Flat Periwinkle

Littorina obtusata, flat periwinkles, are sea snails. They grow to 1.5 cm in height. They can be brown, yellow, olive green, and black. They are mostly found in the colors of brown, reddish brown, or yellow, with few being red or white because of the abundance of brown seaweed. L. obtusata have the name flat periwinkle because they have a flattened spire and a large teardrop-shaped aperture. The last whorl of the shell makes up 90% of their height. The maximum recorded shell length is 13.5 mm. The operculum is yellowish to brown. This species is mainly littoral, but also has gill respiration. This means that they can survive both in water, and extended amounts of time on land.

This marine species occurs wherever brown seaweeds grow. It is widely distributed: the Baltic Sea, in European waters from Norway down to Southern Spain, in the Mediterranean Sea, and in the Northeast Atlantic Ocean along the Gulf of Maine.

Littorina obtusata are herbivores that primarily eat algae. They scrape algae from surfaces using their radula, which contains thousands of tiny teeth. They are commonly associated with, and feed upon, the dominant intertidal brown algae, Ascophyllum nodosum.

Female Littorina obtusata deposit spawn masses on Ascophyllum nodosum, Fucus serratus, Fucus vesiculosus, and sometimes on rock surfaces. Their eggs are whitish and oval, or kidney shaped. They are encased in a mass of transparent jelly. Littorina obtusata eggs take up to four weeks to fully develop and for the snail to emerge. Sexual maturity is reached at about two years. They continue to reproduce all year round, with their maximum from spring to fall

Littorina obtusata are very good at adapting to new areas and predators. Their largest predator is the Carcinus maenas, the green crab. As the threat of these crabs has grown, Littorina obtusata's shells have thickened to protect themselves. Wader birds also threaten Littorina obtusata's population. They forage in intertidal zones, preferring the low-water mudflats, but when they are submerged, they use the Ascophyllum/Fucus belts.