REARING OF BIVALVE MOLLUSKS
85
ence larvae of these clams have never approached the size of 320 p,
as reported by Sullivan (1948). In a letter to us Miss Sullivan explained
that she had confused larvae of Pitar morrhuana with those of M . mercenaria and ascribes the error to the circumstances that newly set spat
of both species were present in her collection (Loosanoff et al., 1951).
Our observations on maximum size of larvae reached before metamorphosis also show that it is highly improbable that Stafford (1912)
was actually describing larvae of M . mmenaria because he stated that
veligers of this clam grew to a size of 448.6 p before setting. Furthermore, when the shapes of the larvae shown in Stafford's figures are
compared with those of larvae grown from eggs at our laboratory, it
becomes clear that Stafford mistook larvae of some other bivalve for
those of M . mercenaria.
Food requirements of larvae of M . mercenaria are less restricted
than those of some other species, such as C. virginica. Clam larvae
can be grown to metamorphosis on a diet consisting almost exclusively
of a unialgal culture of Chlorella sp. However, such foods as I . galbana
and M . lutheri promote better growth.
Larvae have been grown at temperatures ranging from 15" to 33OC.
At the latter temperature setting occurred 6 to 8 days after fertilization.
During early stages the color of the larvae can be significantly
changed by differently colored foods. However, as the larvae grow and
their shells become darker, this effect becomes less noticeable. Larvae
of the same cultures greatly differ in size, especially if they are not
properly attended or are too crowded.
Larvae of M . mercenaria are not too selective with regard to the
substratum on which their metamorphosis is completed. They metamorphose in containers made of a variety of substances, including
glass, plastics, earthenware, Plexiglas, concrete and several other
materials. This is contrary to observations on other forms, such aa
echinoderms (Mortensen, 1938) and polychaetes (Wilson, 1948), where
it was found that mature larvae postponed metamorphosis until they
located a substratum possessing the necessary physical and chemical
characteristics. In our cultures setting usually began when some
larvae were approximately 180 p in length, regardless of the size of
containers or material of which they were made.
Setting of clam larvae occurred at approximately the same time,
whether they were grown in containers from which all light w a
excluded or in transparent glass vessels. Turner and George (1955)
found that clam larvae were rapid swimmers capable of moving upward
at the rate of 8 cm/min. However, they showed no detectable reaction
to light.
85
ence larvae of these clams have never approached the size of 320 p,
as reported by Sullivan (1948). In a letter to us Miss Sullivan explained
that she had confused larvae of Pitar morrhuana with those of M . mercenaria and ascribes the error to the circumstances that newly set spat
of both species were present in her collection (Loosanoff et al., 1951).
Our observations on maximum size of larvae reached before metamorphosis also show that it is highly improbable that Stafford (1912)
was actually describing larvae of M . mmenaria because he stated that
veligers of this clam grew to a size of 448.6 p before setting. Furthermore, when the shapes of the larvae shown in Stafford's figures are
compared with those of larvae grown from eggs at our laboratory, it
becomes clear that Stafford mistook larvae of some other bivalve for
those of M . mercenaria.
Food requirements of larvae of M . mercenaria are less restricted
than those of some other species, such as C. virginica. Clam larvae
can be grown to metamorphosis on a diet consisting almost exclusively
of a unialgal culture of Chlorella sp. However, such foods as I . galbana
and M . lutheri promote better growth.
Larvae have been grown at temperatures ranging from 15" to 33OC.
At the latter temperature setting occurred 6 to 8 days after fertilization.
During early stages the color of the larvae can be significantly
changed by differently colored foods. However, as the larvae grow and
their shells become darker, this effect becomes less noticeable. Larvae
of the same cultures greatly differ in size, especially if they are not
properly attended or are too crowded.
Larvae of M . mercenaria are not too selective with regard to the
substratum on which their metamorphosis is completed. They metamorphose in containers made of a variety of substances, including
glass, plastics, earthenware, Plexiglas, concrete and several other
materials. This is contrary to observations on other forms, such aa
echinoderms (Mortensen, 1938) and polychaetes (Wilson, 1948), where
it was found that mature larvae postponed metamorphosis until they
located a substratum possessing the necessary physical and chemical
characteristics. In our cultures setting usually began when some
larvae were approximately 180 p in length, regardless of the size of
containers or material of which they were made.
Setting of clam larvae occurred at approximately the same time,
whether they were grown in containers from which all light w a
excluded or in transparent glass vessels. Turner and George (1955)
found that clam larvae were rapid swimmers capable of moving upward
at the rate of 8 cm/min. However, they showed no detectable reaction
to light.