Flatworms are the simplest organisms with
a righthand and lefthand side. In the phylum
of the flatworms (Platyhelminthes), turbellaria or planarians are the class of free-living organisms. You'll
see them referred to by both names. The smallest
turbellaria are "microturbellaria."
The visible free-living planarians are sometimes
called Triclads.Planarians are secretive and nocturnal in
their habits, uncolorful and small. Most
of the flatworms in the aquarium are less
than a millimeter long, and we never notice
them. These "microturbellaria"
eat bacteria, graze on algae and organic
detritus, and may attack the smallest protists,
roles in which they are competing with unicellular
ciliates, which may be about as big as the
smaller microturbellaria. Dugesia is the large planarian genus supplied for
high-school bio labs.
The planarians you see have a flat grayish
to whitish elongated body: you'll need a
magnifier to see that sometimes they have
a pointed or broadened head. The harmless
free-living flatworms creep around on the
glass, which is where you'll notice them
first, and also all through the gravel, on
plants and plant debris and on every other
surface. They stick to surfaces with adhesive
glands. Flatworms glide over surfaces on
their invisibly small hairlike cilia, which
stirr up the little eddies of detritus that
gave them the name "turbellaria."
It's a smooth flowing ride, like a snail's
or a slug's, lubricated by mucus, without
stretching and contractions or thrashing
and wiggling. You should be able to identify
flatworms from their movements. A hapless
planarian may be blown in the current, but
flatworms don't swim. They'll even glide
along the underside of the water surface,
supported by surface tension. At that minute
scale, forces like surface tension easily
overcome gravity.
Planarians are hermaphrodites; some of them
are even self-fertile. They produce invisibly
small sticky eggs, singly or in clusters
and strings. The eggs hatch into minute versions
of the adult; there aren't any flatworm larval
stages. Their lifespans are probably short,
so that a population may explode and die
away again, reflecting a temporary windfall
of available nutrients, like the "bloom"
of planktonic protozoa that causes temporary
haziness.
You won't notice planarians, unless there
are suddenly lots of them. They could easily
seem to have come from "somewhere else,"
all of a sudden. Fact is, too copious feeding
with flake feed has encouraged them, and
the unnoticed local population has boomed.
In the ecology of the aquarium, the flatworms
have scavenger roles comparable to those
played by snails. Some planaria are predators
of animals as small as rotifers and nematodes,
but not of any organism big enough for you
to see. When you read in a widely-used book
that "flatworms have been found on young
fish," be assured that the fry concerned
had died before the flatworms began to scavenge
a meal. Even the largest, visible Triclad
flatworms just never attack a living fish,
and I seriously doubt that they could be
a danger to even the smallest wrigglers on
the bottom of a tank. But planaria will rapidly
assemble in the dark to scavenge a dead fish.
And like snails they are predators of fish
eggs laid on gravel--— one reason some fishes
scatter their eggs among plants.
Controlling planarian populations. Though Clout (one tablet per net gallon)
will decimate planaria (nothing eliminates
them), don't panic and reach for it, nor
for copper-based poisons. Planaria thrive
in water that is highly enriched in organics
and debris, so some increased housekeeping
on your part is a better approach. Once the
flatworm population is back under control,
consider reducing the amount of flake feed
you're introducing to the system. Give the
fish a "starve-day" once a week,
for their own good. And feed some live food.
Live food critters compete with planarians
instead of feeding them. If Gouramis and
Paradisefish are left unfed a couple of days
to get hungry enough, they will eat flatworms.
Most other fish that taste them will spit
them out again.
To collect planarians, tie up a smidgen of
hamburger or chicken liver in a little piece
of cheesecloth (to keep fishes off the meat)
attached to a string hung over the tank edge.
Leave this baitbag on the gravel in the evening.
Planaria have fine chemical senses of smell,
and they will swarm over the bait. After
a few hours of darkness, smoothly lift the
baitbag on its string. Slip a brine-shrimp
net under it before you lift it free of the
water. Plunge everything, net and all, into
boiling water. Planaria have amazing abilities
of regeneration, one reward of an undemanding
lifestyle, I suppose. So don't go chopping
at them: a severed head will grow a new body.
Planarians aren't parasites. Since flatworms form a whole phylum (Platyhelminthes)
and have probably been around since before
Cambrian days (though there's no fossil record,
understandably), it's not surprising that
they have developed highly diversified lifestyles.
Besides these free-living planarians, many
other flatworms are adapted to parasitic
existence as "tapeworms" (those
are the Cestodes,
some of which dwell in fish intestines)
or as "gill flukes" or "skin
flukes" (those are the
Trematodes). In fact, most kinds of Platyhelminthes
are parasites, so fishkeepers can get a little
edgy when they see any flatworms at all.
But you aren't ever going to see any of the
parasitic forms, because their only life-cycle
stage outside a fish is as a microscopic
egg. So the moral is, don't worry about the
flatworms you can see; they're not going
to attack the fish in any way. The rule "If
you can see it, it's not a parasite,"
applies to flatworms.
Flatworm links. You can get an outline of the basics on
planaria at the Planarian HomePage.
More focused on the forms and lifestyles
of freshwater flatworms and their roles in
the freshwater ecology is S.M. Mandaville's
Zoobenthos pages in the huge Soil and Water Conservation
Society of Metro Halifax website.
