or sea gooseberries as they are also known, are one of the most beautiful
animals the ocean has to offer. They are not related to jellyfish but form
a group of their own: the Ctenophores.
The Ctenophore depicted in this article is
Pleurobrachia pileus. It grows not much bigger than two centimetres.
Therefore you can't really call it a microscopic organism. But it has many
features that are interesting to study under the microscope. Ctenophores
can be found in every ocean. Some even reach considerable sizes.
|The sea gooseberries
can be easily collected. They often swim close to the surface. With a little
net or even a glass jar they can be scooped from the water. When put in
a glass container they can be kept alive for many days.
One of the most obvious characteristics are
the locomotory organs, eight rows of comb-like structures that show irridescent
colours when they reflect direct sunlight.
They are real predators. With it's long tentacles
it captures food like copepods, the larvae of many organisms and other
small creatures floating in the plankton. It brings the food to it's mouth
which is shown in the pictures at the head of this article. Most textbooks
however show the organism with it's mouth facing downward. I always see
them with the tentacles pointing down.
One row of combs under the microscope.
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|When I was recently
studying some plankton samples I saw this very tiny egg. It was no more
than half a millimetre but to my surprise contained a larva of a ctenophore.
All body organs were already there. Also the comb plates can be seen. I
didn't see eight as in the adult but four. Even the tentacles are visible.
(This larval development is so different from that of jellyfish that it
is clear that they are not closely related animals)
Most striking is the aboral sense organ.
In the pictures of the adult organism it is difficult to see because it
is so small compared to the size of the animal. The picture of the larva
clearly shows the sense organ. A small dome contains a little round structure
that is connected to the eight combs. With this the organism can control
it's position in the water.
This picture was taken with oblique
illumination to give some contrast to the very transparent organism.
By adding a few dots of vaseline under each corner of the coverslip I prevented
the egg capsule from being squashed by it. Because of the shallow depth
of field you see only a circle of what is in fact a perfect sphere.
Footnote: Thank you to Claudia Mills
for kindly telling us the species name of the Pleurobrachia
above. Visit Claudia's web page on Ctenophores at http://weber.u.washington.edu/~cemills/Ctenophores.html
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