386 JOHNSON AND BKINTON [CHAP. 18 



In the case of the strongly migrating euphausiids, sub-adults and larvae of most 

 species are present above 200-300 m during the day-time, while adults are 

 found in the upper layers at night. Samples obtained below 300 m, including 

 those from multiple hauls using opening-closing nets, are used to plot gen- 

 eralized profiles of vertical distribution. Such hauls serve also to establish 

 which of the species distributions can be reliably mapped on the basis of the 

 standard collections — that is, whether presence or absence at a locality can be 

 determined from collections made above 300 m. 



B. The Pacific Subarctic Species 



The Pacific subarctic sustains a high biomass, although its geographical 

 extent is small (six million square miles) compared with that of the North 

 Pacific central (10 million square miles) and Pacific equatorial zones (15 million 

 square miles). In the northwestern Pacific, the subarctic zone is characterized 

 by the copepod species Calanus tonsus, C. cristatus, Eucalanus bungii bungii, 

 the amphipod Parathemisto japonica and the euphausiid Euphausia pacifica 

 (Brodskii, 1955; Bogorov and Vinogradov, 1955; Beklemishev and Bilrkov, 

 1958). Eucalanus bungii bungii, the northern variety of this species, is also 

 characteristic of the eastern portion of this zone and the northern portion of the 

 California Current (Johnson, 1938 et seq.), as is Calanus cristatus. 



The distribution of the euphausiid Thysanoessa longipes is representative of 

 the subarctic zone (Fig. la). Banner (1949) showed that a large form of T. 

 longipes bears spines on the posterior abdominal segments. A smaller form has 

 keels on those segments. The "spined" form is more northern than the "un- 

 spined" form. Both occur off Hokkaido, Japan, though their densities diminish 

 sharply at the convergence where the cold water of the Oyashio submerges 

 beneath the Kuroshio system. The "spined" form is predominant in the Gulf 

 of Alaska while only the "unspined" form is found in the California Current, 

 southward to 39°-40°N. 



Water-mass envelopes for the two forms of T. longipes (Fig. 2) confirm the 

 subarctic nature of the water in which both live. Water inhabited by the 

 "spined" form is more nearly uniform with respect to the T-S property than 

 that inhabited by the more southern "unspined" form. The environment of 

 the latter, while remaining nearly subarctic in character below 300 m, is strongly 

 influenced above that depth by admixture of the North Pacific Drift. (The 

 layer of maximum density of T. longipes during both day and night is 0-280 m.) 

 Populations of "unspined" T. longipes are intermingled with the transition 

 zone species T. gregaria (Fig. 4) in the region 41°-45°N. 



An area of abundance is evident in the distribution pattern of each of the 

 forms of T. longipes. The areas are separated from each other in mid-ocean by 

 a belt of lower density having an east-west axis near 50°N, but converge off 

 northern Japan at the western limit of both ranges. The two forms are regarded 

 as ecophenotypes. It is not known whether they are genetically separate. The 

 morphological distinctions, not evident in the young, appear to be brought 



