The mussel is a filter-feeder; it filters bacteria, plankton, and detritus from the water (3). When large beds of this gregarious species form, individuals are bonded together with threads of byssus. Predation is the greatest cause of mortality; a range of predators take mussels, including dog-whelks (Nucella lapillus), crabs, sea urchins, star-fish, and birds such as the oystercatcher (Haematopus ostralegus) (3). Although mussels seem fairly defenceless, remarkably they are able to fend off marauding dog whelks and other predatory gastropods; a number of mussels work together to immobilise the predator with bysuss threads (3). Organisms that attach to mussels, such as seaweeds and barnacles, may increase the risk of the mussel becoming detached by wave action; however, mussels are able to sweep their foot over their shell, which may help to minimise the likelihood of such an organism becoming attached (3). The sexes are separate, fertilisation occurs externally and spawning peaks in spring and summer (2). The larval stage is free-swimming and planktonic for around 4 weeks (2), before settling first on filamentous organisms such as seaweeds (3). After growing for a while, they detach and drift in the water on a long byssal thread; a mode of dispersal likened to that of young spiders floating through the air on a silk thread (2). After four weeks or so, the young mussel will have settled again, this time on a mussel bed (2). Young mussels are thought to have evolved primary settlement on filamentous substrates in order to avoid having to compete with adult mussels (3). Mussels are host to the pea crab (Pinnotheres pisum), and a copepod (Mytilicola intestinalis), both of which are not parasites, as was once thought, but commensal organisms (they benefit from living with the mussel, but the mussel is not affected) (2). Furthermore, mussel beds provide habitats for a variety of marine life, and support higher levels of biodiversity than surrounding mudflats (3). The biodiversity of the bed increases with its size and age (3).