Ocean
Watch
Monday, March 16, 1998
Sponge’s way of cloning
requires only seawater
A reader recently sent me a newspaper clipping (source
unknown) that said, "Break a sponge into thousands of pieces. Scatter
them in a tank. Next day, they'll be reformed in the exact shape of the
original sponge."
"I will be grateful," wrote the reader,
"if you can tell me how it is possible that such a thing can
happen."
Such a thing can happen because sponge biology is like
the biology on "The X-Files": weird. I mean, if Sojourner had
found water on Mars, there probably would have been sponges in it.
On this planet, sponges rank one notch above
single-celled animals, such as amoebas, on the evolutionary scale. A
sponge has no nervous system and no muscles. This means no stomach, no
eyes, no pain, no movement.
In spite of this, sponges manage to do some remarkable
things, beginning with building unusual skeletons.
Some sponges keep their shape by forming spicules, tiny
rods that brace up the sponge's cells. Spicules are made either of calcium
carbonate, like the skeletons of stony corals, or of silica, a
glasslike material that can stick into, and hurt, human
skin.
Other kinds of sponges stand up with the support of
tough, rubbery fibers called spongin.
Sponges containing this soft type of skeleton are the
bath and cleaning sponges familiar to us all.
What do sponge cells do in the midst of all this
supportive scaffolding? Most of them line up to form a network of canals
covered with tiny, beating hairs. These hairs move water into the sponge
through pores at the surface, and out through holes.
One of the things that comes into a sponge on these
self-made currents is food in the form of plankton: bacteria, microscopic
seaweed, one-celled animals, and sex cells released by plants and other
animals.
Although some sponges grow to immense size, reaching
more than six feet in diameter, they can still eat only food no larger
than what one cell can swallow at a time. That's one disadvantage of not
having a stomach.
Another item sucked in by the sponge's continual
current is sperm from other sponges.
Most sponges are hermaphrodites, each individual
bearing both eggs and sperm. Sperm gets shot out through one of the
sponge's openings, then inhaled by another and transferred to its eggs.
There, the fertilized eggs remain protected and fed.
When mature, the youngsters get ejected, swimming until they find suitable
sites to settle down and become full-fledged sponges.
But that's not the only way sponges reproduce, and
herein lies the answer to my reader's question.
If you press a sponge through a fine silk cloth, the
sponge's cells fall out singly. If these cells are in a dish of seawater,
they begin creeping around the bottom. If one runs into another, the two
stick together and continue crawling. Eventually, tiny masses of cells
form which then organize themselves into new sponges.
So.
It's true that when a sponge is broken up into
thousands of pieces, each piece will become a tiny new sponge exactly like
the original, and exactly like each other. Such asexual reproduction is
called cloning.
Other sponge facts:
Hawaii hosts about 63
species of sponges, ranging from tide pools to depths of over 300 feet.
Twenty-four of those species live only in Hawaiian waters.
Hawaii's fire sponges have
a chemical on their surface that burns human skin. It's best not to touch
living sponges, particularly red or yellow ones.
One large sponge from the
Gulf of Mexico was found to be hosting over 17,000 animal guests. About
16,000 of those were snapping shrimp.
