Another
Flabellina sp., common in the
Indian Ocean side of Thailand
Amazing
colour, beautiful form, funny appetites, unique defenses
and one that practices feminism and masculinity, the
nudibranch has it all.
While the sights of larger reef inhabitants amaze other
divers, my hubby and I dive with magnifying lens strapped
on to our BCD. Many a time, I have got amused laughs,
curious stares and a flow of questions regarding this
strange piece of dive equipment. Why? Playing Sherlock
Holmes underwater has many rewards especially when hunting
down nudibranchs.
Nudibranchs have fascinated divers and continue to hold
their audience captive with dazzling colours and shapes
thus earning the distinction of being trendsetters.
Oddly enough, many divers have passed one by without
recognition, as these trendsetters are masters of forms.
Some have two horned tentacles (rhinophores) on their
head, others feathery gills on their back. Many are
brightly coloured though some blend into their environment.
While most are small (1-8 cm) there is the Spanish dancer
that can grow to 50 cm. With at least 3,000 species
in the world’s oceans, there are no strict rules as
to where they may be found. However, the vast majority
inhabit the warm waters of the Indo-Pacific reefs.
Spanish
Dancer (Hexabranchus sanguineus)
is found among soft coral at Anemone
Reef, off Phuket.
What Are They?
Nudibranchs, more commonly known as the true sea slug,
are closely related to Gastropods (shell bearing organisms).
The species lacks a shell and are soft-bodied creatures
that use their foot-like appendage to crawl over the
surface of the reef in search of food. If you have ever
seen one move, they do not seem to be in much hurry
to find the next meal at all!
Nudibranch literally translates as ‘naked gills’, a
perfect description of its breathing apparatus. In contrast
to their relatives the gastropods, who have their gills
well hidden from prying eyes, most nudibranchs take
great pride in displaying their gills in the form of
feathery structures on their back or sides. Despite
their variety, all nudibranchs can be grouped into 4
main types which is indeed a blessing with 3,000 of
them on the reefs!
Chromodoris
quadricolor also known as
the ‘ pyjama ‘ nudibranch
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Harlequin
Nudibranch (Doridaceans)
The largest group, these nudibranchs are characterized
by the combination of a pair of tentacles on the top of
their head and a tuft of feathery gills on the rear part
of the back. But beware, just when you thought that's
simple, here come the phyliids who also belong to this
group but lack conspicuous gills and have lumpy ridges
on theirs backs!! Did I say it was easy?
Glossodoris
symmetricus seen around Koh Phi Phi is a colourful nudibranch
inhabiting the intertidal rocky reefs.
Tubercular
Nudibranch (Aeolidaceans)
The second largest group are the easiest to recognize
with their long tapering bodies covered with numerous
finger-like appendages (cerata) that are non-retractable.
Members
of Risbecia tyroni are generally found crawling in pairs,
one attached to the back of the other.
Side-gilled Nudibranch (Dendrotaceans)
These have several feathery gills along each side of the
back as well as a cup-like sheath around each rhinophore
on the head that can be retracted.
The
flamboyant Nudibranch Nembrotha kurbayana is my favourite
nudibranch.
Veiled
Nudibranch (Arminaceans)
With a fleshy extended lobe on the head, some members
possess longitudinal ridges on their back while others
have well-developed cerata.
Life
As A Nudibranch
Nudibranchs lead a nomadic life appearing when and where
they please, from 100m abyssal walls to shallow, sandy
flats. It is because of this characteristic that they
often elude biologists. How can you study them
Comparison
of size: the magnifying glass was 75mm in diameter and
the nudibranch Nembrotha kurbayana was about 2.5cm
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Glossodoris
atromarginata is quite common in shallow reefs around
Koh Samui and on wrecks in shallow waters.
when
they cannot keep an appointment at a certain place and
time?
What do they eat? All nudibranchs have a
brittle tongue called the radula to feed and use their
tentacles to locate food and potential mates. Surprisingly
enough, most are carnivores with a craving for hydroids
(organisms with stinging cells called nematocyst), corals,
sponges, fish eggs, crustaceans and tunicates (sea squirts).
Others feed on algae and bryozoans (moss-like animals).
But the most industrious have taken up farming by growing
algae (zooxanthellae) on their cerata which produce carbohydrates
for the nudibranch through photosynthesis. They carry
breakfast, lunch and dinner with them as they travel.
Such talent deserves applause!
Reproduction, as with all living organisms also takes
place in the life of the nudibranch. The nudibranch is
hermaphroditic meaning that both sexes are present in
a single nudibranch but self-fertilization does not occur.
Eggs are deposited on the reef as colourful ribbon strands,
often on favourite food such as sponges. Once hatched,
the larva is free swimming and may travel with oceanic
current until a suitable surface is found. How long do
they live for? Nudibranchs that feed on slow growing organisms
such as sponges and sea pens can live up to a year and
others that feed on transitory organisms such as bryozoans
have short life spans of a few weeks.
Survival for the nudibranch is dependent on chemical warfare.
Their bright colors serve as a warning to would be predators
of their inedible qualities. Their flesh is frequently
toxic or distasteful due to various chemical secretions.
The aeolids feed on sea anemones and hydroids with an
ulterior motive! They accumulate the nematocysts (stinging
cells) from these animals, concentrate them in their finger-like
appendages and use them as weapons.
Eating for a reason comes takes a new meaning with these
critters!
Designers, farmers, warriors and survivors, remember
to look out for these colorful reef inhabitants with their
amazing abilities on your next dive.
abcdivers@hotmail.com
The
Phyllidiidae family of nudibranchs are the most commonly
seen nudibranch in the Gulf of Thailand. Picture shows
Phyllidia varicosa
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