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Factors Affecting Local Distribution
ОглавлениеMarine mussels on a local scale dominate the intertidal to subtidal regions of rocky shores (see earlier). Mussels extend 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, mussels attach to the rock surface and crevices, and in sheltered harbours and estuaries they attach on rocks and piers, often occurring 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. It comes as no surprise that temperature and salinity, which play a key role in geographic distribution, are also important in determining species distribution on a local scale. For subtidal species, additional factors such as water depth, substrate type, currents, turbidity and predation and competition play an important role. Upper distributional limits in the intertidal zone are believed to be determined primarily by desiccation and temperature stress, while lower limits are determined by biological factors, particularly predators. Anthropogenic factors such as waterborne pollutants, introduced species and disease can also be significant in determining local distribution patterns.
Rocky shore ecosystems are governed by the tidal movement of water, which creates zonation patterns from high to low tide. The area above the spring high‐tide mark is called the supralittoral (splash) zone, which is regularly splashed but not submerged by seawater, except during storms with high tides (Purcell 2018). The area around the high‐tide mark is known as the intertidal fringe; organisms in this sparse habitat include anemones, barnacles, crabs, lichens, limpets, mussels, periwinkles and whelks, with very little vegetation. Below this zone is the intertidal, with three zones: high, mid and low, based on the average exposure. Each of these zones has its own characteristic animal and plant community. The high intertidal area is only flooded during high tides; common organisms are anemones, barnacles, chitons, crabs, isopods, mussels, sea stars and snails. This zone can also contain rock pools inhabited by small fish and larger seaweeds. The mid intertidal zone is covered and uncovered twice a day with saltwater from the tides. Therefore, temperatures are less extreme due to shorter direct exposure to the sun, and salinity is only marginally higher than ocean levels. However, wave action in the mid intertidal zone is generally more extreme than in the high tide and splash zones. Typical organisms are snails, sponges, sea stars, barnacles, mussels, crabs and brown and green algae. The low intertidal is only exposed on unusually low tides. Common organisms in this region are brown and green seaweed, crabs, hydroids, mussels, limpets, sea cucumber, sea urchins, sea stars, shrimp and snails. These organisms are not well adapted to long periods of dryness or to extreme temperatures. Below the low‐tide mark is the littoral zone, which can again be divided into three zones based on areas of tidal action, from shallow to deep: the supralittoral zone, the eulittoral zone and the sublittoral zone. The littoral zones are much more stable than the intertidal zones. Different types of oysters, star fish, sea urchins, coral, crabs and anemones live in the littoral zones, some of which are significant predators of marine mussels (see later).