This is the Common Periwinkle (Littorina littorea), a hardy marine gastropod widely distributed along the rocky shores of the North Atlantic, particularly abundant around the coasts of the British Isles. Its shell is typically thick and rounded, with a pointed spire and fine growth lines. Adults usually range in colour from dark grey to brown or black, while juveniles can sometimes appear pink or reddish, especially in sheltered or algae-rich environments. Most individuals grow to between 1.5 and 3 cm in height.

Location:
Native to the northeastern Atlantic, including the British Isles; also introduced to parts of North America. Found on rocky shores, in crevices, and among seaweed from the upper to lower intertidal zone.

Average Size:
Typically 1.5 to 3 cm in height.

Family:
Littorinidae (Periwinkles)

Species:
Littorina littorea

Ecological Role:
Well adapted to the challenging intertidal environment, the Common Periwinkle can tolerate exposure to air, changes in temperature, and varying salinity. It uses its muscular foot to cling tightly to rocks. When threatened or exposed at low tide, it withdraws into its shell and seals the entrance with an operculum—a tough, plate-like structure attached to the foot that helps prevent water loss and deters predators.

Feeding:
Periwinkles graze on microscopic algae and biofilms using a radula—a ribbon-like tongue covered in rows of tiny, rasping teeth. This grazing helps to maintain ecological balance on rocky shores by controlling algal growth.

Reproduction:
Unlike some marine snails that release eggs directly into the water, Littorina littorea reproduces through internal fertilisation. Females deposit fertilised eggs in gelatinous capsules, which later hatch into free-swimming larvae.

Ecological Importance:
The Common Periwinkle is a key species in intertidal ecosystems, serving both as a grazer of algae and as prey for shorebirds, crabs, and other marine invertebrates.