Constructed sea walls, breakwaters, piers, docks, tidal canals, islands and other coastal infrastructure create habitat for marine plants and animals around ports, harbours, and other intensively settled coastal areas. The waters may be polluted by urban runoff or industrial outflows. Opportunities for introduction of invasive species e.g. through shipping mean that non-native species are common. Foodwebs are simple and dominated by species able to opportunistically colonise these structures through dispersal of eggs, larvae and spores. Commonly, these include algae and bacteria, and filter-feeders like sea-squirts and barnacles.