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

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'co d:tying conditions than D. wilZana because it has relatively thick
g'ela'cinous cell walls, and because it produces much larger quem ti ties
of mucilage. Although both species lost water at about the same
rate, it is possible that much of the water lost by D. wilZana came
from the cell protoplasm, whereas most of 'che ~la.ter lost by D, antarctica
tllas probably from cell walls.
Algal clea:cing experiments carried out a'c different levels on the
sOl1'chern 8hot'e of the I sle of Man showed thai: the removal of one alga
allows the vertical ranges of others to extend. When AscophyZlum
nodoswII (L.) Le J"ol. was cleared during the fruiting period of FUCU8
vesicuZo8U8 L. the Pucus extended down the shore and colonised the
cleared areas (Burrows and Lodge 1951).
In Ne'Yl Zealand removal of one Durvi Z laea species did not al ter
the vertical range of the other. The same species recolonised cleared
sites, and the original zonation pattern was clearly established before
the recolonising Durvillaea become visible to the naked eye.
In some areas in Britain, such as Port StMary on the Isle of
Man, experimentally cleared sites were recolonised by FUCU8 hybrids
(Burrows and Lodge ibid). I have found no evidence of hybrids between
D. antarctica and D. wilZana recolonising cleared areas, or for that
matter growing in virgin stands.
The presence of distinct graze lines at about MLWN in beds of
young D. antarctica indicates fish grazing is one of the prime factors
determining the lower limits of D. antarctica. Yet Lewis (1964:216)
stated that predation and grazing do not usually set clear limits
to distribution; more co~nonly they affect the abundance of a
species at a particular level. Perhaps there is some physiological
reason why D. antarctica does not grow subtidally. 'l's; factor(s)
delimiting the lower extent of D. willana are not knoltm. It extends
deepest i.n permanently clear water suggesting that the quantity and
quality of light may be the limiting factors.
Stephenson and Stephenson (1949) proposed (;! univ8l:.'sal scheme
for describing the zonation of organisms on rocky shores. They
divided the shore into the
'The lONer