Inter-tidal Ecology at Narrow Neck Reef — Dellow 
363 
sia verrucosa and Enteromorpha procera f. 
minuta are growing above their normal level. 
Together with Volsella neozelanicus they 
colonize wide stretches of the relatively flat 
rock about M.S.L. Slight elevations are 
marked by a local increase in abundance of 
the large barnacle Elminius plicatus. The 
chief difference between St. Leonard’s Point 
and Narrow Neck Reef lies in the presence 
of the Mytilus — Attached Algae Community 
at the former locality about E.L.W.S. (see 
p. 370). 
From data obtained in the leveling survey, 
Figure 7 was constructed by plotting the ver- 
tical ranges of 25 of the more important 
plants and animals against the curve for the 
percentage of annual exposure at each level. 
The average amount of exposure and sub- 
mergence endured by each species can there- 
fore be calculated from this chart. (Each 
limit represents the average of several read- 
ings on the leveling staff. ) 
BIOTIC COMMUNITIES’ 
These are described roughly in their order 
of occurrence from high to low water. It 
should be observed, however, that the upper 
limit of one association may overlap the 
lower limit of the one immediately above: 
for example, the lower limit of the Corallina 
— H or mo sir a association is given as 0.8 feet, 
while the upper limit of the Encrusting Coral- 
line association is 1.7 feet. This may be ex- 
plained by local differences in such factors 
as topography and exposure to wave action 
which tend to allow the replacement of one 
community by another. Thus, a horizontal as 
well as a vertical sequence with changed con- 
ditions of habitat may be distinguished. The 
range of exposure undergone by each com- 
munity is expressed as a percentage of the 
total possible exposure per annum. 
Littorina Formation 
1. Calothrix — Melaraphe Association 
Vertical range: 12.9-9.2 feet. Between 
E.H.W.S. and M.H.W.N. 
Exposure: 100-78 per cent. 
Calothrix scopulorum (d) 4 
Rhizoclonium riparium (Id) 
Enteromorpha procera f. minuta (f) 
Microcoleus acutissimus (o) 
Melaraphe oliveri (d) 
Lyngbya lutea (a) 
Lophosiphonia macra (If) 
Mono stroma latissimum (r) 
Two separate consociations are recogniz- 
able: 
a. Calothrix Consociation 
The dull, blackish-green crusts of Calothrix 
scopulorum cover extensive areas of exposed 
rock about the highest levels of the inter- 
tidal region, especially on flat or gently slop- 
ing surfaces exposed to strong insolation. 
After a high spring tide or a heavy rain, the 
thick sheaths surrounding the trichomes be- 
come extremely gelatinous. Calothrix does 
not tolerate stagnation because in small, 
water-filled depressions, in which Lophosi- 
phonia macra is often present, growth of 
Calothrix ceases abruptly. During periods of 
continuous exposure, the encrusting mat be- 
comes dry and cracked and peels off easily. 
At such times the rock colonized by Calothrix 
may become white because of salt efflores- 
cence. The upper limit of the consociation, 
which is much more clearly defined than the 
lower, is probably correlated with sudden ex- 
treme changes in salinity and microclimate. 
An example of sudden temperature changes 
in the Calothrix belt was recorded on Febru- 
ary 8, 1948. At noon, rock temperature 
reached 33.3° C. After a brief shower it fell 
to 24° C. 
b. Melaraphe Consociation 
As Calothrix becomes sparser in the lower 
2 feet of its vertical range, the small gastro- 
pod Melaraphe oliveri assumes dominance. 
4 d = dominant o = occasional 
a = abundant 1 = local 
f = frequent r = rare 
