All Creatures on this page
are of the Class GASTROPODA Univalves
CLICK HERE FOR THE SEA SLUG
FIELD GUIDE
Order: NUDIBRANCHIA
(NUDIBRANCH) - True Sea Slugs
Nudibranchs represent a fascinating and colourful
life form. Their name is pronounced nudi - brank and it
means "naked gill". As the name suggests the vast
majority have gills outside their body and they have
evolved without the protection of a shell. Illustrated in
image one. They can be found throughout the world's
oceans at all depths with the largest range in the Indo
Pacific region.
The only place you will not find them are high
energy areas, such as heavy surf close inshore where
their soft body could be damaged.
Instead of a shell for protection from predation
they use highly effective chemical warfare. If distressed
some species will secrete toxins so powerful that within
a 50cm radius most marine life would be killed. They
obtain this toxin from their food source containing
chemical compounds which they store. There are many
different species of Sea slugs feeding on a wide variety
of different foods, including corals, sponges, ascidians,
bryozoa and hydroids packed with single celled alge
called zooxanthellae. "Solar Powered Slugs" store and
farm Zooxanthellae which supplies them with nutrients,
for more information please visit the Glaucidae section
within the Sea Slug Fieldguide.
Some species accumulate the poison in their Cerata,
tentacle type sacks located along their backs away from
their head, others store in the dorsal body wall. In
addition to extracting the corals poison they will often
obtain the same striking colouration for both camouflage
and as a warning to anything that might want to eat them
that they are poisonous or pretending to be.
Image 3 shows a purpureomaculosa Nudibranch feeding
on a sponge, leaving the white fibre skeleton of the
sponge remaining. Image 4 displays a Nembrotha Nudibranch
feeding on ascidian and
Roboastra in image 5 feeds on bryozoan. All are
carnivorous, some cannibalistic.
All have a fleshy foot, which runs the length of
their body. They produce a slimey mucus and using muscle
or hair movement on the foot they travel. Their
Rhinophores - chemical sensors are located on the head as
can be seen in image 6. They are used for detecting food
odors and other Nudibranchs.
They are hermaphroditic possessing both male and
female reproductive systems. Their gonad is located on
the right side of their body close to the head, this is
clearly visible in image 7. When mating they position
themselves head to tail and join organs, as seen in image
8. They are rarely able to fertilize themselves. Once
mating has finished they crawl off and only one of the
pair will lay eggs on coral of a similar colouration,
which is usually their preferred food.
This cryptic behavior offers some protection to the
eggs before hatching. The egg mass, which in most cases
is laid in an anticlockwise spiral is made up of
thousands of small eggs giving a speckled appearance,
illustrated in image 9.
Once hatched they are planktonic for a period which
varies among species. Nudibranchs consuming hydroids and
bryozoans live quite short lives, 4-5 weeks. Others
eating slow growing sponges and sea pens live around one
year. There are approximately 3000 described species of
Nudibranchs and they vary in size from 4 - 600mm.
Order: CEPHALASPIDEA
Family: Aglajidae - Headsheild Slugs
Often confused with Nudibranchs they appear to be
absent of a shell but most species do possess one which
is usually internal and quite small. As their name
suggests Headsheild Slugs have a well developed head
shield offering protection as they plough under the
surface of the sand where they live. During the day they
can sometimes be observed on rocks whilst mating. They
are hermaphrodites and mate in chains of three or more
taking on the role of male, female or in some cases both
at the same time. This can be seen on image 10.
Order: CAENOGASTROPODA
Family: Velutinidae (Lamellarids)
Very little is known of this species. Documented on
Sipadan with different species found throughout the Indo
Pacific. We photographed Coriocella sp (image 11) in the
Cocos keeling Islands. Similar to Nudibranchs they have
evolved and lost their shell or when present in some
species it is buried in the fleshy mantle wall. They too
use chemical warfare.
Order: SACOGLOSSA - Sap
Sucking Slugs
Family: Elysiidae
Unlike Nudibranches all but a few are herbivorous
feeding on algae. Found in all the worlds oceans and
seas, tropical and temperate. Usually they are well
camouflaged some blending to the same colour as their
food source others are brightly coloured.
Order:
NOTASPIDEA - Sidegill Slugs
Family: Pleurobranchidae
Members of this family vary a great deal in body
form, however they all have gills located between the
mantle and foot and this is usually on the right side of
the body - hence their common name. They are grazing
carnivores living mostly in shallow coastal oceans and
seas.
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FIELD GUIDE
