252 
BULLETIN OP THE BUREAU OF FISHERIES 
several months previous. Probably the specimens in question had drifted thither 
around Cape Cod from the center of abundance in the southwestern part of the gulf. 
Granting that M. Tonga is able to breed in the gulf to some extent, its periodic 
disappearances are sufficient evidence that it does so only sporadically and tempo- 
rarily. Perhaps it is only able to carry on through one or two generations in the 
high temperatures in which it must exist there, and failing accessions of new stock 
dies out until there is a fresh invasion from the north. Evidently such fluctua- 
tions in local reproduction and migrations mirror the physical features of the water 
in which this little crustacean lives, but it is not yet possible to state the precise 
relationship which its temporary appearances in the Gulf of Maine bear to tempera- 
ture and salinity there or in the waters to the east and north, or to the seasonal or 
annual variations in the flow of the currents. 
There is every reason to class it a cold-water species in the gulf, and it has 
actually been taken there in water a fraction cooler than zero (at St. Andrews, 
February, 1917; Willey, 1921); but having been found widespread in the summer 
and autumn of 1915 in temperatures as high as 8 to 10°, it can survive and perhaps 
even breed over a wider range than has generally been supposed in European seas, 
where 6.75° is the highest temperature of record for it (Farran, 1910), and where 
most of the captures have been from water of 2.25 to 3.25°. M. Tonga was in 
comparative abundance and apparently in good condition off Marthas Vineyard at 
14.5° (station 10331), but it is hardly conceivable that it could have lived long there. 
Minimum, temperatures at any depth at stations where Metridia longa is recorded for August, September, 
or October, 1915 
Station 
Date 
Minimum 
tempera- 
ture in 
degrees C. 
Station 
Date 
Minimum 
tempera- 
ture in 
degrees C 
10304 
Aug. 6 
4. 78 
10325 
Oct. 4 
5.28 
10306 
Aug. 31 
5. 78 
10326 
do 
5. 39 
10307 
5. 1 
10327 - 
Oct. 9 
9.4 
10309 
5. 72 
10328 - 
do. 
9.4 
10311 
Sept. 2 
9.4 
10329 
.do 
8. 95 
10315 
Sept. 7 
10 
10331 
Oct. 21 
14.5 
10318 
Sept. 16 
8. 61 
10333 
Oct. 22 
11. 89 
10319 
8.5 
10337 
Oct. 26 
30. 39 
10321 __ 
11. 22 
10338 
Oct. 27 
9.4 
10323 
6 
10339 ___ 
do 
7.28 
10324.. 
do 
6. 78 
More information is needed before the relationship between the salinity of 
the water and the occurrence of M. Tonga in the gulf can be traced. Most of the 
records for this species in the northeastern Atlantic have been from salinities rather 
higher than those of the Gulf of Maine, where it has been taken most commonly 
in water of 32 to 33.5 per mille; but Nordenskiold’s account (p. 248) suggests that in 
the very low temperatures of the polar sea it may be able to exist in water but slightly 
saline, and we took it in salinities of 31 to 32 per mille on several occasions during 
the spring of 1920 and once in 29.94 per mille (station 20096, surface haul). Probably 
M. Tonga is never plentiful enough to be of much importance in the natural economy 
of the Gulf of Maine, but no doubt it serves to some extent as fish food, having been 
