Spawn mass is laid on weed, stone or rock as a gelatinous cylinder about 25mm long, containing about 1000 ova. Uniquely among British prosobranch gastropods, it is wound in a tight spiral of two to four turns and up to 3mm diameter. The spiral can be wound clockwise or anti-clockwise. A single female may lay 10 000 ova in seven masses over a two week period.

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