A biological study of Durvillaea antarctica (Chamisso) Hariot and D ...

90
The most southern tip of South Africa lies just north of 35°S,
so that south and west coasts are north of the main influence of the
West Wind Drift.
'rhe Benguela Current is partly derived from a
branch of the West Wind Drift and the Falkland Island Cm'rent.
Presmnably drifting D. antarotica could be carried to cold
South African coasts, although there are no records to confirm this-
If drifting D. antarctica do reach South African shores, then
the only obvious (nl:planation left is that other organisms in the ION
Ii ttoral py'event. the kelp becoming established.
The large laminarian kelp Laminaria pallida Grev. grows abundantly
on the cold temperate South African \'1est coast. According to
stevenson (1944) it is mainly found in the upper sublittoral on exposed
coasts; Nhile in more sheltered areas it e}ctends int.o the lower
littoral, If D. antaratica becane established, then one would expect
it to form a band above the Laminaria.
On exposed coasts there would
appear to be no competition between the kelps for space on the rock, but
a t high tide any small pioneer D. antarctica would be shaded by the
huge laminarian.
The low littoral of South African shores is dominated by large
limpets.
stevenson called this the Cochlear-Argenvi llei zone after
the limpets Patella cochlear Boni and Patella argenviZLei Krauss.
Apparently this zone is characteristic of regions where wave force
is strong. Only in very sheltered localities are Laminaria and
MaoY'ocystis plants able to penetrate this zone. Stevenson (1939)
thought it possible that in exposed places these herbivm:'es prevented
large algae from becoming established in the low littoral.
The density of these large limpets on the South African coast
is exceptionally high. stevenson (ihid:515) recorded 300-1400 P.
argenvillei/m2.
In New Zealand I measured the density of two much
smaller limpets that occur commonly amongst D. antal'otiea (Siphonaria
zelandica QUoy and Gaimard and Cellana ~adians
Cmelinl as between
68~'364/m 2 •
'i'his limpet mosaic does not occur in areas where wave force is
exceptionally strong, where instead of the limpets, there is a band
of mussels (Mytilis e:r>enatus Lamark and M. meridionaZis Krauss) and
a band of the anemone Bunodactis reynaudi Milne Ed\I1ards. 'l'hese three
species occupy almost all available space in the ImoJer littoral.
Du:t>viZlaea genfiling's attempting to colonise South African
shores would have to compete against Laminaria and this dense covey
of mussels and anemones, They mus~c also he subjected to grazing hy