algae animal background beach bivalve coast England Great Britain kent mollusc natural nature sea seashell seaweed shell shore texture aquatic bivalves canada life line low marine mark Molluscs ocean pacific rock starfish tide Vancouver Island water blocks cave chalk cliff coastline face hole landscape seashells tideline white bare exposed mussel Mytilus edulis rockpool shellfish stone Arion ater creep english garden invertebrate orange pest red repulsive revolting shell-less Slime slimy slug snail blue coastal Cornish cornwall mussels Mytilus rocks seascape view waves band barnacles Cirripedia low tide bleached cluster coiled shells colored coloured exoskeleton faded gastropods land snails metal fence post small snails spirally coiled shells terrestrial snails