The Fauna of the Algal Zone of the Swan River Estuary. 57 
Estimation of Population. 
Although the importance of the algal fauna has long been recognised 
and such phrases as “among sea-weed” frequently occur in records, few 
studies of the fauna of aquatic vegetation have been made. Those that have 
been made are concerned almost entirely with fresh water plants (Ward, 
1896; Moore, 1913; Richardson, 1921; Percival & Whitehead, 1929; Krecker, 
1939), usually with flowering plants. Descriptions of the algal zones of 
the sea have rarely been accompanied by data as to the small members 
oc the communities on and among the algae. The record is mostly of the 
dominant animals, usually Molluscs, occurring on the same substratum as 
the algae and sometimes passing on to the vegetation. (Flattely & Walton 
1922; Colman, 1933; Kitehing, 1935; Bright, 1938; Stephenson & Bright, 
1938; Bokenham, 1938; Eyre, 1939; Stephenson & Day, 1940). A method 
for the quantitative analysis of the fauna of aquatic vegetation is wanting. 
Moore (1913) used the general terms “abundant” and “scarce.” Bokenham 
(1938) used “Dominant,” “Plentiful” and “Present.” Richardson (1921) 
made a quantitative count of the upper nine inches of aquatic plants. The 
count was based on the animals washed off the plants collected from a 
definite area. Other workers (Needham, 1928, 1929; Pate, 1932) have failed 
to differentiate between the animals of the plants and those of the underlying 
substratum, except in the case of the two authors mentioned next. 
Krecker (1939) working on fresh- water phanerogams took as a basis the 
population per ten linear feet of plant. He conducted a plant to plant 
examination. The only alga investigated in the Swan estuary which lends 
itself to such treatment is Cystophyllum muncatun which attains lengths of 
up to seven feet. But the majority of the algae are relatively short, occur- 
ring as more or less branching filaments or intertwining strands densely 
crowded together. The broad flat Ulva lactuca provides a third type. 
Colman (1940) working on the fauna of the intertidal sea-weeds, used 
the number of animals per unit weight of sea-weed. The method adopted in 
the present research was to calculate the fauna per 100 c.c. of the alga 
measured by displacement of water. As few, if any, of the animals present 
used the algae as food, but rather as a substratum it was considered that 
the space occupied by the alga was of more significance than the weight, 
since the volume bears a constant relationship to the surface of the plant 
(though admittedly the relationship varies to a degree from plant to plant 
according to irregularities of shape) whereas the weight is not a function 
of the surface, since it varies with the density. The displacement method 
adopted is also advantageous in being rapid. 
By this means a picture was gained not only of the algal fauna as a 
whole, but enabled comparison between different algae. 
