Spawn ribbon probably expands immediately on contact with seawater in the mantle cavity because it appears fully expanded as it comes into view from the shell.
Fertilization is external after the spawn leaves the female. Close proximity of an attendant male during laying is not required ( D. Kipling and C. Rickard, pers. com. 2017 and Holmes, 1997). Males in the general vicinity release milky white puffs of sperm and seminal fluid a few minutes after spawning starts and are probably stimulated to action by the release from the spawn of soluble chemicals. The released sperm swim to the spawn, probably in response to material from within the egg mass (Holmes, 1997).
Oban, Scotland. Sublittoral. April 2017. © D. Kipling.