62 
J. M. Thomson. 
feet depth. But the population density varied in its maximum from deeper 
than two feet to shallower, probably according to the physical conditions 
at the particular time. A point to be noted is that in November, Gladiofe - 
Tens imparipes was present in large numbers below a depth of two feet, but 
absent entirely above; whereas Mesochra parva was present in large num- 
bers in the surface layers, but few in numbers below two feet. 
Biotic Influence from Outside the Community. 
It has been mentioned earlier that certain of the species recorded in these 
collections are more properly regarded as only occasional intruders into the 
algal association. Such are Gammarid spp. Cyclograpsus audouinii, Modi- 
olus sp., Sagitta sp . Besides these there are occasional intruders which are 
not recorded in collections but whose presence was noted. This group includes 
both aquatic and non-aquatic creatures. 
Aquatic intruders, 
Trockus sp. Abundant on rocks above and below water level, and 
occasionally found amongst the algae. Also on the rocks occur Balanus 
nigrescens and Balanus amphitrite which possibly influence the abundance 
of the algae. Sphaeroma quoyana with its attendant commensal Iais pube- 
scens (var. longistylis) burrows into the sandstone which forms the sub- 
stratum for the algae. Whether it has any influence on the algal fauna 
above is unknown. Similar remarks apply to various Gammarids, Poly- 
chaets and Turbellarians found on the rock, together with various Isopods. 
Hippocampus tuberculatiis Castelnau has been taken in this weed area. 
Dunker (1910) reports Trachyrramplius brevicauclis Castelnau from the 
Swan estuary also, and this species may also occur amongst the algae. 
The blue Serrated Swimming crab Scylla serrata has also been observed 
amongst the algae. Swarms of little fish periodically appear, and these 
were particularly abundant during September-October. The common “jelly- 
fish,” Aurelia aurita is sometimes driven in large numbers among the algae,, 
with what effect, if any, upon the algal fauna is problematical. 
Extra-aquatic intrude rs. 
These generally appear when the water is particularly low, exposing 
the algal beds to the air. Some of the birds also pick through the weeds in 
shallow water. The following have been observed apparently feeding on 
or among the algae or the Molluscs on the rocks beneath. 
Phalacrocorax atm. Pisobia minuta 
Phalacrocorax varius. Tringoides hypoleucus. 
Phalacrocorax carbo. Laras novae -hollandiac. 
M i croca rbo mclanoleu cu s . 
When the algae are exposed, numbers of ants, flies, wasps, spiders, land 
isopods and beetles (e.g., Ophodinus sp.) make their way thither, retreating 
again as the water rises. Most of the algal fauna doubtless retreats as the 
water level falls, but some at least remain in the damp weed. 
