Hydra are solitary polyps in the Cnidaria
phylum, the same phylum as sea jellies, sea
anemones and corals; hydra are almost the
only cnidarians that have managed to invade
freshwater. (Well, okay, there's a little
medusa of temperate lakes called Craspedacusta.) Hydra are not parasites, they are little
predators. Such little predators, it's true,
that you may not even notice them until they
catch enough baby brine shrimp to turn pinkish.
Like many small translucent invertebrates,
they take on the colors of their most recent
meal. The kind of fishkeeper who jumps to
conclusions can become quite convinced that
the hydra came in somehow with the dried brine shrimp cysts, but you'll
quickly realize that is impossible for these
creatures that are limited to fresh water.
In fact, to eliminate hydra from daphnia
or moina cultures, the only dependable technique
is to let the substrate dry out completely.
The daphnids will regenerate from their resting
cysts, but the hydra will be gone until they
are re-introduced. And in a planted aquarium,
they pretty surely will be.
Hydra are strictly carnivores. Flake feed
and other detritus don't tempt them at all,
but they do ingest the ciliates and other
minute organisms that uneaten flakes support.
A 10x magnifier may show you their basal
disc, attached to the tank glass. On a good
day, at 10x I can just make out the cnidoblasts,
which contain the unique nematocysts, closely
dotted along the tentacles. These discharge
stinging barbs into the hydra's minute prey.
Hydra multiply by budding off polyps and
have amazing powers of regeneration, hence
their scientific name. The original Hydra
of Antiquity was a pre-Hellenic multi-headed
water-dwelling snake god that was already
ancient when Hercules battled it as one of
his Labors. At each sucessful stroke of Hercules'
sword, two heads grew where one had been.
(The resourceful Hercules took a flaming
torch and swiftly cauterized each cut in
turn, which saved his ass.)
Most hydra species are colorless, like the
patriotic Hydra americana. Green hydra, Hydra viridis, contain symbiotic single-cell algae, Chlorella, which thrive within vacuoles in the hydras'
cells and give them a bright lime-green color.
Though other Chlorella species take up residence inside freshwater
protozoa and sponges in chilly lakes or turn
the Spotted Salamander's egg masses green,
H. viridis is probably the closest you'll come to witnessing
a freshwater version of the symbiotic zooxanthellae
that reefkeepers nurture in the tissues of
their Tridachna clams. In bright light the
algae derive CO2, which they need for photosynthesis, from
the constant output of
cellular respiration,
and they scavenge the ammonia
that's another
cellular by-product. In
exchange the algae
give off oxygen. The hydra
are too primitive
to organize any respiration
more ambitious
than the cellular respiration
of each individual
cell, but in a way the
symbiotic algae function
as a substitute for a green
hydra lung.
Hydra have been known to get into a culture
of Moina (miniature Daphnia) here at my place and
cause the population to
crash, and they are
definitely dis-commended
in fry tanks. Hydra
can decimate the small
fry of rainbowfish,
too. Adrian Tappin sterilizes rainbowfish fry tanks beforehand
with household chlorine bleach, or eliminates
hydra in fry tanks with flubendazole. I don't
know how dangerous hydra would be to baby
guppies, but they can certainly catch and
eat gourami fry. In turn, some grown gouramis
will eat hydra, which seems like rough justice,
doesn't it? But hydra don't transmit any
diseases to fish. None.
Besides Adrian Tappin's flubendazole, hydra
can be controlled, if you must, with Formalin
or with Clout or Fluke-Tabs, at low dosages
that won't stress grown fishes. Most anti-hydra
measures will be very stressful to fish fry.
If you have no fish in the aquarium, sodium
metabisulfite (Na2S2O5) or "Campden tablets" are being
used to counter hydra by Killifish pro Tim Addis. "Campden tablets" are better
known for removing extra chlorine from drinking
water or eliminating unwanted bacteria and
yeasts in brewing and winemaking, rendering
tannin extracts more soluble, etc. I'm just
reporting here, as I've never personally
gone on a hydra-exterminating campaign.
Other hydra "remedies" I've seen
suggested included ammonium nitrate, limewater,
potassium permanganate, hydrogen peroxide,
even electrocution! Easy does it! Though
copper-containing medications are more credible,
it's hard to control the toxicity of copper
in freshwater, and I've also been reading
somewhere that copper doesn't actually kill
hydra, it merely makes them retract into
a little nub, so that we don't notice them.
Yet in this retracted state they aren't able
to extend their feeding tentacles, so they
are temporarily being starved. Frankly, assuming
your planted aquarium hasn't been poisoned
with medications recently, I think that if
you went over your finer-leaved plants leaf
by leaf with a magnifier you'd often find
multitudes of hydra, leading a blameless
existence.
Hydra links. The pros at theKrib.com offer lots of hydra postings, with the widest
imaginable array of anti-Hydra
measures (maybe
a sign that nothing really
works).
I. Sinclair describes his modestly successful
attempts at eradicating hydra at the Livebearers site. Perhaps you'll agree that he exaggerates,
for sake of a better tale, the dangers hydra
pose
In the 1740s Abraham Trembley was collecting Hydra and other plankton
from the garden ponds of Sorgvliet, near
The Hague, to instruct his aristocratic pupils,
and was cultivating them in glass jars, the
direct precursors of glass aquaria. A 1791 engraving of Trembley's hydra jar was included in an exhibition of historic
naturalists at Johns Hopkins University Library.
