The Fauna of the Algal Zone of the Swan River Estuary. 61 
sented by immature specimens only during August, September and November. 
The following were the dominant species (numerically) from month to 
month : — 
March — Tmais cavolinii. Caprella penantis. 
April — Caprella penantis; Tanais cavolinii. 
May — H arpa cticus gra cilis. 
June. — Erichthonius pugnax ; Melita sp. (Immature). 
July — Caprella scaura ; Tanais cavolinii. 
A ugu s t — Gladiof evens imp a rip es. 
September — Gladiof evens imparipes . 
October — Mesochra parva; Glacliof evens imparipes. 
November — Mesochra parva ; Glacliof evens imparipes. 
December. — Harpacticus gracilis ; Tisbe tenera. 
The population density of the algal zone fell from March to August 
and then rose again. (See text fig. 1.) 
The number of species present also dropped from 3d in March to six 
in September, rising again to 25 in December. 
Species Density. 
Following Hesse, Allee, Schmidt (1937), “Species Density” is taken to 
mean the number of species present in unit area or unit volume. The 
figures are given at the bottom of Tables 1 and 2. It will bo seen that 
although Cystophyllum had the greatest population density, in March and 
June its species density was not as great as that of Enteromorplia , and 
in April and May there was no significant difference between the two. The 
highest species density is 25, recorded for Cladophora in December. En- 
ter omorpha follows with 22 in March. Like the population density, the 
species density fell from March to August and September and then rose 
again. The lowest species density recorded was 1 ( Viva- in August, and 
Enteromorplia in September). Species density does not necessarily corres- 
pond with the population density. Thus in March Cystophyllum had tho 
greatest population density, but the lowest species density. 
Again, although two species of alga may have similar species densities, 
the species making up the community are not necessarily the same. Thus in 
July Enteromorplia and Ulva had species densities of 13 and 12 respectively 
but had only five species in common. The apparent preference for one alga 
or another is doubtless extremely complex in its causation; all the factors 
that influence environmental distribution probably play a part, food, com- 
petition with other species, vulnerability to attack and so on. 
Variations with Depth. 
Apart from the collections summarised in Table 1, on three occasions 
separate collections were made to gain an idea of distribution by depth. The 
results are shown in Table 2. The four inches nearest the surface are least 
densely inhabited, particularly where open to buffeting by the wind-driven 
waves. In positions sheltered by large rocks the top few inches are more 
thickly populated. Most species were taken between four inches and two 
