Marine Mussels. Elizabeth Gosling
Magellan or ribbed mussel, Aulacomya atra, is native to South American coasts, from southern Brazil to Tierra del Fuego on the Atlantic coast and from there to El Callao, Peru on the Pacific coast (Pérez et al. 2013). The species is also found on the coasts of New Zealand, from Namibia to Port Alfred in South Africa and in the Kerguelen Islands in the Indian Ocean. Tierra del Fuego, Argentina is the southernmost limit of the species distribution. A. atra has been introduced into the North Sea and to UK coasts (www.marinespecies.org). Like Aulacomya atra, Choromytilus meridionalis, the black mussel, is native to the west coasts of Namibia and the west and south coasts of South Africa (Grant et al. 1984). The Asian date mussel, Arcuatula (= Musculista) senhousia, is native to the Malay Peninsula and northward to Siberia. It has invaded estuaries in the Mediterranean, Australia and New Zealand and the NE Pacific from San Diego, California to British Columbia, Canada. Such globally widespread introductions were probably through ships’ ballast water, by transport of planktonic larvae or adults rafting on debris or boat hulls, or via oyster mariculture (details in Asif & Krug 2012). The green crenella, Musculus discors, is common around most of the British Isles from Shetland to the Channel Isles, but also has a wider global distribution. It is found from the Arctic Circle south through the Bering Sea to Japan or to the Puget Sound in the Pacific, and south to New York in the Atlantic, including Norway, Sweden, Denmark, The Netherlands, Belgium, France, the North Sea, the western Baltic Sea and the Mediterranean Sea (www.marinespecies.org).
Bathymodiolus spp. are found in hydrothermal vents (hot springs) in the deep ocean. These typically form along the mid‐ocean ridges, such as the East Pacific Rise and the Mid‐Atlantic Ridge (MAR), located where two tectonic plates spread apart and new magma rises and cools to form new crust and volcanic mountain chains. Species such as Bathymodiolus azoricus and B. puteoserpentis dominate hydrothermal vent sites and cold seeps (vents with cooler, weaker flows), while small‐sized mussels in the genera Idas, Adipicola and Benthymodiolus inhabit sunken wood, vegetal debris and bones on the ocean floor. Another genus, Giganticus, inhabits warm water seeps near active volcanoes in the Southern Hemisphere (see Chapters 1 and 9).
Local Distribution
On a local scale, Mytilus mussels dominate the intertidal to subtidal regions of rocky shores. M. edulis has the widest distribution pattern in the genus, extending from high intertidal to shallow subtidal (1–5 m) and even deeper waters, from estuarine to fully marine conditions, and from sheltered to extremely wave‐exposed shores. On rocky shores of open coasts, the species attaches to rock surfaces and crevices, and in sheltered harbours and estuaries it occurs on rocks and piers, often as dense masses of one or more layers, with individuals bound together by byssus threads. Young mussels colonise spaces within the bed, increasing spatial complexity and providing numerous niches for other organisms (Seed 1996). At exposed sites, the mussel prefers gently sloping, slow‐draining platforms to steep rock faces (Seed 1976). On such shores, dense beds of small mussels, often several layers thick, blanket the rocks (Figure 3.1). Local distributions on the upper and lower shores can be very erratic on a seasonal and annual basis, due to irregular cycles of settlement, competition, predation, wave action, exposure to air and desiccation, and detachment during storms (see later). Death of underlying individuals may detach the mussel bed from the substratum, leaving the bed vulnerable to tidal scour and wave action (Seed & Suchanek 1992). On sheltered shores and in estuaries, fewer but larger mussels are found (Figure 3.2). In these habitats, infaunal beds may occur on gravel or even quite sandy areas, although it is likely that some harder substratum embedded within more sandy areas is required. Subtidal populations on pier pilings, oil platforms and culture ropes enjoy a virtually predator‐free environment, and this, combined with continuous submersion, allows M. edulis to reach a large size in a relatively short period of time (e.g. 50 mm shell length in six to eight months; Page & Hubbard 1987). In its native range, M. galloprovincialis is found from exposed rocky outer coasts to sandy bottoms (Ceccherelli & Rossi 1984). As an invader, it typically requires rocky coastlines with a high rate of water flow. The mussel is able to outcompete and displace native mussels and becomes the dominant mussel species in certain localities. For example, survivorship and tolerance to air exposure and desiccation in M. galloprovincialis are reported to be double those of any of the indigenous species in South Africa (Branch & Steffani 2004). Experiments in which mussels were held for 42 weeks at the high tide mark, where they experienced seven days of continuous exposure to air, resulted in 92% survivorship in M. galloprovincialis compared to 78% for Perna perna, 37–46% for Choromytilus meridionalis and only 0–10% for Aulacomya atra. Also, M. galloprovincialis grows faster than native mussels and has a reproductive output between 20 and 200% greater than that of indigenous species (Van Erkom Schurink & Griffiths 1993).
Figure 3.1 Layers of small mussels (Mytilus) on barnacle‐covered rocks on a semi‐exposed shore at Clogherhead, Louth, Ireland.
Figure 3.2 A cluster of mussels (Mytilus) on a sheltered shore at Lough Hyne, Cork, Ireland. Mussels in this habitat can reach shell lengths of 50–60 mm or greater.
In New Zealand, P. canaliculus is distributed widely throughout the two main islands, but it is more common in the warmer northern parts of the country. Dense beds of up to 100 individuals m−2 can be found in northern coastal areas, which can include rocky reefs, wharf piles and soft bottom habitats. Intertidal (mid‐littoral1) populations are limited by aerial exposure, while subtidal (down to 50 m) populations are limited by predation pressure. Environmental parameters, such as temperature and salinity, are strong determinants of the distribution of this species. While its temperature range is from 5.3 °C in the south to 27 °C in the north, this mussel appears to prefer the warmer waters of bays and estuaries in northern coastal areas. A wide range of salinities are tolerated by the species, although the optimum range is 30–35 psu (Alfaro et al. 2011 and references therein). P. viridis mussels generally inhabit intertidal, subtidal and estuarine environments with high salinity. They form a thick carpetlike growth (up to 35 000 individuals m−2) on rocky surfaces and submerged structures like wharves, pilings, breakwaters and buoys, and they play a very important role in rocky shore ecosystems. The species colonises submerged rocks, wood, concrete, metal, old submerged logs, boats, PVC pipes, ropes, muddy sea bottoms and even seagrass beds and aerial roots of mangroves. P. viridis tolerates a temperature range of 15–32.5 °C without much problem. For example, the species can survive a temperature of 39 °C for about 200 min (Rajagopal et al. 1995), which is considerably greater than the thermal tolerance of P. perna (see later). Also, P. viridis thrives well at winter water temperatures as low as 12 °C and has a higher degree of adaptability to salinity changes than P. perna (Rajagopal et al. 2006).
P. perna, a native of tropical and subtropical rocky shores, colonises jetties, navigation buoys, petroleum platforms, wrecks and other artificial hard substrata. Hicks & McMahon (2002)