Identity of Breeding Temperatures in Southern and 
Northern Hemisphere Species of Mytilus (Lamellibranchia) 
F. E. Allen 1 
Mussels of the genus Mytilus are distributed 
throughout the temperate seas of the world 
and are among the most conspicuous animals 
of the shore and shallow seas. The Australian 
representative, Mytilus planulatus Lamarck, is 
one of the commonest bivalve molluscs of 
this region, and in many respects exhibits a 
marked similarity to the northern M. edulis 
Linne. This similarity is not only in general 
appearance, size, and vertical distribution, but 
seems to extend even to the temperature re- 
quirements for successful settlement. M. 
planulatus occurs throughout southern Aus- 
tralia, Tasmania, and New Zealand. On both 
east and west coasts of the Australian main- 
land, the northern limit of distribution is 
about 32° S. latitude, although outlying popu- 
lations may be found a little further north. 
Fremantle on the west coast and Port Stephens 
on the east may be considered the northern- 
most strongholds of M. planulatus\ the mean 
monthly summer temperatures being 23.4° C. 
and 24.6° C. respectively. M. edulis ranges 
throughout the northern hemisphere; Hut- 
chins (1947) has shown that its southern 
limit of distribution corresponds approxi- 
mately with the maximum monthly mean 
(summer) of 80° F. (26.6° C). 
The information presented here was ob- 
tained during the course of marine fouling 
1 Marine Biological Laboratory, Division of Fisher- 
ies, Commonwealth Scientific and Industrial Research 
Organization, Cronulla, New South Wales. Manuscript 
received July 26, 1954. 
investigations on the east coast of Australia 
The type of collector used has been described 
by Allen and Wood (1950). In each of the 
five years in which investigations were made, 
the season of settlement was clearly defined. 
Settlement in Port Jackson, New South Wales, 
began in June or early July, and terminated 
late in December. Settlement outside this sea- 
son is almost unknown. In April 1947 a few 
juveniles were found, but in view of the 
amount of information now available this 
occurrence is considered adventitious. Figure 
1 shows the counts of young M. planulatus 
settled on plates which were exposed for 14 
days at The Spit, Port Jackson, in 1949. No 
settlement took place prior to July 8 nor after 
December 9, which gives a season extending 
from midwinter through spring to early sum- 
mer. Temperatures ranged from 12.3° C. on 
June 24 (the minimum for the year) and 15.1° 
C. on July 8 to 21.3° C. on December 9, the 
end of the last fortnight during which settle- 
ment took place. 
For this season of settlement there is a 
striking parallel in the northern mussel, My- 
tilus edtdis L., as reported by Engle and Loos- 
anoff (1944). Their observations were made 
at Milford Harbor, Connecticut, U.S.A., 
which is in approximately 41° N. latitude. 
Settlement of M. edulis took place there from 
June to August (Fig. 2), the peak being from 
the middle of June to the middle of July in 
1942. During this period the temperatures 
ranged from about 12.5° C. on June 1, when 
107 
