NEWS 115
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Large numbers of
Holothuria edulis
- the name means "edible sea cucumber" - are processed into
trepang.
Holothuria edulis
looks a bit like human faeces.
Pseudocolochirus
sp.
Planet Earth, just like us.
Breathing via the backside
The better-known sea stars and sea urchins
have only one important body opening,
sited in the center of the underside of the
body. This opening is used to eat food,
excrete feces, and release sex cells. Sea stars
and sea urchins don't have any respiratory
organ in the normal sense.They take inwater
via a sieve-like plate (the so-called
madreporite plate) and pump it through the
body using a sort of hydraulic system. This
not only serves to supply oxygen, but also
provides mobility. The numerous little feet
found on the bodies of starfishes, sea
urchins, and sea cucumbers, the so-called
ambulacral feet, are operated via a hydraulic
system!
A sea cucumber is fundamentally a very
elongated starfish lying on its side. The
change in body form resulted in the
madreporiteplate endingupon the inside of
the body. So how does water get inside?
Apparently it was impractical for sea
cucumbers to do everything via a single
body opening. It is probably more beneficial
to process the ingested sediment along the
length of the body as if on a conveyor belt
and excrete it at the rear: the anus was
invented by echinoderms! In addition the
existence of an anus improves the efficiency
of food intake. If the indigestible sediment
residues were to leave the sea cucumber via
the mouth opening, then a comparatively
large amount of this redundant material
would be picked up again. Sea cucumbers
also breath via their anal opening! They have
developed sac-like internal organs, so-called
respiratory or cloacal trees, whose
extremities are finely branched.These organs
are pumped full of water via the anus, and
extract oxygen from it. Sea cucumbers also
require water for their hydraulic system.
Beware,highly poisonous!
Sea cucumbers have no sensory organs
worth mentioning and only a very simple
nervous system. Flight from predators is not
an option. Because sea cucumbers are so
numerous and widespread everywhere, one
might think that many fishes and
crustaceans would have specialized on this
easy prey. Not so! All sea cucumbers possess
an extremely potent poison, known as
holothurin. This is concentrated in the so-
called Cuvierian tubules, threadlike
appendages in the end of the gut. In the
event of a serious attack the sea cucumber
expels these Cuvierian tubules through its
anus towards the aggressor. The Cuvierian
tubules are not only highly toxic for fishes,
but also very sticky.
Despite this sea cucumbers are eaten in the
Tropics! If you continue to irritate a sea
cucumber after it has expelled its Cuvierian
tubules then it will expel its entire innards,
which smell atrocious to us but are eaten by
people in the South Seas,either rawor as the
basis of a soup. The entire emptied and
