Marine Mussels. Elizabeth Gosling
of patches and of individuals within patches, can provide detailed mechanistic understanding of patch structure and dynamics.
Minchinton et al. (1997) monitored the recovery of intertidal algae and sessile macrofauna after a rare occurrence of scouring sea ice denuded the intertidal area of an exposed rocky shore in Nova Scotia, Canada. They found that barnacle cover was restored soon after the ice scour and macroalgae cover in about two years, but that it took considerably longer (four to six years) for the mussel beds (M. edulis and/or M. trossulus) to recover. Similar results have been reported by Brosnan & Crumrine (1994), who trampled 250 experimental plots in the intertidal zone for a period of one year and then monitored recovery. The algal–barnacle community recovered in the year following trampling, but mussel beds (M. alifornianus) and their associated species had not recovered within two years following cessation of trampling, and over this period no mussel recruitment was recorded. Two trampling studies (Beauchamp & Gowing 1982; Goldstein 1992) were performed 14 years apart at the same intertidal site, Santa Cruz on the west coast of California, with areas differing in intensity of trampling. There was little evidence of impact or change in either studies, although similar studies elsewhere have documented clear trampling effects (Ghazanshahi et al. 1983; Durán & Castilla 1989; Underwood & Kennelly 1990; Keough et al. 1993; Keough & Quinn 1998; Clark et al. 2002), as have others on the California coast (Beauchamp & Gowing 1982; Ghazanshahi et al. 1983; Goldstein 1992; Addessi 1994). In 1995, Van De Werfhorst & Pearse (2007) carried out a new study at the same Santa Cruz sites as Beauchamp & Gowing (1982) and Goldstein (1992), using a different strategy sampling regime, and were able to demonstrate trampling effects. Mussel bed cover, with the associated higher number of species within it, decreased with increased human trampling, while the frequency of bare rock areas increased. Consistent with the previous studies, the two brown algal species that were abundant on the less trampled sites were completely absent at the most trampled. These results, in conjunction with those of the earlier studies, indicate that rocky intertidal assemblages can be resilient to human trampling, and that sampling scale and design are important in evaluating and monitoring trampling impacts (Van De Werfhorst & Pearse 2007). Micheli et al. (2016) evaluated the separate and combined influences of disturbance from storm waves and disturbance associated with human trampling of rocky shores by conducting an experiment mimicking controlled levels of trampling at sites with different wave exposures and before and after a major storm event in central California, United States. Their results show that trampling and storm waves affected the same taxa and had comparable and additive effects on rocky shore assemblages. Both disturbance types caused significant reduction in percent cover of mussels (M. californianus) and erect macroalgae, and resulted in significant reorganisation of assemblages associated with these habitat‐forming taxa. A single extreme storm event caused similar per cent cover losses of mussels and erect macroalgae as 6–12 months of trampling, which combined additively rather than synergistically. Mussel beds in wave‐exposed sites were more vulnerable to trampling impacts than algal beds at protected sites. Mussels and erect macroalgae recovered within five years after trampling stopped. These results suggest that impacts from local human use can be reversed in relatively short periods. See Mendez et al. (2019) for a similar study on Brachidontes spp. in Patagonia, Argentina.
Waves are not only associated with hydrodynamic stress but can also carry heavy loads of sand, periodically disturbing intertidal shores through sand burial or sand scour (reviewed in McQuaid et al. 2015). Acting as an agent of disturbance, sand removes plant tissue, epiphytes or invertebrates with poor attachment to the rock surface through scouring and decreases light, oxygen and substratum available to organisms through burial. It can lead to temporary species impoverishment by selective elimination of maladapted species, although in the longer term it may also enhance species richness by increasing habitat heterogeneity, allowing within‐shore coexistence of sand‐intolerant species and those associated with sand deposits (McQuaid & Dower 1990).
Unexpectedly, sand stress strongly affects the survival of M. galloprovincialis and P. perna individuals but is not related to their physiological tolerances and does not explain their vertical zonation. When buried under sand, P. perna mortality rates are higher than those of M. galloprovincialis in both laboratory and field experiments, yet it is P. perna that dominates the low shore where sand inundation is recurrent (Zardi et al. 2006). Although both species accumulate sediments within the shell valves while still alive and sand buried, the quantities are much greater for P. perna, causing intense visible damage and clogging of the gills, which explains its higher mortality rates. Presumably, the accumulation of sand within the shell of P. perna is linked to its gaping behaviour. Wave and sand stress vary also in time, altering the timing and mortality rates of the two mussel species (Zardi et al. 2008). During periods of high sand accumulation in mussel beds, the indigenous species has increased mortality rates that are higher than those of M. galloprovincialis, while the pattern is reversed during winter, when wave action is high (Zardi et al. 2008). When sand stress is high, the less stable secondary substratum of sand and shell fragments weakens the attachment strength of mussels living within a bed. Consequently, the indigenous species loses its advantage in attachment strength over the invasive species, and this results in a seasonal shift in the competitive balance between the two.
There has been considerable interest in the ecology of patches, especially concerning the successional6 events leading to their recolonisation. In the case of M. californianus, the patches are initially colonised by diatoms, then by ephemeral algae such as Ulva and Porphyra spp. and then by perennial algae. These successional stages are followed by acorn barnacles, Balanus glandula and Semibalanus cariosus, then goose barnacles, Pollicipes polymerus and sometimes M. trossulus, and then by M. californianus. The latter needs secondary space (as opposed to primary space, i.e. rock surface), such as substrates like algae, barnacles and byssus threads, for larval settlement, but later becomes competitively dominant over all other sessile species. These events take about five years, depending upon degree of wave exposure (Dayton 1971; Paine & Levin 1981; Wootton 2002).
The mechanism driving these successional events is generally thought to be competition with larger, later‐colonising species assuming competitive dominance over smaller, early‐succession species, until finally sea mussels dominate. M. californianus exhibits competitive dominance: small barnacles are smothered, larger barnacles such as Semibalanus cariosus are overgrown and abraded and goose‐neck barnacles, Pollicipes polymerus, are slowly crushed to death. Both M. californianus and Semibalanus cariosus have refuge in size from predation by whelks Nucella spp., and thus could potentially monopolise all space were it not for predation by sea stars, Pisaster ochraceus, from lower positions on the shore and for creation of new colonisable spaces by log damage. Studies on the Olympic Peninsula, Washington, United States also show that by consuming the early successional stages, predators such as whelks and birds and herbivores such as chitons and limpets actually accelerate succession (Suchanek 1981; Paine 1984).
The importance of large‐scale ocean currents in the global distribution of bivalves has already been dealt with in this chapter. Locally, areas with strong currents usually provide favourable feeding conditions for bivalves. However, very strong currents can have an inhibitory effect on feeding and consequently growth. Also, strong currents may prevent larval settlement and byssal attachment of spat, ultimately resulting in local variability in recruitment.
Fishing methods can affect bivalve abundance directly by causing significant mortality and indirectly by causing shell damage. In Spain, mussel seed from intertidal exposed rocky shores is the method most used by farmers to seed ropes in mussel culture areas (Peteiro et al. 2007). This practice, while legal, must have a detrimental effect on mussel beds and their community structure, although to date there is no documented evidence of damage.
Biological Factors
Just as humans greatly appreciate the delicate flavour of bivalves, so also do a whole range of other organisms from groups as diverse as fish, birds, mammals, crustaceans,