ea cucumbers make up the class
Holothuroidea
in
zoological
systematics. Around 600 species are known
from all over the world. They live exclusively
in the sea and all species feed on small
organisms. Fundamentally they use two
methods of doing this. The first group
consumes vast quantities of sand and mud,
sorts the digestible from the indigestible
inside the body, and then excretes the
indigestible as purified sand. The second
group catches plankton from the water
using modified mouth tentacles.
Extra-terrestrials?
All non-sessile*, multicellular animals on
Earth are bilaterally symmetric - except for
the echinoderms! Bilaterally symmetric
means that there is a matching right and
left half to the body. By contrast
echinoderms - which include the crinoids,
brittle stars, starfishes, sea urchins, and, of
course, the sea cucumbers - are radially
symmetric, and have five body axes or a
multiple of that number.
Life on Earth probably arose on just one
occasion, and for this reason all life forms -
be they bacteria, fungi, plants, or animals -
are related to one another in some way.
This can be seen most clearly in
biochemistry, where the obtaining of
energy and metabolism are based on the
same principles in all life forms. But there are
also fundamental agreements in anatomy.
So how did the radial symmetry of the
echinoderms come about? Are they
ultimately extra-terrestrial life forms that
NEWS 115
11
Hand on heart: did you answer the opening question with "sea cucumbers"? And yet it is true! Sea cucumbers represent
some 90%of the biomass of the ocean depths, and around 70%of the surface area of the Earth lies beneath the oceans,
so the calculation is correct.
Do you know what are the commonest
animals in the world?
Sea Cucumbers are a model of evolutionary success!
by Levin Locke
Sea water
S
Colochirus crassus
is a gorgeous plankton-feeding sea cucumber.
The mouth tentacles of
Pseudocolochirus
look like soft corals. All photos: F. Schäfer
originally came here from another galaxy?
No, probably not. As the larvae of
echinoderms are perfectly normal bilaterally
symmetric. The fundamental development
of the animal egg cell after fertilization has
even been studied using the eggs of sea
urchins, as sea urchin eggs are so large,
damage-resistant, and above all transparent.
The initial development of a human embryo
doesn't differ from that of the embryo of a
sea cucumber. Sea cucumbers are natives of
*Many sponges and a number of other sessile
animals are amorphous, i.e.without a regular form.
