270 
BULLETIN OF THE BUREAU OF FISHERIES 
Relationship to temperature and salinity. — The geographic distribution of P. parvus 
n the ocean in general points to moderately high temperatures as most favorable 
for it, justifying Farran’s (1910) characterization of it as a tropical and temperate 
species. The many records of it in the Red Sea, around Ceylon, and in the Malay 
Archipelago, often from hauls no deeper than the intake pipe of a steamer’s pump 
(A. Scott, 1902), make it probable that no temperature ever prevailing in the open 
sea is fatally high or even unfavorably so for it. Toward the other extreme, the 
presence of P. parvus at so many localities in the Gulf of Maine in February and 
March (table, p. 299) proves it able to survive cooling down to 3 to 5°. In fact, the 
actual localities and depths of capture locate it in water fractionally cooler than 2° 
at three different stations; 45 but most of these February-March records are from 
localities where the temperature was above 3° at some level between the surface and 
the bottom (stations 20044, 20045, 20046, 20048, 20054, and 20081). Specimens 
drifting into colder regions or levels of the gulf in early spring may perish, as any 
animal finding its optimum environment in high temperature probably would. 
Thus, the zone close to the coast may well be a death trap for this copepod during 
the coldest season, but the stock living in the basin can avoid winter chilling by 
sinking to the deeper levels, where it would not experience a temperature lower than 
4 to 5° in most years. Therefore, it would not be surprising if more extensive study 
proves its zone of maximum abundance in the gulf to lie at a greater depth during 
the coldest season than during summer and autumn. Tending to corroborate this 
prediction is the fact that the richest catches for March and April (stations 20054 
and 20115) were in vertical hauls from 250 and 295 meters, respectively, where the 
temperature below 150 meters was 5° or higher; and that the vertical nets fished 
through zones of water warmer than 10° (below 100 meters) at the localities of the 
“rich” catches off the southwest slope of Georges Bank for February (stations 20044 
and 20045). 
Previous records locate P. parvus in salinities higher than 40 per mille in the 
Arabian Gulf and as low as 19.33 per mille in the Kattegat. In addition it appears 
indifferently oceanic or neritic, occurring from the open sea, on the one hand, to 
tide pools, on the other ( fide Dr. C. B. Wilson). Therefore, it is not likely that the 
variations in salinity which obtain in the Gulf of Maine are an important factor in 
influencing its distribution there. Perhaps no member of the crustacean plankton 
of the open sea can accommodate itself to greater fluctuations in the salinity of the 
water than this little copepod. 
Endemicity and immigration. — The spawning of P. parvus has not actually been 
recorded in the Gulf of Maine, but the fact that the species occurs there throughout 
the year and is about equally widespread from month to month, though with a 
definite periodic cycle in its abundance and in the regularity of its distribution, is 
strong evidence that P. parvus does reproduce successfully in the gulf, and that 
enough of the stock survives the winter to multiply to the frequencies recorded for 
summer and autumn. The monthly averages for the percentages of stations at 
which the species has been taken and for the numbers of specimens per square 
meter both point to May as the commencement of the breeding season in the gulf; 
45 Station 20056, whole column cooler than 1.19°; station 20058, whole column 1.39 to 1.43°; station 20081, surface 1.95°. 
