The flat periwinkle grazes on detritus and microorganisms. It also feeds on green seaweeds such as sea lettuce (Ulva lactua) and species of Enteromorpha (2). Breeding may take place throughout the year, but tends to vary depending on temperature (4). The sexes are separate (individuals are either male or female), and fertilisation occurs internally after copulation (2). Egg laying is timed to coincide with the spring tide; the eggs are laid in the sea in gelatinous capsules that usually contain around three eggs (although up to 9 eggs per capsule has been known) (4). The egg capsules float in the sea and free-swimming 'veliger' larvae hatch after a few days. After 6 weeks spent in the ocean, the larvae settle on the shore. The young periwinkles attain sexual maturity at two or three years of age and may live for up to five years (2).
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