In an article by Prof. Karl Mobius on "The Oyster and Oyster Culture,"
reproduced in the recently issued report of the U. S. Commissioner of
Fish and Fisheries, the author says:
A mature egg-bearing oyster lays about one million of eggs, so that
during the breeding season there are upon our oyster beds at least
2,200,000,000,000 young oysters, which surely would suffice to transform
the entire extent of the sea-flats into an unbroken oyster bed; for if
such a number of young oysters should be distributed over a surface 74
kilometers long by 22 broad, 1,351 oysters would be allotted to every
square meter. But this sum of 2,200,000,000,000 young oysters is
undoubtedly less than that in reality hatched out, for not only do those
full-grown oysters which are over six years of age spawn, but they begin
to propagate during their second or third year, although it is true that
the young ones have fewer eggs than those which are fully developed. At
a very moderate estimation, the total number of three to six year old
oysters which lie upon our beds will produce three hundred billions of
eggs. This number added to that produced by the five millions of full
grown oysters would give for every square meter of surface not merely
1,351 young oysters, but at least 1,535. In order to determine how
many eggs oysters produce, they must be examined during their spawning
season. This begins upon the Schleswig-Holstein beds in the middle of
June, and lasts until the end of August or beginning of September. The
spawning oyster does not allow its ripe eggs to fall into the water, as
do many other mollusks, but retains them in the so-called beard, the
mantle, and gill-plates until they become little swimming animals. The
eggs are white, and cover the mantle and gill-plates as a semi-fluid,
cream-like mass. As soon as they leave the generative organs the
development of the germ begins. The entire yolk-mass of the egg divides
into cells, and these cells form a hollow, sphere-like body, in which an
intestinal canal arises by the invagination of one side. Very soon the
beginnings of the shell appear along the right and left sides of the
back of the embryo, and not long afterward a ciliated pad, the velum, is
formed along the under side. This velum can be thrust out from between
the valves of the shell at the will of the young animal, and used by the
motion of its cilia as an organ for driving food to the mouth, or
in swimming as a rudder. During these transformations the original
cream-white color of the germ changes into pale gray, and finally into a
deep bluish-gray color. At this time they have a long oval outline, and
are from 0.15 to 0.18 of a millimeter in breadth. Over 300,000 can find
room upon a square centimeter of surface. If an oyster in which the
embryos are in this condition is opened, there will be found upon its
beard a slimy coating thickly loaded with grayish-blue granules. These
granules are the embryo oysters, if a drop of the granular slime be
placed in a dish with pure sea water, the young animals will soon
separate from the mass, and spread swimming through the entire water.
When the embryos are at this stage their number may be estimated in the
following manner: The whole mass of embryos is carefully scraped from
the beard of the mother oyster by means of a small hair brush. The whole
mass is then weighed, and afterward a small portion of the mass. This
small portion is then diluted with water or spirits of wine, and the
embryos portioned out into a number of small glass dishes, so that they
can be placed under the microscope and counted. Thus, knowing the weight
of the small portion and the number of embryos in it by count, we can
estimate the total number of embryos from the weight of the entire mass,
which is also known. In this manner I estimated the number of embryos
in each of five full grown Schleswig-Holstein oysters caught in August,
1869, and found that the average number was 1,012,956.
Notwithstanding this great fecundity, but an extremely small proportion
of the young oysters produced during the course of the summer arrive
at maturity, 421 only out of 500,000,000 escaping destruction. The
immolation of a vast number of young germs is the means by which nature
secures to a few germs the certainty of arriving at maturity. In order
to render the ideas of germ-fecundity and productiveness more easily
understood, Prof. Mobius makes the following comparison between the
oyster and man:
According to Wappaus, for every 1,000 men there are 347 births.
According to Bockh, out of every 1,000 men born 554 arrive at maturity,
that is, live to be twenty years or more of age; thus, on an average,
347 children are produced from 554 mature men, or 626 children from
1,000 mature men. Since 1,000 full-grown oysters produce 440,000,000 of
germs, then the germ fecundity of the oyster is to the germ fecundity of
man as 440,000,000 to 6.26, or as 7,028,754 to 1. On the other hand, the
number which arrive at maturity is 579,002 times as great with mankind
as with the oyster; for of 1,000 human embryos brought into the world
554 arrive at maturity, or of 440,000,000 newly born 243,760,000 would
live to grow up, while of 440,000,000 young oysters only 421 ever become
capable of propagating their species. The proportion is then 421 to
243,760,000, or as 1 to 579,002. I am fully persuaded that these figures
represent the number of oysters which arrive at maturity more favorably
than is really the case, since from every thousand of full grown oysters
it is certain that, on an average, more than 440,000,000 young are
produced.
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