Biology Reference
In-Depth Information
temperature changes have been identified by comparing the temperature
requirements of tropical and cold-water seaweeds from both hemispheres in relation
to the climatic history of the various regions (Wiencke et al.
1994
). Since Mesozoic
times [251-65.5 million years before present (My)] there was a continuous warm
water girdle around the earth so that cold-water seaweed species probably only
evolved after the glaciation events in the Tertiary (65 My) (Luning
1990
). An upper
survival temperature of 33-35
C may still be found in representatives of temperate
and tropical seaweed species, indicating that this thermal trait is rather deeply
entrenched and not subject to fast adaptation (Luning
1990
). While strictly tropical
seaweed species are stenothermal and may survive 30-37
C and grow best between
25 and 30
C, they do not have the ability to live below temperatures of 7-12
C
(Pakker et al.
1995
; Bischoff-B
asmann et al.
1997
). The first steps in the adaptation
of seaweeds to lower temperatures are an increase in cold tolerance and an increase
of growth and reproduction rates at lower temperatures leading to eurythermal
species. This temperature trait is apparent in tropical to warm-temperate species
which acquired a better lower temperature tolerance with survival temperatures
between 6 and
€
1
C without losing the upper temperature tolerance (Yarish et al.
1984
; Pakker et al.
1995
; Bischoff-B
asmann et al.
1997
). Later, the ability to survive
€
15-20
C was lost. This type of
temperature response is typical for endemic Arctic and Arctic to cold-temperate
distributed seaweeds exposed to low temperatures for about 3 My (Briggs
1995
).
The last steps in the adaptation to low temperatures are the loss of the ability to grow
and reproduce at
20
C and to grow and reproduce at
temperatures
5-10
C and to survive temperatures
6-13
C (Wiencke et al.
1994
; Bischoff-B
asmann and Wiencke
1996
). This type of temperature response is
exemplified in endemic Antarctic species exposed to cold water for at least 14 My
(Crame
1993
; Briggs
1995
). So, the climatic history during species evolution
determines the temperature requirements of seaweeds in all biogeographical
regions. All these cases give moreover an insight into the time periods needed for
adaptation to changing temperatures. Physical barriers and differential environmen-
tal gradients along coastlines also may produce ecotypic adaptation. There has been
a wealth of studies tackling this question. It became evident that upper tolerance
limits seemingly are quite stable within several seaweed species (Luning
1990
and
references therein). True temperature ecotypes have only been found in a few
species yet (e.g.,
Ectocarpus siliculosus
: Bolton
1983
,
Saccharina latissima
: Gerard
and Du Bois
1988
).
€
18.3.2 Changes of Seaweed Distribution and Oceanic
Temperature in the Geological Past
Interesting examples for migration in the geological past are present-day
amphiequatorial species such as
Acrosiphonia arcta
(Chlorophyta) or the species
pair
Desmarestia confervoides/D. viridis
(Phaeophyceae), which are absent in the
