age of the algal cultures, their density, chemical coml)osition, hactrrial
flora and, of course, the media in which they arc grown. These dificulties have been eliminated, to some extent, in our more recent experiments where production of food cells has been standardized (Davis
and Ukeles, 1961).
As has previously been showii for adult bivdves (Loosanoff and
Engle, 1347), their larvae can be killed either by a heavy concentration
of algal cells alone, by the filtrate of algal cultures or by a combination
of the two (Loosanoff et al., 1953). I n other words, dense concentrations
of certain food organisms, such as Chlorella, affect larvae of M . mercenaria, as well as those of several other species, both mechanically,
by interference of food cells with larval swimming and feeding mechanisms, and chemically, by producing external metabolites which are
toxic to larvae. As an illustration, the larvae grew comparatively well
when control cultures received approximately 100000 cells per ml of
large Chlorella, even though this concentration was somewhat above
the optimum for this strain of algae. However, cultures of larvae
receiving cells of Chbrella, t h a t had been removed from the culture
medium by Millipore filters and later resuspended in sea water at the.
rate of 1 million cells per ml, were rapidly killed. Similarly, larvae
receiving the filtrate only from a certain volume of algal culture
originally containing one million cclls of Chlorella per ml also quickly
died. These studies further showed that a filtrate containing heavy
concentrations of metabolites of Chlorella cells may be even more
detrimental to larvae than heavy concentrations of the resuspended
cells themselves. As has already been mentioned, the ecological
effects of external metabolites have long been recognized by aquatic
biologists (Lucas, 1947).
Recently, Davis and Guillard ( 1958) conducted extensive experiments to determine the relative value, as larval food, of representatives
of ten different genera of microorganisms. I . galbana and M . Eutheri
were the best foods and were approximately of equal value. I n some
experiments, nevertheless, Chlorococcum sp. was the best food for
larvae of M . mercenaria. Clam larvae were also able to utilize several
species of Chlorellu, Dunaliellu euchlora, Dunaliella sp., P l a t y m o w sp.,
Chlamydomonas sp. and Phaeodactylum tricornutum. However, they
could not utilize one species of Stichococcw or Prymnesiuni parcum.
Experiments also demonstrated that, as in the case of larvae of
C. virginica, a mixture of I . yalhana, M . lutheri, Platymclnas. sp. and
D. euchlora promoted somewhat more rapid growth of clam larvae
than did equal quantities of any of these food8 separately.
Some of the algae tested at Milford Laboratory are given below with
flora and, of course, the media in which they arc grown. These dificulties have been eliminated, to some extent, in our more recent experiments where production of food cells has been standardized (Davis
and Ukeles, 1961).
As has previously been showii for adult bivdves (Loosanoff and
Engle, 1347), their larvae can be killed either by a heavy concentration
of algal cells alone, by the filtrate of algal cultures or by a combination
of the two (Loosanoff et al., 1953). I n other words, dense concentrations
of certain food organisms, such as Chlorella, affect larvae of M . mercenaria, as well as those of several other species, both mechanically,
by interference of food cells with larval swimming and feeding mechanisms, and chemically, by producing external metabolites which are
toxic to larvae. As an illustration, the larvae grew comparatively well
when control cultures received approximately 100000 cells per ml of
large Chlorella, even though this concentration was somewhat above
the optimum for this strain of algae. However, cultures of larvae
receiving cells of Chbrella, t h a t had been removed from the culture
medium by Millipore filters and later resuspended in sea water at the.
rate of 1 million cells per ml, were rapidly killed. Similarly, larvae
receiving the filtrate only from a certain volume of algal culture
originally containing one million cclls of Chlorella per ml also quickly
died. These studies further showed that a filtrate containing heavy
concentrations of metabolites of Chlorella cells may be even more
detrimental to larvae than heavy concentrations of the resuspended
cells themselves. As has already been mentioned, the ecological
effects of external metabolites have long been recognized by aquatic
biologists (Lucas, 1947).
Recently, Davis and Guillard ( 1958) conducted extensive experiments to determine the relative value, as larval food, of representatives
of ten different genera of microorganisms. I . galbana and M . Eutheri
were the best foods and were approximately of equal value. I n some
experiments, nevertheless, Chlorococcum sp. was the best food for
larvae of M . mercenaria. Clam larvae were also able to utilize several
species of Chlorellu, Dunaliellu euchlora, Dunaliella sp., P l a t y m o w sp.,
Chlamydomonas sp. and Phaeodactylum tricornutum. However, they
could not utilize one species of Stichococcw or Prymnesiuni parcum.
Experiments also demonstrated that, as in the case of larvae of
C. virginica, a mixture of I . yalhana, M . lutheri, Platymclnas. sp. and
D. euchlora promoted somewhat more rapid growth of clam larvae
than did equal quantities of any of these food8 separately.
Some of the algae tested at Milford Laboratory are given below with