478
NOTES AND COMMEKT
An Observation
on the Spawning of the Sea Scallop, Placopecten
(Gmelin), on Georges Bank
magelhznicus
The spawning of the sea scallop, Placo(Gmelin), was observed
in late September in the natural habitat of
the scallop on Georges Bank at 41” 15’ N.
Lat. and 66” 22’ W. Long. The northeastern part of Georges Bank is the most
productive and most intensely fished sea
scallop ground on the Atlantic coast. Between 1944 and 1956, 77 million pounds of
sea scallop meats (adductor muscles only)
representing about 2.5 billion individuals
were taken from these beds.
From September 21 to 25, 1956, an average of 250 individuals was examined each
day. None of those examined on September 21 had begun to spawn, but’ spawning
began during that night. The smaller
scallops, ranging in size from 50 to 90 mm,
began spawning first and were nearly spent
when most of the scallops larger than 90
mm began spawning. Eighteen hours later,
on September 22, 48 per cent were either
spawning or already spent. At the close
of the observation period on September 25,
92 per cent were either spawning or already
spent.
All the sea scallops examined later at
other locations on the bank were spent.
Bottom temperatures were between 8’ and
11°C. The trigger mechanism
of t,he
spawning is not known.
In many instances, eggs being released
by a spawning female had adhered along the
margin of its upper, rounded valve. Although these adherent eggs were not positively identified, they were assumed to be
scallop. In all these instances, the eggs on
the valve and within the gonad were the
same size and color.
The condit,ion of the gonad in t’he sea
pecten magellunicus
scallop during spawning proceeds through
three distinctly different’ stages which can
be observed macroscopically.
A ripe male
gonad is firm and plump with a milky-white
color. When spawning begins, this coloration becomes invaded with irregular, grayish,
clear channels. These clear channels gradually engulf the entire gonad. When completely spent, the gonad is collapsed and is
clear or translucent.
A ripe female gonad
is a brilliant coral red. As spawning begins,
the color fades to a pale pink and the eggs
seem to be somewhat larger. Clear channels appear and invade the entire gonad as
in the males. At this st’age, nearly spent,
the sexes become difficult to distinguish
without close examination.
The time of spawning observed here
closely agrees with the late September and
early October period menbioned by Posgay
(1950) for Cape Cod Bay. The period of
spawning, however, is considerably shorter
than that reported for the Digby beds by
Dickie (1955).
REFEREKCES
in abundance
DICKIE, L. M. 1955. Fluctuations
of the giant scallop, PLacopecten magellanicus
(Gmelin)
in the Digby area of the Bay of
Fundy . J. Fish. Res. Bd. Canada, 12: 797857.
of the sea
POSGAP, J. A. 1950. Investigations
scallop.
Third Report on Investigations
of
the Shellfisheries
of Massachusetts.
Div.
Mar. Fish., Dept. Conservation,
Commonwealth of Massachusetts,
pp. 24-30.
J. A. POSGAY
I<. DU-\XE
U. S. Fish and Wildlife Sercice
TJ’oods Hole, Massachusetts
Influence of Oxygen Tension on Respiration
The Winkler method of determining oxygen is often used to measure the rate of
photosynthesis of freshwater and marine
phytoplankton
communities.
In order to
find the actual rate of carbon assimilation,
the amount of oxygen lost in the dark bottle
NORMAN
of Phytoplankton
is added t’o the amount of oxygen produced
in the light bottle.
Many investigations
have been carried
out regarding the question whether or not
respiration is influenced by light.
In the
light-dark
bottle method of Gran men-
479
XOTES -44NDCOMMENT
tioned above it is assumed that respiration
in the dark bottle is equal to or not much
different from respiration
in the light.
However, not only illumination
but also
the amount of oxygen available to the
Durrespiring cell may be of importance.
ing photosynthesis respiration takes place
in a very high tension of oxygen liberated
by photosynthesis, although not in pure
oxygen . The effect described in this paper,
however, is not the same as photooxidation,
since it occurred in dark bottles.
It may be asked, therefore, if oxygen
tension influences respiration.
To answer
this question measurements of respiration
were made by the Winkler method for
0,
1N PER
CENT
OF
SATURATION
FIG. 1. Respiration
rates of natural
diatom
communities.
Four
series
in supersaturated
water, showing the increase of Og uptake with in
creasing C)? tension.
a
d
IT
L
100
I
0
0,
IN
PER
I
I
200
CENT
300
OF
400
SATURATION
FIG. 2. Respiration
rates of a natural
-4nabaena communit~y in under- and supersaturated
water.
Two series of experiments
on successive
days.
Each point on the curves is the average of
10 single determinations.
TABLE 1.
dnabaena
Respiration
com?nunity
Each value
100
230
373
over a 5-hour period for an
at various saturation
levels
0-foxygen
is the mean of 10 determinations.
2.584 f
3.560 f
5.088 f
0.11
0.16
0.24
several natural limnic phytoplankton
communities in water super- and undersaturated
with oxygen. Replicate samples of natural
acus delicatissima
and
diatom (Synedra
,4sterionella
formosa)
and Anabaena
(A.
$0~ aquae, ,4. spiroides, and A. inaequalis)
phytoplankton
communities were placed in
250-ml glass-stoppered bottles and allowed
to remain in the dark at constant temperature after first having been brought to a
known level of oxygen supersaturation.
Control experiments with water that had
been ultrafiltered to remove all microorganisms showed no change in oxygen content
over several days.
In the experimental bottles the rate of
respiration increased as the per cent saturation of the water was increased, as shown
in Figures 1 and 2. Note in Figure 2 that
the relationship between rate of respiration
and per cent saturation apparently holds
for undersaturated water as well. The rates
of respiration in the figures are reported in
relative terms, using the respiration at 100
per cent saturation as equal to unity.
This
was necessary because no attempt was made
to relate respiration to number of cells,
volume of cells, surface area, etc. Absolute
values for the 4nabaena series of 16 August
1957 are given in Table 1 as an example.
Thus at roughly 400 per cent saturation,
respiration in Anabaena was about twice as
great as at 100 per cent.
This considerable influence of oxygen
tension on respiration is surprising, because
it is known that an OZ tension of 10V4 atm.
is suflicient for oxidation by the enzymes of
the cell. We must suppose, therefore, that
the rate of diffusion is the limiting factor
even in unicellular organisms the size of
and not only in the big
phytoplankton
480
NOTES AND COMMEXT
thalli of marine algae and the bodies of
higher aquatic plants (Gessner and Pannier 1958).
This interpretation
is substantiated by
results from the M’arburg method. The
same Anabaena popula,tion in Warburg vessels shaken 100 times a minute for 20 minutes
showed no difference in respiration between
samples kept under 100 and 400 per cent
oxygen .I Thus, with shaking, an oxygen
tension of 100 per cent (water in equilibrium
with air) is sufficient for maximal respiration. The same result was obtained by
Franck and French (1941) with round pieces
of Hydrangea leaves. Using the Warburg
method they found 20 per cent of oxygen
Beto be enough for maximal respiration.
tween 20 and 100 per cent saturation no
further increase of oxygen uptake could be
demonstrated.
On the other hand, in bhe present experiments when the Anabaena flasks were not
shaken during the experiments but only
briefly at the end to establish an equilibrium
between the gaseous and water phases, the
Warburg manometers showed the same
differences in respiration between 100 and
500 per cent saturation as had been found
with the Winkler met,hod. These differ1 We are indebted to Dr. 0. Kandler
determinations
of Anabaena respiration
Warburg method.
for these
by the
ences disappeared only in strong water movement. Bottles rotated slowly in a klinostat
gave the same differences by the Winkler
method as stationary bottles. Obviously
only the Winkler method approaches nattural conditions, because even in stormy
weather plankton cells surely are not shaken
100 times a minute as in the Warburg
vessels.
Values obtained by the Winkler method
for the net (not gross) rate of photosynthesis are somewhat too low. For obtaining the real values of the net photosynthetic
rate, determinations
of respiration in the
dark bottle should probably be carried out
in water that is in equilibrium with pure
oxygen and not, with air.
REFERESCES
FRANCK, J., AND C. S. FRENCH. 1951. PhotoJ. Gen. Physoxydation
processes in plants.
iol., 25: 309-321.
GESSNER, F., ASD F. PANXIER.
1958. Der
Sauerstoffverbrauch
der Watiserpflanzen
bei
Sauerstoffspannungen.
Hyverschiedenen
drobiologia.
10: 323-351.
FRITZ GESSNER
Botanical Institute
University of Munich.,
Germany
FEDERICO PANNIER
Botanical Institute
University of Caracas,
I’enexuela