here are so many living
things - so many ways of living! And, some of them
illustrate better than any textbook the long route
that life took from molecule to mammal. One of these
is the genus Clarias, the 'Walking Catfish'.
They are true fish. But,
they breath air!
Their air breathing organ is in their brachial
cavity, just like our lungs are. It contains so
much air that they often swim as though they were
tail-heavy. Their gills have atrophied so much that
they suffocate where water freezes over and air
is unobtainable. However, they can "walk"
on land to move from one water body to another.
You've seen those drawings of primitive sea creatures
crawling up onto lifeless land - this drawing is
One species, Clarias
batrachus, is occasionally found in tropical
fish stores. It has a body shaped like a fish, scaleless
like an amphibian, and four pairs of barbels. It
comes in a wide variety of colours: white, tan,
gray, brown, or albino. Males have spots on their
dorsal fin and females don't. It moves on land by
curling its tail forward then jumping, like a frog
with only one rear leg, keeping itself upright with
its stiff front fins. Mine is less than a foot long,
but can move so fast it might get away from me if
I don't have a butterfly net in hand.
Clarias are not appreciated
by fastidious aquarists, for they dismember and eat
any fish smaller than they are. It's not a quick process
either. Clarias have a small mouth, with no
teeth - their prey has to be reduced to swallowing
size by shaking. They normally live on muddy bottoms,
and can't see more than a few centimetres, so they
have to locate food almost entirely by smell and motion.
And, in the artificial environment of a small aquarium,
smells can spread so evenly that even that fails to
help them locate food - they have to just charge around
until they run into it. I feed mine dried food, and
earwigs in season.
They are also a useful example of the resilience of
biodiverse ecosystems. The June 1969 National Geographic
(135:846-851) contains photos of the panic-driven
campaign that occurred when they were discovered in
the Everglades. The living things of thousands of
pools were killed with rotenone until it was discovered
that everything except the catfish were being killed
- they simply climbed out of the poisoned pools and
walked to the next one. The campaign was abandoned
- the Everglades given up as lost forever by the authorities.
Then, it was discovered that alligators love Clarias
- the alligators of the Everglades have never been
better fed! I am not aware of a single species whose
existence in the Everglades was endangered by the
addition of Clarias. (Incidentally, the same
is true of house sparrows, starlings and purple loosestrife,
all of which have engendered equally panic-stricken
campaigns around me in Canada.)
As do all experienced aquarists,
I underfeed rather than overfeed for best health.
However, when Clarias feel underfed, they try
to head for the next pond, by thumping and pushing
on the aquarium cover. Once out of the aquarium, they
will roam a building in search of an exit until they
are almost totally desiccated. (Or, meet a household
cat or dog - they are helpless to defend themselves
on land.) They are strong fish - I've got a weight
on the cover!
They grow to 40 cm long, so
need a large aquarium - 150 litres or more. The pH
should be slightly acidic, 6.5-7.0, and water temperature
15-25C, for best health. They are very hardy, do not
need a heater in a centrally heated building, and
eat anything usually offered to fish. They can eat
until their belly swells to the size of a golf ball,
then do without food for the next week - no fish sitter
needed. They are common in tropical Asia and are used
for food there.