The flat periwinkle is actually two species; Littorina obtusata and L. fabalis Despite coming in a range of colours from bright orange through yellow, green and even muddy brown or striped, the colour is not species specific. This fingernail-sized mollusc can be found on wracks on the mid to low shore, it is a grazer. It is believed the colour comes partially from the food choices but is also due to predation. Different colours appear to attract different predators and the degree of shelter (from waves) could limit the predators present so a colour that is not attractive would dominate.
Whatever the reason for the colour variation it is fun to gather a few together, the end result is like a bag of sweets. Place them in a pool and you might find some grow legs and scuttle away. These tough little shells are a prime location for hermit crabs. A green alga is often on the shell and this might inadvertently aid camouflage. If you are patient the true owner will appear and if you have a camera it is well worth taking a video, despite being basically a snail, it is surprisingly cute.
Easy to identify as they have a distinct lack of spire to the shell, are smooth unlike rough periwinkles and much larger than small periwinkles. The difference between the two forms is not as simple as you might hope, they do vary. One distinctive feature, however, is the penis. If like me you prefer to respect the privacy of this fabulous little creature, then get a few together turn them over and the more rounded (fatter and higher up the shore) are likely to be L. obtusata. The sexes are separate and they lay egg masses on wracks. These hatch into adult-like crawling young after 4 weeks. Sexual maturity is reached after two years and an individual can live for three or more years. L. fabilis only lives for one year, it is thought this is due to greater crab predation as it is more frequent lower down the shore than L. obtusata forcing the mollusc to reproduce younger.
Periwinkles have an operculum, that covers the shell entrance when the snail retracts, it also reduces water loss when out of the water. They are normally found on fucoids and seem happy out of the water. They do have gills though and so prefer not to live higher up the shore where they would be out of water for longer periods.