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Shellfish

The most important shellfisheries in the Southern Irish Sea have traditionally been for crustaceans (edible crab and lobster) and bivalve molluscs (mussels, cockles, oysters and scallops). Smaller fisheries also occur for other species including whelks, periwinkles, clams and velvet and spider crabs. Except for the Pacific Oyster (C. gigas) all are native to the Irish Sea and are widely distributed through the Eastern North Atlantic although some, such as the European mussel (Mytilus edulis) and the European (native or flat) oyster (Ostrea edulis), occur more widely. On a more local level the commercially-important shellfish have varying habitat requirements and between them occupy most of the coastal habitats available. Some species, in particular mussels and edible crab, are found in most habitats from rocky shore to mud-flats. Others are more discerning; such as cockles and scallops which prefer softer substrates of mud, sand or gravel or lobsters which inhabit rocky areas.  Mussels and cockles are the most restricted to coastal areas, preferring shallow waters up to around 5m below mean lowtide. In contrast, scallops, oysters, edible crabs and lobsters can also thrive in deeper waters over 50m. All of these species have a free-swimming larval stage so are able to disperse considerable distances: a valuable characteristic in adapting to the effects of climate change. In contrast, life-span and generation times vary substantially between species. Mussels and cockles have generation times of on average 1 - 2 years yet for edible crabs this may be as longas 20 years. While all are thought to have the potential to be long-lived, lower-shore mussels survive on average for just 2 -3 years as a result of high predation but the better-defended and deeper edible crab can survive for up to 100 years. While many of their needs overlap, these species of bivalve and crustacean have distinct feeding habits. As filter feeders, many bivalves inhabit estuaries and similar places where suspended particles and dissolved nutrients are abundant. This habit creates two important effects: bivalves are vulnerable to accumulating any toxins and diseases present, which has implication for human health when sold as food; on the other hand, mussel beds in particular are gaining recognition for their ecosystem services .  For example, they perform a valuable role in cleaning the sea, helping to ensure cleaner waters and beaches in the vicinity as a result. In contrast, crabs and lobsters are predators, often preying on shellfish as well as other marine life. As a result, they are far less likely to accumulate pathogens and toxins and pass them into the human food chain. Additionally, they arguably provide less valuable ecosystem services, although they are active scavengers and so play a part in nutrient recycling. Given that the major shellfisheries target species occur throughout the common habitats of the Irish Sea, it is likely that most effects of climate change in the region will impact on at least some sectors of the industry. For example, increasing run-off due to precipitation on land is likely to more strongly impact coastal and estuarine species such as mussels, while the effects of ocean acidification will be felt by all. However, this diversity may also provide more opportunity within the sector to adapt to the challenges as they arise.