38 
PACIFIC SCIENCE, Vol. VI, January, 1952 
Scytothamnus australis f 
Apophlaea sinclairii f 
Hormosira hanksii o 
This is the characteristic mid-tidal com- 
munity of rocky shores on Rangitoto. The 
upper limit of this association coincides with 
the lower limit of the preceding association. 
The relative proportions of the two dominants 
vary with the environment. Thus, certain en- 
vironmental conditions, which may differ for 
the two organisms, appear to be required for 
the optimum expression of each. As a result, 
in the extreme case, two consociations may 
be recognised. 
Elminius plicatus, Apophlaea sinclairii, and 
Scytothamnus australis are scattered through the 
association. It is probable that the importance 
of Apophlaea above the upper limit of Saxo- 
strea may be related to an absence of compe- 
tition. Scytothamnus assumes local dominance 
in more exposed localities. 
With an increase in physical exposure, 
Chamaesipho hrunnea enters the association, to 
the extent of dominance. Its upper limit is 
above that of C. cohmna. 
6. Hormosira banksii consociation 
Hormosira hanksii d 
Scytothamnus australis f 
Corallina officinalis f 
Vermilia carinifera o 
Microdictyon sp. o 
The Hormosira community, classed under 
the mid-tidal formation, occupies approxi- 
mately the same limits as the Chamaesipho- 
Saxostrea association of less protected shores. 
This plant, which is associated with physical 
shelter, may form a consociation on flat or 
broken surfaces. The community is very 
characteristic of scoria shores. 
Vermilia carinifera may completely occupy 
such special substrates as isolated rocks and 
steep surfaces (Fig. 3). 
7. Elminius-Modiolus fasciation 
Elminius modestus d 
Modiolus neozelanicus d 
Gelidium pusillum o 
Caloglossa leprieurii o 
Scytothamnus australis o 
This mid-tidal community occurs on the 
special substratum provided by wharves. 
8. Enteromorpha bulbosa-Monostroma 
crepidinum aspect 
A chlorophycean community occurs in the 
upper part of the levels of the formation be- 
tween June and January. It is associated with 
physical shelter, but has no apparent relation 
to fresh-water drainage. 
Lower Littoral Mixed Algal Eormation 
This formation occupies that part of the 
littoral which lies roughly between M.L.W. 
N.T. and M.L.W.S.T. It is noteworthy for 
the fact that all the dominants in the various 
communities are plants. A considerable vari- 
ation in species composition is partly asso- 
ciated with changes in physical exposure. A 
basic association may, however, be recog- 
nised. 
9. Corallina-Hormosira association (Fig. 
2 ) ^ 
Corallina officinalis d 
Hormosira hanksii d 
Codium adhaerens a 
Colpomenia sinuosa a 
Elminius modestus a 
Cystophora torulosa a 
Xiphophora chondrophylla var. minus a 
Aeodes nitidissima a 
Splachnidium rugosum f 
Hormosira hanksii exhibits a much-reduced 
thallus form as compared with the Hormosira 
plants from the upper part of its range. This 
appears to be associated with relative toler- 
ance of water loss (see page 44). 
Several important subordinate species ap- 
pear to have ecological requirements fairly 
similar to those of the dominants. These are 
Codium adhaerens, Splachnidium rugosum (sea- 
sonal), and Colpomenia sinuosa (seasonal). 
Elminius modestus covers many pebbles and 
stones down to M.L.W.S.T. 
