Eggs - extra-capsular yolk
Upper: Part of egg ribbon of Thuridilla neona showing colourless veliger larvae and apparently unutilised orange-red extra-capsular 'yolk'. Photo: Corey Pittmann.
Lower: Section of egg ribbon of Chromodoris coi showing streaks of yellowish extra-capsular 'yolk'. Photo: Kheong Sann Chan.
Most sea slugs lay each egg in separate capsules which are attached to each other in a long string. To feed the developing embryo, a portion of yolk is usually deposited the egg in each capsule. In some groups, a substance which appears to be yolk is deposited in the egg ribbon, outside the capsules. In some chromodorids this extra-capsular yolk is deposited as a little cap on each capsule, while in others it is scattered randomly throughout the egg ribbon. In sacoglossans with extra-capsular 'yolk' it is arranged in a continuous string or an orderly series of blobs.
It has been thought that this 'yolk' is an extra source of food for the developing larvae but as the yolk is 'extra-capsular' it is clearly not available to the developing embryo-larva until it hatches from the capsule. Boucher (1989) observed the veligers of Chromodoris albopunctata [see message below #1972] eating the extra-capsular yolk in that species but that appears to be the only actual observation. There are some indications that the 'yolk' gradually vanishes as the larvae develop but Corey Pittmann's observations on Thuridilla neona [see ##9720] and Rachel Przeslawski's from Elysia maoria [#9583] show that in those species the 'yolk' volume remains virtually unchanged.
See relevant messages on:
Chromodoris coi #7229, #14013, #19259
Chromodoris collingwoodi #15530
Chromodoris albopunctata #1972
Chromodoris geometrica #5694, #20190
Chromodoris kuniei #16465
Chromodoris splendida #21334
Thuridilla neona #9719, #9720
Thuridilla hopei #7677,
Thuridilla livida #4571
Elysia pratensis #8143
Elysia maoria #9583
Elysia subornata #10198
Boucher, L.M. 1983. Extra-capsular yolk bodies in the egg masses of some tropical Opisthobranchia. Journal of Molluscan Studies, 49(3): 232-241.
Ortea, J.A., Luque, A.A. & Templado, J. (1988) Elysia picta Verrill, 1901, and Geitodoris pusae (Marcus, 1955), two amphiatlantic opisthobranch gastropods. J. Moll. Stud., 54: 243-247.
Reid, J.D. (1964) The reproduction of the ascoglossan opisthobranch Elysia maoria. Proceedings of the Zoological Society, London, 143(3): 365-393.
Rudman, W.B., 2004 (July 27) Eggs - extra-capsular yolk. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/extracaps