REARING OF BIVALVE MOLLUSKS
33
is formed, but the hinge line, instead of being straight, is concave,
characterizing “ saddleback larvae ”, or convex, typical of “ humpback larvae ” (Fig. 16). In both of these abnormalities much of the
ventral pcrtion of a larva’s body will extend beyond the shell. The
results of overcrowding on development and growth of larvae will be
more fully described in a later section of this article.
Occasionally, in some cultures many larvae have abnormally small
vela. This abnormality may, sometimes, be due to mechanical injuries
to the velum when larvae are screened before their shells are fully
developed to protect the soft parts. In other cultures it has been
aasociated with tho presence of numerous ciliates. It is possible that
velar deformities in these c a m were the results of injuries by ciliates,
but it is more probable that the ciliates were feeding on particles of
vela cast off by larvae in response to adverse conditions, such aa
wtificially-created concentrations of certain chemicals.
The same type of abnormalities, as observed in overcrowded
cultures, occurs when eggs are cultured in sea water in which adult
oysters have previously been kept. Probably because of the same
reason, eggs carried along with water from tanks or trays in which
a mass spawning has occurred seldom develop into normal larvae
unless the original water is greatly diluted with fresh sea water.
Failure of larvae to develop normal shells when overcrowded, or
when grown in water in which adult oysters had previously been kept,
may indicate a depletion of certain substances, normally present in
sea water, that are needed for shell formation. In experiments devised
to verify this possibility some of the eggs discharged by a single female
were placed in fresh sea water, while others, fertilized with sperm
from the same male, were placed in set3 water taken from an aquarium
in which adult oysters had been kept. The latter group of eggs gave
a much lower percentage of normal larvae.
One interesting clms of abnormal larvae consists of those that do
not feed, even though they do not show clear-cut anatomical malformations. These apparently normal larvae, which developed to straighthinge stage under our standard conditions and are kept in the same
cultures with other larvae which are feeding and growing normally,
seem unable to feed, do not grow, and eventually die., Before death,
the larvae become emaciated so that most of the space inside their
shells is empty with only the retractor muscles, a small velum and a
shrunken visceral maas remaining. In some cultures this abnormality
is found in more than 25% of the larvae.
In several experiments this type of abnormality has been associated
with the kind of food given. In these instances approximately 50%
33
is formed, but the hinge line, instead of being straight, is concave,
characterizing “ saddleback larvae ”, or convex, typical of “ humpback larvae ” (Fig. 16). In both of these abnormalities much of the
ventral pcrtion of a larva’s body will extend beyond the shell. The
results of overcrowding on development and growth of larvae will be
more fully described in a later section of this article.
Occasionally, in some cultures many larvae have abnormally small
vela. This abnormality may, sometimes, be due to mechanical injuries
to the velum when larvae are screened before their shells are fully
developed to protect the soft parts. In other cultures it has been
aasociated with tho presence of numerous ciliates. It is possible that
velar deformities in these c a m were the results of injuries by ciliates,
but it is more probable that the ciliates were feeding on particles of
vela cast off by larvae in response to adverse conditions, such aa
wtificially-created concentrations of certain chemicals.
The same type of abnormalities, as observed in overcrowded
cultures, occurs when eggs are cultured in sea water in which adult
oysters have previously been kept. Probably because of the same
reason, eggs carried along with water from tanks or trays in which
a mass spawning has occurred seldom develop into normal larvae
unless the original water is greatly diluted with fresh sea water.
Failure of larvae to develop normal shells when overcrowded, or
when grown in water in which adult oysters had previously been kept,
may indicate a depletion of certain substances, normally present in
sea water, that are needed for shell formation. In experiments devised
to verify this possibility some of the eggs discharged by a single female
were placed in fresh sea water, while others, fertilized with sperm
from the same male, were placed in set3 water taken from an aquarium
in which adult oysters had been kept. The latter group of eggs gave
a much lower percentage of normal larvae.
One interesting clms of abnormal larvae consists of those that do
not feed, even though they do not show clear-cut anatomical malformations. These apparently normal larvae, which developed to straighthinge stage under our standard conditions and are kept in the same
cultures with other larvae which are feeding and growing normally,
seem unable to feed, do not grow, and eventually die., Before death,
the larvae become emaciated so that most of the space inside their
shells is empty with only the retractor muscles, a small velum and a
shrunken visceral maas remaining. In some cultures this abnormality
is found in more than 25% of the larvae.
In several experiments this type of abnormality has been associated
with the kind of food given. In these instances approximately 50%