Spawning female with no closely proximate male. Unlike the trochid Phorcus and Gibbula species which release individual unattached eggs, the female C. zizyphinum lays, indiscriminately over rock and weed, a long, thin spawn-ribbon, 250 to 750mm long by 2 to 5mm wide, attached at one end to the substrate. The ribbon has up to 700 large ova with mean diameter 0.47mm (Holmes, 1997).
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). Ova have large yolks. Trochophore and veliger stages, passed within the spawn mass, can be seen rotating and then crawling within ova. Young hatch as crawling snails. Unusually for a British trochid, there is no planktonic phase.
Oban, Scotland. Sublittoral. April 2017. © D.Kipling