learn only 3 things about them ...
They are NOT vegetables or worms!
Like sea stars, they have 5-part symmetry but lengthwise.
cucumbers make up 90% of the deep sea ecosystem and can
be considered the dominant lifeforms of the planet!
Sea cucumbers are commonly seen on many of our shores. On sandy patches
large sea cucumbers may move about on the surface, while smaller ones
burrow. In coral rubble areas, they may hide under or cling to the
rubble, seaweeds or other animals such as sponges and ascidians. On
the rocky shore, some hide under the rocks. Many lie buried in the
sand, only their feeding tentacles emerging above ground.
What are sea cucumbers? Sea cucumbers
are animals and NOT vegetables as their common name suggests. Often
mistaken for worms, sea cucumbers are related to sea stars and belong
to Phylum Echinodermata. Class
Holothuroidea has about 1,200 known species. Some are colourful and
easily seen, others are well camouflaged or hidden under stones or
in the sand. Sea cucumbers can be round as balls, long and worm-like,
or even U-shaped. They are found almost everywhere from shallow areas
to the deep sea, from tropical to the Arctic and the Antarctic.
Wormy Echinoderms: Sea cucumbers
look quite different from their echinoderm cousins such as sea stars.
Sea cucumbers don't have arms, obvious spines or a hard skeleton.
They are instead soft and worm-like. But like other echinoderms, they
are symmetrical along five axes. Instead of lying on their mouths
like sea stars, brittle stars, sand dollars and sea urchins, sea cucumbers
lie on their sides with their mouths on one end and backsides on the
Sometimes mistaken for worms or
other sausage-shaped animals. Here's more on how to tell apart worm-like
animals and sausage-shaped
animals. Sometimes, the feeding tentacles of a buried or hidden
sea cucumber is mistaken for a sea anemone or other feathery animals.
Here's more on how to tell apart feathery
Feeding with their feet! Most
sea cucumbers have tube feet. The tube feet around the mouth are modified
into branched or feathery feeding tentacles. There can be 10-30 such
tentacles, which can be completely retracted into the body. Most have
tiny tube feet which are used to cling to surfaces and things. Only
a few use their tube feet to creep on. In some, tube feet emerge in
rows concentrated on the flattened lower half that faces the surface
(called the sole). In others, the tube feet may be scattered all over
their body. Sea cucumbers that lack tube feet include synaptid sea
cucumbers and those sea cucumbers that burrow.
Morphing sea cucumbers: Instead
of a hard skeleton, the bodies of sea cucumbers are mostly made up
of mutable connective tissue that they can rapidly change from soft
to rock hard. This tissue plus well-developed layers of muscles (around
the body and along its length) helps them to move about; flow into
narrow places to hide; or disuade predators from taking a bite out
of them. They also have ossicles (hard pieces of calcium carbonate),
but these are microscopic and widely distributed in their mutable
connective tissue. In some sea cucumbers, their ossicles can give
their skin a stiff and rough texture. Sea cucumber ossicles take on
a wondrous variety of delicate shapes and are used to identify sea
Breathing through their backsides!
A unique feature of some sea cucumbers is an internal breathing system
of branching tubes along the length of their bodies. Called respiratory
trees, most large sea cucumbers have a pair of these, each connected
to the opening on the backside. To breathe, the sea cucumber pumps
water in through its backside and up through the respiratory trees.
The water is then flushed out through the backside again. With this
constant flow of water, some tiny creatures find the backside of a
seacucumber a cosy and safe place to be! These include pea crabs and
the Pearlfish. Small or thin-walled sea cucumbers, however, simply
breathe through their skins.
Dust Busters of the Sea: Most
sea cucumbers eat detritus, collecting this in their sticky, mucus-covered
branched or feathery tentacles by sweeping the sea bottom or waving
their tentacles in the water. The tentacles are then inserted one
by one into the mouth and sucked clean. Some simply shovel sediments
into their mouths with their tentacles and process the edible bits,
leaving behind them a trail of sausage-like lumps of processed sediments.
Some sea cucumbers have been estimated to process 130kg of sediments
Repulsive vomiting: Being soft
and slow, sea cucumbers protect themselves by hiding or being unpleasant
to deal with. Some have toxins or distasteful substances in their
bodies. To repel and distract predators, some sea cucumbers vomit
their entire digestive system and other internal organs such as the
respiratory tree and even their reproductive organs. Depending on
the species, these can emerge from the front or back end of a sea
cucumber. Some species eject toxic or sticky strings (called Cuvierian
tubules) from their backsides. These immobilise the predator in a
gummy mess or release toxins. The sea cucumber eventually regrows
its innards and arsenal of Cuvierian tubules.
Please don't make sea cucumbers expel their intestines. Not all do
this. Those that do cannot eat until they re-grow their innards. The
guts or other noxious substances may irritate your skin and may hurt
you if these get into your eyes or mouth.
Cucumber babies: Most sea cucumbers
have separate genders and are usually either male or female. Their
reproductive organs are near the front of their body. In most species,
sperm and eggs are released simultaneously for external fertilisation.
Some spawning sea cucumbers raise their front end in a cobra-like
posture when releasing their eggs and sperm. Observations suggest
spawning is synchronised, sometimes more than one species spawning
cucumbers undergo metamorphosis and their larvae look nothing like
The form that first hatches from the eggs are bilaterally symmetrical
and free-swimming, drifting with the plankton. They eventually settle
down and develop into tiny sea cucumbers.
Role in the habitat: Sea cucumbers
dominated the deep sea ecosystem and can make up 90% of the creatures
found there. Here, some have very long tube feet to stilt walk in
the mud, others have webbed tentacles to briefly swim. Since this
ecosystem makes up about 70% of the surface of the earth, sea cucumbers
can be considered the dominant organisms on earth! Being so numerous,
sea cucumbers are important to the ecosystem. Their larvae are probably
important in plankton-based food chains. The constant processing of
sediment by countless sea cucumbers possibly plays a role in nutrient
Human uses: Some large sea cucumbers
are considered delicacies and harvested for food. The harmless Garlic
bread sea cucumber (Holothuria scabra) is among those species
of sea cucumbers that are collected as a Chinese delicacy. They are
gutted and dried for sale as 'trepang' or 'beche-de-mer'. Sandfish,
which can grow up to 40cm and weigh 1.5kg, are the most widely collected
and among the more valuable sources of beche-de-mer. Tests indicate
these sea cucumbers contain toxins. They must be properly prepared
before they are safe to eat. Others may be collected for the live
aquarium trade. Scientists are also studying the toxins of sea cucumbers
for possible medical and other applications.
Status and threats: Collection
of the garlic bread sea cucumber has been a traditional activity for
centuries in many coastal peoples in many parts of the world ranging
from Madagascar to the Philippines. However, the recent high market
price of this delicacy has resulted in increased collection in last
20 years. Some edible sea cucumbers are globally threatened by over-collection.
In some areas, such sea cucumbers have become scarce. In others, specimens
collected are smaller and have to be harvested from deeper waters.
Efforts to culture these edible sea cucumbers have only just started.
Many of our sea cucumbers are on the list of threatened animals in
Singapore. In Singapore, the main threat is habitat loss due to reclamation
or human activities along the coast that pollute the water. Trampling
by careless visitors and overharvesting can also have an impact on
striking colours of the Sea apple
warns of its toxic nature.
Pulau Sekudu, Dec 03
These tiny red sea cucumbers are
often mistaken for worms.
Chek Jawa, Aug 02
Feeding tentacles of
the Thorny sea cucumber.
Changi, Dec 03
Feeding tentacles of the
Purple sea cucumber.
Chek Jawa, Jan 03
Tube feet used to cling to the stone.
Changi, Aug 05
The Ashy pink cucumber can eject
its intestines to deter predators.
Beting Bemban Besar, Jun 09
Poop produced by a sea cucumber.
Pulau Semakau, Aug 11
Some sea cucumbers are
smooth and transparent!
Chek Jawa, Sep 03
Often mistaken for worms,
synaptid sea cucumbers are
commonly seen on a sponge.
Pulau Sekudu, Jul 03
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- Ong J. Y. and H. P. S. Wong. Sea cucumbers (Echinodermata: Holothuroidea) from the Johor Straits, Singapore. 10 July 2015. The Comprehensive Marine Biodiversity Survey: Johor Straits International Workshop (2012) The Raffles Bulletin of Zoology 2015 Supplement No. 31, Pp. 273-291.
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