108 
Fig. 1 . The number of young Mytilus planulatus 
settling at The Spit, Port Jackson, New South Wales 
during the breeding season 1949. 
settlement commenced, to over 22° C. when 
the main season of settlement ended at the 
beginning of August. The season of settle- 
ment is much shorter than that of M. planu- 
latus , but the range of temperatures over 
which settlement is successfully achieved is 
almost identical. The season of settlement in 
both M. edulis and M. planulatus corresponds 
to a temperature range of about 12.5° C. to 
about 19° C. with relatively insignificant set- 
tlement above 22° C. 
This statement is strengthened by informa- 
tion from several sources. Graham and Gay 
(1945) found that the season of attachment 
of M. edulis to test panels at Oakland, Cali- 
fornia, was only from March to May. Tem- 
peratures were rising from 14° C. at the 
beginning of March to about 19-5° C. in May 
(op. cit. fig. 1, p. 376). Further evidence that 
the higher temperature imposes a limit to 
breeding is the fact that M. planulatus has a 
longer breeding season in the colder waters 
of Victoria than it does in Port Jackson. Miss 
J. H. Macpherson of the National Museum 
of Victoria states (personal communication) 
that she found young mussels in the plankton 
all the year round, and that "there is a lull 
about June to September, but even in this 
period there are a few breeding." In Port 
Phillip, where these observations were made, 
PACIFIC SCIENCE, Vol. IX, April, 1955 
Fig. 2. The number of young Mytilus edulis settling 
at Milford Harbor, Connecticut during the breeding 
season 1942. (After Engle and Loosanoff, 1944, figs. 
1, 3.) 
temperatures for this June-September period 
are usually below 12° C., while the summer 
temperatures only occasionally reach 20° C., 
and rarely exceed this figure. Ralph and Hur- 
ley (1952) found an extended season of at- 
tachment for M. planulatus at Port Nicholson, 
Wellington, New Zealand, where maximum 
temperatures did not exceed 19° C. 
The evidence supports the view that the 
season of settlement of the two species is 
governed to a great extent by minimum and 
maximum temperatures which are almost 
identical for both species, and that outside 
the limits mentioned little, if any, reproduc- 
tion occurs. Breeding appears to be initiated 
by a rise of temperature to about 12°, and to 
continue as long as the upper limit of about 
19° is not exceeded. However, the attainment 
of this upper limit seems to inhibit further 
breeding, as settlement does not recommence 
when the temperatures drop once more to the 
breeding range. This may be indicative of an 
innate breeding rhythm which, in some local- 
ities, is modified by the seasonal temperature 
trends. Also this apparent spring breeding 
may occur only when these organisms are 
approaching the tropic-ward limit of their 
geographic range. 
The species M. planulatus and M. edulis are 
very similar in appearance and the additional 
