REARING OF BIVALVE MOLLUSKS
39
the chief controlling factors have been food and temperature. The role
of these conditions will be more fully discussed in special sections later
on, and also in the sections dealing with development and growth of
larvae of different species ; therefore, here it will be sufficient to mention
only general observations.
Our experiment8 have shown that larvae of different bivalves display
different food requirements (Loosanoff and Davis, 1951). Until they
reach a length of about 125 p larvae of C. virginim, for example, are
quite restricted in types of food they can utilize (Davis, 1953). Certain
naked flagellates are the only organisms, thus far tested, that may be
included in this category. Chlorella is one of the many genera of algae
having thick cell walls that oyster larvae either cannot utilize or
utilize only to a very limited extent during early stages, although it
seems to be quite a satisfactory food for older larvae (Davis and
Guillard, 1958). Thus, if during the early stage of development of
oyster larvae specific food organisms are either entirely absent or are
uncommon, the larval free-swimming period may be greatly prolonged
or the larvae may never reach metamorphosis. For example, during
our earlier efforts of raising larvae of C. virginica, when little was
known about their food requirements, approximately 50 days were
required before the most advanced individuals began to metamorphose
in some cultures. In similar cases, with larvae of C. gigas, the cultures
were discarded after 53 days because the largest larvae at the time
measured only about 100 p. Now, using good food organisms, such as
Ieochryeis galbana and other naked flagellates, and mairitaining the
temperature at about 23"C, larvae of C. iiirginica have been reared to
metamorphosis in our laboratory in 18 days. This is, probably, the
approximate time required by larvae to grow to setting size under
natural conditions in Long Island Sound (Loosanoff and Engle, 1947).
At 30°C well-fed oyster larvae, grown under laboratory conditiong,
began to metamorphose 10 days after fertilization.
The importance of the second factor, water temperature, on length
of larval period of bivalves has also been well demonstrated in our
etudies of larvae of M . mercenaria (Loosanoff et al., 1951). These
studies showed that, under identical conditions, larvae kept at a temperature near 30°C began to set as early as the 7th day afkr fertilization,
while cultures maintained at 18°C contained the first metamorphosing
individuals only after 16 days.
In the caae of larvae of all the species we reared it has been
clearly demonstrated that even though larvae originate from the same
spawning and, sometimes, from the same parents, and are kept in the
same vessel under identical conditions, individuals grow at widely
39
the chief controlling factors have been food and temperature. The role
of these conditions will be more fully discussed in special sections later
on, and also in the sections dealing with development and growth of
larvae of different species ; therefore, here it will be sufficient to mention
only general observations.
Our experiment8 have shown that larvae of different bivalves display
different food requirements (Loosanoff and Davis, 1951). Until they
reach a length of about 125 p larvae of C. virginim, for example, are
quite restricted in types of food they can utilize (Davis, 1953). Certain
naked flagellates are the only organisms, thus far tested, that may be
included in this category. Chlorella is one of the many genera of algae
having thick cell walls that oyster larvae either cannot utilize or
utilize only to a very limited extent during early stages, although it
seems to be quite a satisfactory food for older larvae (Davis and
Guillard, 1958). Thus, if during the early stage of development of
oyster larvae specific food organisms are either entirely absent or are
uncommon, the larval free-swimming period may be greatly prolonged
or the larvae may never reach metamorphosis. For example, during
our earlier efforts of raising larvae of C. virginica, when little was
known about their food requirements, approximately 50 days were
required before the most advanced individuals began to metamorphose
in some cultures. In similar cases, with larvae of C. gigas, the cultures
were discarded after 53 days because the largest larvae at the time
measured only about 100 p. Now, using good food organisms, such as
Ieochryeis galbana and other naked flagellates, and mairitaining the
temperature at about 23"C, larvae of C. iiirginica have been reared to
metamorphosis in our laboratory in 18 days. This is, probably, the
approximate time required by larvae to grow to setting size under
natural conditions in Long Island Sound (Loosanoff and Engle, 1947).
At 30°C well-fed oyster larvae, grown under laboratory conditiong,
began to metamorphose 10 days after fertilization.
The importance of the second factor, water temperature, on length
of larval period of bivalves has also been well demonstrated in our
etudies of larvae of M . mercenaria (Loosanoff et al., 1951). These
studies showed that, under identical conditions, larvae kept at a temperature near 30°C began to set as early as the 7th day afkr fertilization,
while cultures maintained at 18°C contained the first metamorphosing
individuals only after 16 days.
In the caae of larvae of all the species we reared it has been
clearly demonstrated that even though larvae originate from the same
spawning and, sometimes, from the same parents, and are kept in the
same vessel under identical conditions, individuals grow at widely