QUESTION:
How does Antarctic krill differ from warm-water krill? 
How have they adapted to the cold climate?

        Richard A., Mr. Zahn's Class
        Crockett Jr. High School, 7th grade
        Irving, Texas


ANSWER from Karen Haberman, Antarctic Research Team
            Palmer Station, Antartica
            Wed, 26 Feb 1997 03:40:53 -0500

Dear Richard, 

Wow, this a pretty huge question!  The biggest difference between
Antarctic krill and their warm-water relatives is that they have a
longer lifespan and grow to be larger.  The Antarctic krill
(Euphausia superba) can reach up to about 60 mm, whereas most other
species grow to only 25 mm or less.  Also, they do not begin to
reproduce until their third year or later, and may live up to 6-8
years.  In contrast, other individuals of other krill species can
reproduce after their first year. Also, I don't think individual of
many other species (if any?) live much longer than a year.

What is interesting is how similar, physically, Euphausia superba is 
to its pacific coast relative, Euphausia pacifica.  They look very 
similar, but Euphausia superba is much larger.

As for adapting to the cold, you have to think about this from the
point of view of an animal that has a body temperature and salt
content similar to its surroundings.  It is different than a mammal
or bird that must maintain a high body temperature.  Believe it or
not, Antarctic krill probably benefit from being at a low
temperature.  Their energetic costs are actually LOWER because they
are at a low temperature, just the opposite of what is true
for mammals.  Also, their body fluid is close to the salinity
(saltiness) of seawater, so just like the seawater they swim in,
they do not freeze at zero degrees celsius.  As long as the seawater
is fluid, they will be just fine.

However, the polar climate does cause seasonality in the
availability of the phytoplankton, which is food for the krill.
Since phytoplankton requires light, low light in winter means there
is less phytoplankton.  Thus, the krill grow slowly or even shrink
during the winter, when food is scarce.  (This probably is also why 
they take so long to reach maturity.)  However, they can grow just
as fast (or faster?) than the temperate water krill during the summer
when food is available.

Thanks for your question!
Karen H.