Biology Reference
In-Depth Information
tropics. The fast evolving internal transcribed spacer region of the rDNA of
northern and southern hemisphere populations of both entities only exhibit minor
variation, suggesting that a migration across the equator took place at the maximum
of the W
urm/Wisconsin glaciation 18,000 years ago, the so-called last glacial
maximum (LGM; van Oppen et al.
1993
). The hypothesized migrationist transit
of the tropics is supported by the temperature tolerance of both species which is
high enough (25
C) to survive a passage through the tropics at the LGM (Peters and
Breeman
1992
; van Oppen et al.
1994
; Bischoff and Wiencke
1995
). Similarly it is
assumed that a common ancestor of the warm-temperate NE-Atlantic kelp species
Laminaria ochroleuca
and its S-Atlantic sister species
L. abyssalis
and
L. pallida
was able to survive the passage through the tropics during glacial lowering of
seawater temperatures by its gametophytic microstages being able to survive at
least 25
C (tom Dieck
1992
; tom Dieck and de Oliveira
1993
).
During the last glaciations, the Arctic ice cap extended south to ~45-55
N and
there was a considerable southward shift of the Gulf Stream (CLIMAP Project
members
1981
; Bradley
1985
) inducing major latitudinal dislocations of marine
biota in the N-Atlantic between glacial and interglacial periods (van den Hoek and
Breeman
1989
; Breeman
1990
). Some seaweed species were faced with an extreme
reduction in their distribution area. The conditions were particularly severe in the
NW-Atlantic, where distributions became excessively reduced. A pertinent exam-
ple is the cold-temperate green alga
Cladophora sericea
. By comparing tempera-
ture demands of the species with modeled glacial sea surface isotherms (van den
Hoek and Breeman
1989
; Cambridge et al.
1990
), it became evident that the
distribution area of the species was probably very strongly reduced during the
LGM in the NW-Atlantic. In the NE-Atlantic, the distribution was shifted from
the coasts of Scandinavia, Great Britain, and France to the Iberian Peninsula,
Northwest Africa, and even the Mediterranean became hospitable. These proposed
migrational shifts during the LGM within the N-Atlantic have been corroborated by
recent phylogeographic studies of diverse seaweed species, indicating the English
Channel Region as a primary refugium (e.g., Provan et al.
2005
) from which species
redispersed to their current distributional range.
An example for a probable extinction during the LGM is the current restriction
of the kelp species
Laminaria hyperborea
to the NE-Atlantic. The much greater
compression of the distribution areas on the NW-Atlantic during the LGM—if
compared to the NE-Atlantic coasts—suggests that
L. hyperborea
was not able to
survive the inhospitable conditions in the NW-Atlantic during the LGM and
became locally extinct (van den Hoek and Breeman
1989
). Other kelp species
possibly became isolated in cold-water pockets after the LGM. Examples are the
deep-water species
L. rodriguezii
(Huv´
1955
;
ˇ
uljevi´ et al.
2011
) or the isolated
population of the southern European kelp
L. ochroleuca
in southern Italy (Giaccone
1972
). Nowadays both species are only found below the thermocline in selected
Mediterranean habitats.
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