Egg capsules 1.8mm, 2.1mm and 1.9mm diameters, excluding rim, on a leaf of Zostera. Each capsule is a low dome with a flat rim (1) adhering to the leaf, and a central lanceolate opening (2) covered by a membrane that ruptures when the eggs hatch so the young can escape (Fretter & Graham, 1962). The capsule is colourless transparent, almost invisible, so often all that is noticed is the pile of 40 to 400 opaque, white ova within (See 31 Rissoa membranacea. ). Central capsule in this image has 246 ova visible. The parents were collected on 1st April and kept at 6ºC (approximately sea temperature) until 9th April when two containers of seawater each had a few leaves of Zostera and 4 male and 7 female R. membranacea put into them. One container was kept at 6ºC in a refrigerator and the other in a room varying between 10ºC and 13ºC. On 11th April, six egg capsules were found on the Zostera in the warmer pot and none in the cooler. This conforms with Waren (1996) who stated that spawning starts at 8ºC to 9ºC.
The image above was taken on 12th April; the right hand capsule is the most advanced as the ova are slightly yellowed and at the four-cell cleavage stage (visible if image zoomed in on, 3) so they may have been laid within 24hours of experiencing the warmer conditions. Egg capsules laid in captivity by specimens from ELWS, Orkney, Scotland. April 2018. Leg. I. Williamson.
30.1 Rissoa membranacea.
