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Re:Unknown Organism

It sounds like an Ostracod to me.  Ostracods are small crustaceans that
swim about using their antennae, and chew on algae or other plant or animal
matter.  They have a clam-like bivalve "shell" that encloses their body,
and fish do not relish eating them very much, although most aquarium fish
will whittle the population down to low levels in a few weeks.   The
little, roundish ones, only about 1-2 millimeters long are pretty harmless,
although I have seen them get very numerous on rare occasions and actually
kill snails by harassing them to death.

There is a bigger, more oval species, about 3-4 mm long, that does damage
my plants severely when numerous, and it is very hard to eliminate.  It
lays eggs that have variable hatching times, some delaying for over a year.
The small, roundish yellow eggs are stuck in crevices, and held in there
with some glue-like material.  When I first got an infestation of these
larger ones, they spread to many of my plants in various tanks and jars
before I noticed the damage they were doing and got concerned.

I thought I could get rid of them on a plant by placing a piece of the
plant in water in a jar, and changing the water completely every few days,
thus washing out all the ostracods that hatched before they had a chance to
grow up and lay more eggs.  I watched the jar closely, and wherever I saw
any little baby ostracods, I changed the water and rinsed off the plant.
Since the plant was floating in a bare jar, there was no substrate for the
ostracods to hide in, and I was sure that I got all the baby ostracods out
each time I rinsed.  However, the plant continued to produce babies
seemingly indefinitely.  It was still producing babies six months later,
when I finally gave up the experiment.

I have managed to free some plants of them, but it is a long process,
involving keeping the plants in a guppy tank for two to three years.  I
have some C. parva that I think is now free, having been in a guppy tank
for 2 years, during which time it has grown from a single plant to a little
patch of about 10 plants.  It hasn't had the best growing conditions, what
with numerous green water outbreaks alternating with periods of heavy
shading when I let the other plants become dense enough to make the green
water go away.

Paul Krombholz, in cool, dry, central Mississippi.