418 
PACIFIC SCIENCE, Vol. IX, October, 1955 
TABLE 2 
Length of the cephalothorax in Calanus tenuicornis and C. ligbti 
(Measurements in Millimeters) 
STATION 
Calanus tenuicornis DANA 
Calanus lighti , sp. 
NOV. 
Number of 
specimens 
Average 
Range 
Number of 
specimens 
Average 
Range 
Cruise 6 
107 
5 
1.54 
1.49-1.59 
205 
13 
1.54 
1.47-1.59 
405 
15 
1.66 
1.54-1.81 
605 
17 
1.78 
1.64-1.88 
707 
12 
1.66 
1.47-1.76 
807 
13 
1.54 
1.40-1.81 
45 
2.31 
2.17-2.41 
1011 
11 
2.05 
1.88-2.31 
1107 
8 
1.57 
1.52-1.61 
15 
2.02 
1.88-2.12 
1201 
27 
1.74 
1.64-1.88 
1205 
14 
1.64 
1.54-1.71 
100 
2.07 
1.90-2.27 
Cruise 9 
1009 
28 
1.54 
1.37-1.61 
90 
2.01 
1.83-2.14 
C. lighti , mostly from CCOFI Cruise 6 (Au- 
gust, 1949), with one station from Cruise 9 
(November, 1949). It is noteworthy that at 
the five stations where both species were 
taken there was no overlap in size at any 
one station. The absence of intermediate 
forms is evidence that interbreeding between 
the two copepods does not occur. 
DISTRIBUTION IN THE NORTHEAST PACIFIC 
Figure 3 shows the distribution of the 
Calanus tenuicornis and C. lighti encountered 
during four CCOFI cruises. At the stations 
shown, oblique hauls of 20 minutes duration 
were made with one meter plankton nets from 
a depth of about 70 meters (140 meters in 
Cruise 20) to the surface. Details of net con- 
struction and methods by which the hauls 
were made are given by Ahlstrom (1948, 
1952). C. tenuicornis was widely distributed 
over the area covered, while C. lighti was 
limited to the stations south of Point Con- 
ception. It is interesting to note that C. lighti 
extends furthest north at the outermost sta- 
tions, so that the northern boundary of its 
range roughly parallels the surface isotherms. 
This sort of distribution is very common 
among certain planktonts in this region and 
spherical refractile bodies. These represent a 
protozoan parasite which is probably respons- 
ible for the erosion of the furcal setae noticed 
in all specimens examined. It has been prac- 
tically impossible to find specimens with all 
the furcal setae intact, and I have not found 
a single male with an undamaged next-to- 
outermost seta. These parasites are so char- 
acteristic that they can almost be used for 
separating these two species from other 
planktonic copepods. 
SIZE RELATIONSHIPS 
The body length of C. tenuicornis is given 
by several investigators (Dana, 1853; Gies- 
brecht, 1892; Esterly, 1905; Farran, 1929; 
Rose, 1933; Verwoort, 1946). Comparative 
measurements of the body lengths of C. 
tenuicornis and C. lighti are: 
C. tenuicornis , from 
FEMALE 
mm. 
MALE 
mm. 
literature 
C. tenuicornis , CCOFI 
1.8 -2.1 
1.5 -1.95 
specimens 
C. lighti , CCOFI 
1.64-2.39 
1.71-2.02 
specimens 
2.32-3.08 
1.90-2.68 
In Table 2 are given the cephalothorax 
lengths of 413 specimens of C. tenuicornis and 
