Each Nudibranch is both male and female (simultaneous hermaphrodites)
producing sperm and eggs. To mate (copulate) two nudibranchs come
together side by side and (usually both) pass sperm sacs through a tube
(copulatory apparatus) on the right behind the head. Copulation
may take seconds or a whole day, depending on the species. Both then go
their own way and lay 1-6 egg masses in a anti-clockwise spiral which may
contain millions of eggs. Hermaphrodism gives these animals a survival
advantage in that mature animals can copulate and produce eggs.
Nudibranchs are hermaphrodites (having male (testis) and female
(ovary) gonads) and can not self fertilise. The reproductive system
(genitalia) is complicated for several reasons:
To prevent self fertilization.
Ejection of sperm (autosperm).
Reception of sperm (allosperm).
Laying of eggs (oviposition).
The sperm is stored until the eggs develop, when fertilisation occurs.
Egg masses (spawn) can contain millions of mucus sheathed egg capsules
attached to the substrate varying size, shape and colour.
Chemical defence is used to protect the spawn against predation. Nudibranchs
take no part in the rearing of their young. Egg masses take three forms:
Ribbons attached to the substrate along one edge.
Cylindrical capsule filled cords, attached on one-side by a thin capsule
free sheet. Some dendronotaceans produce tougher and basically similar
cords.
Small kidney shaped jelly bags attached on one side.
Development takes two forms:
Direct Development
Eggs are laid on or near a food source and the young (veliger) hatch
fully developed and commence feeding. The eggs are laid in a gelatinous
mass, appear firm and are larger than in other types of development
to provide sufficient food to nourish the embryo through to the juvenile
stage.. Endemic species tend to be direct developers.
Larval Development
Eggs are laid in a filomous mass on any suitable surface. When the young
(veliger) hatch they are carried in the current. They eventually
settle unto a food source and continue developing into adults. A
lot of the veliger don't make it adulthood, hence the high number
of eggs laid. Two forms of larvae development occur;
Planktotrophic in which lots of small eggs are produced and the
larvae (veliger) swim and feed on the plankton.
Lecithotrophic larvae (veliger) swim for a short period in
the plankton and do not feed, although they may assimilate ions. Eggs need
more yolk to give the larvae the energy for the brief period in plankton.
Egg size is larger than that of Planktotrophic developers.
The size, shape and colour of spawn varies greatly and
can assist identification.
