July. August 25NEED
TO KNOWwith Dr Karl Kruszelnicki
Do fish drink water?
IT WAS A DECEPTIVELY simple
question from a listener to my weekly
talkback show on Triple J: “Do fish
drink water?” It’s easiest to answer if
we think of atoms and molecules. The
chemical formula for water is H 2 O. It’s
a molecule made from three atoms –
two of hydrogen, one of oxygen – and
it’s shaped like a tiny boomerang.
In any water body, such as a lake or
river or the ocean, there are usually
some oxygen molecules in between
the water molecules – especially if
the surface of the water is exposed
to the air. Compared with water
molecules there are many fewer
oxygen molecules, which are each
made of two atoms of oxygen and
shaped like a barbell.
We humans drink water that goes
into our gut, then our bloodstream
and then our kidneys. Each day,
the kidneys make a quantity of urine
that’s mostly water and equal to
about 2–3 per cent of our body weight.
So, back to fish: all species ‘drink’,
in the sense that they bring water intotheir bodies to extract
oxygen from it. A fish
brings water into its
mouth, and squirts it
straight out – via its
feathery gills (see
image at right).
Most of the water
molecules go straight
out. But oxygen
molecules from
between the water
molecules can enter a fish’s body via
the gills, and then into the blood.
So all fish do ‘drink’ water to get
their oxygen. But if you’re talking
about fish drinking water via the
mouth–gut–bloodstream–kidney
pathway? Well, some do, but it
depends on where they live.
Saltwater fish swallow water like
crazy. But on the other hand, they
make hardly any urine – only 4 per
cent of their body weight each day.
Due to osmosis, the rest of the water
they swallow exits through their gills
and skin, and into the salty ocean.Freshwater fish do the opposite.
They hardly drink any water but
urinate massive amounts – about
20 per cent of their body weight each
day. The water in their urine comes
mostly from the fresh water they
swim in. It enters their body straight
through their gills and skin – again,
due to osmosis. So, salties swallow
the sea, while freshies fatten and wee.DR KARL is a prolifi c broadcaster, author and
Julius Sumner Miller fellow in the School of
Physics at the University of Sydney. His latest
book, Vital Science, is published by Pan Macmillan.
Follow him on Twitter: @DoctorKarlPHOTO CREDIT: BEN BEADEN/AUSTRALIA ZOO. ILLUSTRATION CREDIT: ALDONA GRISKEVICIENE/SHUTTERSTOCK.
AG podcast is go!
We’ve launched our brand-new podcast series,
Talking Australia, featuring conversations with extraordinary
Aussies from right around this magnificent country.F
ROM WILDLIFE
warrior Terri Irwin
(at left, with podcast
host, AG editor Chrissie
Goldrick) to a true legend
of marine conservation,
Valerie Taylor, and world
record-breaking wingsuit
pilot and medical doctor
Dr Glenn Singleman, spend
a little quality time in the
company of some of our
most inspiring Australians.
Listen as they take you along
on their personal journeys
around this magnificent
country and beyond, wheth-
er battling the elements to
achieve their lifelong dreams
of adventure or working hard
to preserve our unique and
fragile natural world and the
creatures that inhabit it.Each week the Talking
Australia podcast features a
new and lively conversation.
Be sure to tune in.SUBSCRIBE TO TALKING
AUSTRALIAon iTunes,
Spotify or from
wherever you download
your podcasts.buzz