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Nudibranchs
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Nudibranchs are essentially snails without
shells, and their name literally means "naked gill". In most species, the
gills are prominently displayed on their dorsal surface.
Some kinds of nudibranchs have a tuft of gills located on their
backs at the posterior end, which are used only for respiration. Other
kinds of nudibranchs have many finger-like structures on their backs
called cerata, which function for gas exchange and often defence.
Sea slugs have a pair of tentacles (called rhinophores) located on
top of their heads, which biologists believe are used as sensory organs to
assist in finding food and seeking a mate.
Many nudibranchs are brilliantly coloured while others are subtly
coloured to match their background, thereby providing camouflage.
Defence
Since nudibranchs have lost their protective shells, they require
alternative means of defence. The bright colouration seen in many species
is believed to warn potential predators that the nudibranchs contain
distasteful or even toxic compounds. Many of the dorid nudibranchs
(Suborder Doridacea) are strongly scented, which is thought to be a
chemical anti-predator warning.
Many sea slugs feed on sea anemones and hydroids (Phylum Cnidaria),
which contain stinging cells called nematocysts. The nudibranchs are able
to block the discharge of these nematocysts during digestion, and they are
then passed to special storage sacs in the cerata. The adopted stinging
cells form part of a defensive mechanism against any predators that
attempt to prey on the nudibranchs.
Some nudibranchs can also swim short distances when disturbed by
predators - they contract their body muscles and undulate through the water
while flapping their cerata.
Habitat
Nudibranchs are benthic organisms, meaning they live on the ocean bottom.
They can be found crawling over rocks, seaweeds, sponges, corals and many other
substrates.
Sea slugs range from the lower intertidal zone to depths of over 700 m.
They are found all over the world, including the British Isles, the tropics and
even the Antarctic.
Diet
All known nudibranchs are grazing carnivores and as a whole they feed on a wide
variety of animals including sponges, hydroids, tunicates, anemones, corals, sea
pens, bryozoans, barnacles, and sometimes other nudibranchs!
Each species of nudibranch tend to be very selective feeders, and may
only eat one certain prey species (e.g. the giant swimming nudibranch,
Dendronotus iris, only feeds on the tube dwelling anemone, Pachycerianthus
fimbriatus.
Most nudibranchs have a ribbon of teeth called a radula, the structure of
which is usually adapted to the animal's particular prey items. For example,
many dorid nudibranchs (Suborder Doridacea) have broad radulae with numerous
teeth for grazing on sponges, while most aeolid nudibranchs (Suborder Aolidacea)
have narrow radulae and strong jaws for feeding on hydroids and bryozoans.
Reproduction and Development
Nudibranchs are simultaneous hermaphrodites, which means that they
possess both male and female sex organs at the same time. This strategy
increases the probability of finding a mate, since every mature individual of
the same species is a potential partner (self-fertilization is very rare).
After mating, nudibranchs lay their egg masses either on or near the
organism on which they feed. These egg masses vary in shape, size and colour
depending on species. Some sea slugs lay single coils of eggs, while others are
in the shape of a thick ribbon wound into a spiral. The egg masses are often
white, but they can also be red, pink, orange or any other colour depending on
the species.
Egg development can take between 5 and 50 days, and is strongly
influenced by temperature. Warmer waters generally result in a shorter embryonic
period.
Usually the eggs develop first into a larval form called a veliger,
which drifts in the ocean currents as plankton. Specific environmental
conditions trigger the larvae to settle and metamorphose into the adult form.
This larval dispersal is important in the successful exploitation of new areas,
since adult nudibranchs move very slowly and cannot travel long distances.
Click on image to see in 1024 x 768
Hypselodoris apolegma
When I saw this little
fellow in the shop aquarium set up I had to find out more about this
creature. I took these images thumb nailed above and searched the internet
for information on it. I found an Canadian site which was quite detailed,
this can be found at the following link:
I sent the site an email
a while back but received no reply. So, with his link firmly on the page,
I hope the author does not mind me going ahead and reproducing the article
online here in the UK. As this page is for information purposes only I am
sure that I fall into the "educational" bracket.
Interesting Facts
Some nudibranchs in tropical waters feed on corals (Phylum Cnidaria),
which contain photosynthetic single-celled algal symbionts called
zooxanthellae. These nudibranchs are able to remove the algae intact from
the coral's tissues and store them in their cerata, where the algae
photosynthesize and provide the host nudibranch with a continuous supply
of manufactured sugars.
Beachwalkers can search for nudibranchs in tidal pools at low tide
- try looking under loose rocks or near animals that they are known to
feed on (e.g. sponges and bryozoans). One of the best ways to locate
nudibranchs is by spotting their egg masses, which are usually quite
visible - the parents are likely nearby!
If you are lucky enough to find nudibranchs in tide pools or while
diving, please do not attempt to pick-up or touch them. These creatures
are very soft and easily damaged, and you may hurt them inadvertently.
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