REARING OF BIVALVE MOLLUSKS
75
had taken place. This conclusion is supported by measurements of the
inner prodissoconch shells surrounded by the band, whiah were usually
approximately 200 p in length. This is the size at which setting of tho
majority of normal larvae of A. simplex occurred.
L. Diseases of larval and juvenile mollusks and their treatment
Even under the best conditions that can now be maintained in the
laboratory there have been occasional heavy mortalities of larval and
juvenile mollusks that cannot be accounted for by experimental treatment or errors in technique. Although in most instances this mortality
was light, weak or dying larvae could usually be seen harboring heavy
populations of bacteria or protozoans: in some cams the mortality
was epizootic, killing nearly all young bivalves within a few days.
The field of diseases and parasites of larval and juvenile mollusks
is entirely new because studies of this nature were impossible until
successful methods for rearing larvae were developed. No information
was available, therefore, as to whether the bacteria, protozoans, etc.,
observed in weak and dying larvae, were the primary cause of their
death or merely secondary invaders and, perhaps, only scavengers.
Although Davis (1953), in attempts to feed oyster larvae on bacteria,
had shown that bacteria retarded growth of larvae and eventually
killed them, it was not until 1954 that a fungus, Sirolpidium zoophthorum (Fig. 26), was found responsible for some of the epizootic
mortalities in our cultures of clam larvae (Davis et al., 1954; Vishniac,
1955). So far, fungus has been found to affect larvae of M. mercenuria,
M . campechiensis, the hybrids of these two species, Teredo nuvalis,
Pecten irradians, Tapes semidecussata and G. virginica. However, since
it was observed and identified only recently, wc have not established
whether it can infect larvae of those bivalves which were reared at
our laboratory prior to its discovery.
Not all of the forms mentioned above are equally susceptible to
fungus. For example, while larvae of M. mcrcenaria and Teredo nuvalis
are easily infected, the infection often assuming epizootic proportions,
infection of larvae of C. virginica is extremely rare. The only case of this
type occurred when a large number of fungus-infected clam larvae was
introduced into a vessel containing young oyster larvae. Larvae of
other forms, such as the bay scallop, Perten irradians, and the Japanese
clam, Tapes semidecussata, frequently became infected, but the disease
seldom reached epidemic proportions.
In M. mercemria and several other species, larvae of all ages, from
very early free-swimming stage to thofie ready to undergo metamorphosis, can be parasitized by fungus. .Juvenile clamu up to 400 p in
75
had taken place. This conclusion is supported by measurements of the
inner prodissoconch shells surrounded by the band, whiah were usually
approximately 200 p in length. This is the size at which setting of tho
majority of normal larvae of A. simplex occurred.
L. Diseases of larval and juvenile mollusks and their treatment
Even under the best conditions that can now be maintained in the
laboratory there have been occasional heavy mortalities of larval and
juvenile mollusks that cannot be accounted for by experimental treatment or errors in technique. Although in most instances this mortality
was light, weak or dying larvae could usually be seen harboring heavy
populations of bacteria or protozoans: in some cams the mortality
was epizootic, killing nearly all young bivalves within a few days.
The field of diseases and parasites of larval and juvenile mollusks
is entirely new because studies of this nature were impossible until
successful methods for rearing larvae were developed. No information
was available, therefore, as to whether the bacteria, protozoans, etc.,
observed in weak and dying larvae, were the primary cause of their
death or merely secondary invaders and, perhaps, only scavengers.
Although Davis (1953), in attempts to feed oyster larvae on bacteria,
had shown that bacteria retarded growth of larvae and eventually
killed them, it was not until 1954 that a fungus, Sirolpidium zoophthorum (Fig. 26), was found responsible for some of the epizootic
mortalities in our cultures of clam larvae (Davis et al., 1954; Vishniac,
1955). So far, fungus has been found to affect larvae of M. mercenuria,
M . campechiensis, the hybrids of these two species, Teredo nuvalis,
Pecten irradians, Tapes semidecussata and G. virginica. However, since
it was observed and identified only recently, wc have not established
whether it can infect larvae of those bivalves which were reared at
our laboratory prior to its discovery.
Not all of the forms mentioned above are equally susceptible to
fungus. For example, while larvae of M. mcrcenaria and Teredo nuvalis
are easily infected, the infection often assuming epizootic proportions,
infection of larvae of C. virginica is extremely rare. The only case of this
type occurred when a large number of fungus-infected clam larvae was
introduced into a vessel containing young oyster larvae. Larvae of
other forms, such as the bay scallop, Perten irradians, and the Japanese
clam, Tapes semidecussata, frequently became infected, but the disease
seldom reached epidemic proportions.
In M. mercemria and several other species, larvae of all ages, from
very early free-swimming stage to thofie ready to undergo metamorphosis, can be parasitized by fungus. .Juvenile clamu up to 400 p in