November 2019, 190 of 195 IPCC member countries have signed the Agreement. Its long‐term temperature goal is to keep the increase in global average temperature to well below 2 °C above pre‐industrial levels and to pursue efforts to limit the increase to 1.5 °C, recognising that this would substantially reduce the risks and impacts of climate change. The Paris Agreement was intended to become fully effective in 2020. Predictions of global environmental conditions for the end of the century coupled