Human imagination is powerful and, as a result, we can
readily be taken in by the power of advertising. One example of which I am
especially fond is that of sea monkeys, sold through advertisements in
children’s comics in the last century (see below), but still available today.
The excerpt from the advertisement shown above sums up the
enjoyment that we are likely to have by watching the antics of sea monkeys and
we hold the miracle of life, being able to create them by adding powder from a
series of bags to an aquarium tank of water. The bags contain salts, nutrients,
and the encysted eggs of brine shrimps (Artemia)
that hatch to produce larvae in the saline water and these metamorphose into
adults (see below) after a few days. The idea of branding the shrimps sea monkeys came from
Harold von Braunhaut, a New York entrepreneur, and he encouraged us to imagine that
the Artemia swimming around in tanks,
some of which had lenses set into the plastic to allow a better view, had faces
and were performing tricks.
I wonder how many of those who had sea monkeys (and we had
some in our household when our children were young) overcame their disappointment
that they were shrimps and not little monkeys, by considering the biology of
these remarkable animals. The name brine shrimps gives away the habitats in
which they are found – small freshwater pools with high levels of salinity, especially
during drying events. Artemia can
tolerate salinities from 25-250 g/L (sea water is usually 35 g/L) and they can
do this because they have an impervious exoskeleton that prevents osmosis and
thus water incursion at low salinities and dehydration at high salinities. That
in itself is something that promotes wonder, as does their ability to reproduce
parthenogenetically.
When their pools begin to dry, encysted eggs are produced
and these withstand extremes of temperature, very low oxygen tension, and the
presence of poisonous gases (although experiments conducted by NASA, and
others, suggest they may be susceptible to the deleterious effects of cosmic
rays [1,2]). They are effectively as dry as the dried pool in which they are
found and are referred to as being is a state of cryptobiosis – meaning that
all signs of life are hidden. They can remain in this state for years and that
is why living Artemia soon appear in
sea monkey tanks after we have added powder to the water. It also explains why
they return after we add fresh water to a dried-out tank.
The encysted eggs of brine shrimps are not the only examples
of cryptobiosis; many seeds and spores can also remain in long-term dormancy
before returning to resume their “normal” life when conditions become suitable.
More unusually, there are creatures that can become dried during other phases
of their life cycle – tardigrades and nematode worms provide examples, as do
the larvae of an African midge (Polypedilum
vanderplanki) that become dried with the temporary rain pools in which they
live. Animals that become cryptobiotic as larvae or adults replace water with organic
compounds that maintain the integrity of tissues and, as with the production of
encysted eggs by Artemia, the
metabolic changes required for survival are triggered by changes in water
quality that indicate that drying is about to occur.
If we are amazed by the antics of sea monkeys, how much more
amazed are we that several different lineages of animals have evolved
cryptobiosis and the ability to become dried? And independently, at that.
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