Flat Periwinkle

Overview

The Flat Periwinkle (Littorina obtusata) is a small marine snail commonly found on rocky shores throughout the North Atlantic. These gastropods are highly specialized, living almost exclusively among the brown seaweeds upon which they feed.

Distribution & habitat

This species is widely distributed across rocky coastlines where fucoid algae, such as Bladder Wrack (Fucus vesiculosus), Egg Wrack (Ascophyllum nodosum), and Serrated Wrack (Fucus serratus), are present. They typically inhabit the mid to lower tidal zones and are occasionally found in sub-littoral areas. They are known to be tolerant of varying levels of wave exposure and lower salinities.

Appearance

The shell is characterized by a flattened spire and a large, tear-drop-shaped aperture, with the final whorl accounting for approximately 90% of the shell's total height. While the surface may appear smooth, it is finely reticulate under close inspection. Their coloration is remarkably variable—ranging from olive green and yellow to brown or black, often featuring banded or chequered patterns—which helps them blend into their seaweed environment. They can be difficult to distinguish from the closely related Littorina fabalis without internal examination.

Biology & behavior

Flat periwinkles are herbivores that graze directly on the seaweed fronds they inhabit. To protect themselves from predators and desiccation, they can seal their shells tightly using a circular, hardened door called an operculum. They are dioecious, meaning individuals are either male or female, and they can be subject to trematode infections which may influence their population dynamics.

Reproduction

Breeding can occur throughout the year, with mating activity often peaking in late spring and early summer. Females lay whitish, oval or kidney-shaped egg masses directly onto seaweed fronds or occasionally on rock surfaces. These eggs typically hatch into crawling young after about four weeks.