498 
BULLETIN OF THE BUREAU OF FISHERIES 
by dikes which hold enough water to protect the oysters from freezing and drying, 
while at high tide they may be covered by as much as 16 or 18 feet of water. Such 
tides involve the movement of great quantities of water and swift currents, but most 
of the beds are located in the upper ends of the bays where currents are not so rapid. 
The interchange of cold water from the very deep portions of the Sound with that in 
the upper ends of the oyster-producing bays prevents the temperature from rising to 
a high level even during an exceptionally warm season. For this reason eastern 
oysters, transplanted to Puget Sound, were never able to spawn, since the high-tide 
temperature on the oyster beds does not reach the critical level of 20° C. 
The spawning season is of several months duration and, although in no case has a 
sudden burst of spawning been observed in which a great number of oysters were 
involved, as occurs frequently with oviparous species, it has been found that smaller 
numbers of individuals often bear embryos or larvae of the same age, indicating that 
favorable conditions may cause spawning to take place in a considerable portion of 
the population at the same time. Alternation of sexual phases (Coe, 1931a, b; 1932a) 
probably is responsible for the rather slowly developed wave of spawning, for different 
individuals are at any time in different stages of maturity. Sometimes as many as 
12 to 15 percent of the adults bear larvae of the same age, so that a system of statistical 
sampling serves to show the rate of growth. Whether sperms or sperm extract will 
stimulate discharge of eggs b} 7 functional females (Galtsoff, 1930b, 1932) has not been 
demonstrated in this species, but it is considered probable. 
It is hardly to be expected that the small native oyster would discharge as many 
eggs as the larger oviparous species (Galtsoff, 1930), not only because of difference in 
size but also because the eggs are held within the mantle chamber where they grow into 
larvae almost twice the diameter of the eggs, and space alone probably acts as a limit. 
Altlioiigh an individual produces in one brood onfy about 250,000 to 300,000 larvae, all 
individuals are capable of bearing at least one brood each season, while the eastern 
oyster is generally functional as only of one sex during a single season (Coe, 1932c, d). 
In some years as many as 150 broods are produced per 100 oysters, indicating that a 
large number bear second broods, while in other seasons as few as 75 broods per 100 
individuals are produced. The degree of success of a spawning season depends upon 
the number of larvae per brood and the total number of broods produced. 
A problem which has never before been attacked is that of the relationship between 
angle of surface and frequency of attachment of larvae, although Prytherch (1934) 
observed that larvae in a dish set more abundantly on the vertical sides than on the 
bottom. It was shown in preceding pages that most larvae attach to under horizontal 
surfaces and that as the angle departs from this the larvae set in smaller numbers. It 
was demonstrated that this behavior of the larvae is not due to the effect of light, but 
the suggestion was put forward that in the normal swimming position of the larva 
the foot is projecting upward and therefore is able to take hold most readily to the 
under horizontal surface. Actual setting, according to Prytherch, is a specific process, 
and larvae may crawl for some time before definitely attaching themselves, but the 
foot must take hold before final attachment. It would seem probable, from considera- 
tions of structure, that the larvae of other species may also attach most abundantly 
to under horizontal surfaces. Various factors, however, may influence the reaction, 
and it would be of interest to determine the activities of other species in this respect. 
Incidentally, Prytherch’s (1934) observation that the pigment spots are not light- 
sensitive organs but have another function is in accord with the present results in that 
no evidence of a directive influence of light was noted. 
