PHYCOLOGICAL SOCIETY OF AMERICA
Life History Morphology
Cold, nutrient rich, low light environments characterize Polar Regions.
Would you guess that the waters of the Arctic have similar flora as
waters of the Antarctic? Even though these two locations have very similar
temperature, nutrient, and light regimes, it turns out that seaweed
communities of the Polar Regions are not so similar. In most cold-water
ecosystems kelp (Laminariales) is the dominant seaweed - but there is
no kelp in the waters of Antarctica. Uniquely, Desmarestialeans dominate
the seaweed flora in Antarctica. Following the opening of the Drake
Passage 35 million years ago, Antarctica’s ice sheets stabilized
and the water temperatures of the Southern Ocean fell. It was only about
3 million years ago that ice sheets developed in the Northern Hemisphere.
Thus marine algae in Antarctica have had a much longer history in a
cold-water habitat than Arctic marine algae. For some helpful information
about the cooling of the Southern Ocean see the Woods Hole magazine
It is hypothesized that the family Desmarestiaceae originated in the
Southern Hemisphere and molecular phylogenetic analysis (Peters
et al.) of nuclear ribosomal DNA from members of this family support
this hypothesis. Desmarestia is a cold-water alga and can’t
survive in warm waters near the equator. How then did this alga get
from the Southern Hemisphere to the Northern Hemisphere? One answer
might rely on ice ages. Perhaps when temperatures fell and the equatorial
waters were not as warm as they are now, certain species of Desmarestia
may have taken this opportunity to "cross the border." However,
comparisons of DNA (van Oppen et
al.)show that individuals from the Arctic and Antarctic do not differ
enough genetically to support the former hypothesis. In other words,
should it be true that Desmarestia crossed the equator during
the last ice age, than it is expected that the genetic sequence determined
from the DNA of the individual from the Northern Hemisphere should be
somewhat distinct from an individual of the Southern Hemisphere given
the amount of time since the last ice age and the average speed of evolutionary
change. Instead, the genetic information of the two species is surprisingly
similar and suggests that the species found today in the North migrated
fairly recently from the South. Perhaps the answer to this riddle lies
in the Life History of Desmarestia.
Since there is a small microthallic phase in the life cycle of this
alga, then deep-water dispersal may be the answer.
Three specimens of pressed Desmarestia that Dr.
Chuck Amsler at the University of Alabama at Birmingham graciously
lent to me so that I might examine one species (Desmarestia antarctica)
that I certainly wont be able to find snorkeling in Monterey Bay! On
the other hand, Desmarestia ligulata is a very common species
in California and seems to have a worldwide distribution. Because D.
ligulata can be found in the Southern Hemisphere and also in the
Northern Hemisphere it is called an amphiequatorial
alga. Additionally you may say that D. ligulata has an amphioceanic
distribution because it occurs in both the Pacific and Atlantic Oceans.
Life History Morphology
© 2005 Ashley Maloney