Life History of Nereis grubei — Schroeder 
(although I do not know whether G astro do - 
nium is abundant at Point Fermin). 
In an ecological study at the Hopkins Marine 
Station, not far from Pescadero Point, small 
numbers of N. grubei were found by Glynn 
(1965) in association with another alga, En- 
docladia muricata. This alga is not included in 
the list given by Reish for the southern Califor- 
nia population. 
OOGENESIS AND METAMORPHOSIS 
Nereis grubei was collected every two weeks 
at Pescadero Point, Monterey, California, for a 
period of 17 months. During the period of 
oogenesis the average oocyte diameter was de- 
termined for a number of specimens from each 
collection; the number of specimens so deter- 
mined varied with experimental requirements. 
All diameters were measured from fresh coelomic 
samples with a calibrated ocular micrometer and 
a compound microscope. Fixation in Bourn’ s 
fluid made up with picric-acid-saturated sea 
water was found to cause an irregular expansion 
of the oocytes (Schroeder, 1966). In some ani- 
mals, oocytes of considerably different diameters 
were present, but in most of these the recorded 
diameters fell into distinct size classes. In such 
cases only the average diameter of the largest 
size class was utilized in the life history study. 
In the few cases in which size classes were not 
evident, the diameters of all the oocytes from a 
single animal were averaged together. It is 
probable that such figures are lower than the 
value truly comparable to the others. The aver- 
age oocyte diameter was used as an index to 
the age of female animals. 
Oocytes could be found in at least a few ani- 
mals throughout the year. Oogonia and tiny 
oocytes appear in June, when very few matur- 
ing females remain from the previous year’s 
spawning group. The oocytes probably reach 
maturity (about 200p) in 7-8 months. The 
young oocytes grow slowly through the fall and 
early winter, and the population gives rise to 
several swarms between mid-February and mid- 
June. The occasions on which nearly mature 
females were found in the field are indicated by 
the short vertical bars at the top of Figure 1. 
(The O in mid-March represents a day on 
which metamorphosing animals were expected 
477 
and sought, but not found.) Such nearly ma- 
ture animals probably swarmed within a couple 
of days of being observed, although swarming 
itself was never observed. 
The fact that maturing animals are found on 
separate occasions, and not in intervening pe- 
riods, implies that specimens within the popula- 
tion mature at different times. It should thus be 
possible, in midwinter population samples, to 
separate animals destined for different spring- 
time spawnings. 
Oocytes have been shown to grow at two dif- 
ferent rates, both in N. grubei (Schroeder, 
1966) and in N. diversicolor (Clark and Rus- 
ton, 1963). Thus in N. grubei the oocyte 
growth rate (rate of diameter increase) seems 
to accelerate when the oocytes are about lOOp 
in diameter, and it is at about this time that the 
first histological signs of metamorphosis can be 
detected (Schroeder, 1967). This fact is utilized 
in Figure 1 in an attempt to follow the develop- 
ment of each monthly swarm within the popu- 
lation as a whole. The dotted lines are projec- 
tions based upon the apparent time of swarming, 
estimates of the growth rate during the two 
phases, and the recorded oocyte diameters. It is 
assumed that the oocyte growth rates are similar 
in the component animals of successive spawn- 
ings. Each month on the graph has been sub- 
divided into 3-day periods, so that the positions 
of the points within a month are significant to 
within three days. It will be seen that during 
the fall animals with oocytes of a wide range of 
sizes are present, but that these are not separ- 
able into distinct groups until December, when 
most specimens can be assigned to a spawning 
group, even though the oocytes are still small 
and growing slowly. It is possible that the dif- 
ferent spawning groups are distinct from the 
initial moment of oogonial differentiation, but 
the variability of oocyte sizes present in young 
animals prevents their recognition. 
A number of animals was maintained in the 
laboratory over a period of weeks and the growth 
of their oocytes was checked by repeated samp- 
ling. These sampling points have also been in- 
cluded in Figure 1 and are connected with 
heavy lines. It will be seen that all but two of 
these animals were derived from the April 
spawning group, while two others metamor- 
phosed sooner and were closer to the March 
