The British Journal of
Experimental
Biology
THE OXYGEN REQUIREMENTS OF CERTAIN
AQUATIC ANIMALS AND ITS BEARING
UPON THE SOURCE OF FOOD SUPPLY.
BY
W. J. DAKIN, D.SC., Professor of Zoology, University of Liverpool,
and CATHERINE M. G. DAKIN, B.Sc.
CONTENTS
SECTION A.
S E C T I O N B.
i. Experiments on the Oxygen Re
quirements of Goldfish .
302
Technique .
3O4
Concluding Experiments
306
3. The Oxygen Consumption of
developing Plaice Eggs .
310
Technique .
3. The Absorption of Oxygen in the
Course of Experiments
.
.315
4. Summary
320
5. References
322
IT is now about twenty years since Professor Putter of the
University of Bonn surprised most zoologists by asserting
that the main source of the food supply of aquatic animals
consisted of organic matter in solution. His theory embraced
the aquatic animals of both fresh and marine waters.
It was natural that in the study of animal physiology
a detailed investigation of animal nutrition should begin
with land animals. In fact our knowledge of this subject
has almost been restricted to mammalia. Leaving aside
special cases, it is characteristic of terrestrial animals that
they capture prey in the form of other organisms ; they feed
upon the vegetable products of the earth, or they live on a
mixed diet of animals and plants. Quite an extensive series
of adaptations have been made out and there has been much
correlation of structure and function.
In that diverse and extensive assemblage of aquatic animals
VOL. 11.—NO. 3.
293
T
W. J. Dakin and Catherine M. G. Dakin
ranging from, say, the Coelenterata to the fishes one meets
with analogous (and homologous) structures apparently for the
perception and capture of paniculate animal and plant food.
Alimentary canals and related organs are present for its
transformation and absorption. It is not surprising, then,
that the character of the food supply of most aquatic animals
should have been (and still is) regarded as not unlike that
of terrestrial animals. It must, however, be admitted that it
was largely on the basis of analogies and not altogether on
experimental data that the ideas of twenty-five years ago
regarding animal food supplies were founded. It was quite
common to find that, although the structure of an alimentary
canal of some aquatic organism was known, there was little
information about the contents which might on occasion fill
it. In whole collections of marine animals the alimentary
canals of the specimens were often quite devoid of contents.
(This gap in our knowledge has been partially filled to-day.1Om)
Supported by a few observations, the ultimate result of the
deductions drawn from the analogies alluded to above was that
biologists and fishery experts came to regard the " plankton "
as of profound importance in the food cycles of seas and lakes.
The smallest floating plant-like organisms (unicellular plants-—
diatoms and plant-like animals, flagellata, and so on) captured
with tow-nets of bolting-silk were regarded as the link between
the dissolved mineral substances in the water and the materials
essential as animal food. The smaller animals of the plankton
fed upon these microscopic organisms, to be devoured in turn
by the larger planktonic animals, which again formed the food
of the fishes and even the whalebone whales. Many series of
this type have been enumerated in works on plankton. A
chain of this kind taken from Johnstone's Life in the Sea is
the following : " Peridians (plant-like organisms)—Copepoda—
sprats—whiting—cod—man."8
With the inventions of Hensen dating from about 1885,
the Kiel school of planktologists initiated their great effort
to estimate quantitatively the actual amount of this floating
life present in the sea at any time and throughout the year.
Their efforts seemed to indicate that the coastal waters and
inland seas at least were no barren water reservoirs. Thus
294
Oxygen and Aquatic Animals
in terms of dry organic substance, whilst cultivated land has
been calculated to produce 1790 kgs, per hectare, the uncultivated Baltic Sea was shown to give 1500 kgs. per hectare
in floating planktonic organisms. It is not surprising that the
planktologists did not doubt the sufficiency of the food supplies
of aquatic organisms if this was sought in the form of living
or dead animal and plant bodies.
Such was the position in 1907, when Putter published a
work14 in which he claimed that actual experiment indicated :—
(1) that aquatic animals require more food to cover their
daily requirements than is available in the form
of paniculate food ("geformter Nahrung" in the
original—i.e. dead or living bodies of animals or
plants, or particles of the same);
(2) that more organic food matter is actually available
in solution in a given volume of sea water than in
the form of particulate food in the same volume;
(3) that aquatic animals find their chief source of food
in the form of organic matter in solution.
Putter's first paper" was based upon chemical analyses
and experimental work giving (a) the amount of carbon
present in the form of organic compounds in solution in the
sea water at Naples; (b) the amount of carbon required in
the form of food to cover the daily requirements of various
marine animals; and (c) the amount of carbon available in
the plankton. Knowing (6) and (c) a simple arithmetical
calculation would give the amount of sea water which the
supposed planktonic feeders .must necessarily filter in order
to obtain their daily requirements. The estimates of the
amount of organic carbon present in solution in the sea
water were obtained by direct analyses. The daily food
requirements of various marine animals were estimated by
first determining the oxygen requirements. The amount of
food available in particulate form was calculated from
tabulations which had appeared in various papers from the
Kiel school of planktologists.
Obviously if the amount of organic carbon present in
solution in sea water is normally very much in excess of
29s
W. J. Dakin and Catherine M. G. Dakin
that found in the form of paniculate food, there would be
considerable reason for regarding the sea as a kind of
nutrient fluid. This conclusion would be confirmed if it could
be shown that the plankton was only sufficient to meet the
food requirements, provided that the animals filtered extraordinary quantities of sea water per hour or day. Putter
thought that he could prove both these statements to be
true. His first estimates were that one litre of sea water
contained 92 mgs. of carbon in solution, and that this was
about 23,000 times as much as was present in the planktonic
organisms in the same volume !
Unfortunately for Putter, Henze 6 was very soon able to
show that the analyses of the sea water were altogether
inaccurate, and that the 92 mgs. of carbon per litre had to
be reduced to something like 3 mgs.! This was confirmed
by Raben of Kiel.19 As a matter of fact so far as most
analyses available to-day are concerned, the amount of carbon
in solution in the form of organic compounds in the open sea
falls within the limits of experimental error.
One might easily assume that this discovery was quite
sufficient to shatter Putter's theory altogether.
However,
although throwing doubt on some of his work, the chemical
investigations which followed seemed to show that the amount
of food available in the form of planktonic organisms was
also very small and equally insufficient to meet the food
requirements which had been calculated from the oxygen
determinations. Some of Professor B. Moore's results gave
only 0.4 mg. of C. per litre in the form of plankton. The
interpretation of these results of physiological and chemical
investigation is by no means easy. Putter, in spite of so
much criticism, still adheres to his belief in the importance of
the absorption of foodstuffs from solution.18 His estimates
might have been more extensively tested, for only too
frequently one finds it assumed that this problem of nutrition
is both quite simple and completely solved.
Investigations have already been made to obtain more
accurate estimates of the particulate food available in both
sea and fresh water; the actual feeding habits of aquatic
organisms have been more closely followed by experiments.
296
Oxygen and Aquatic Animals
Both these lines of inquiry may be pursued further, especially
the latter, in the light of Potts' work on Teredo}5 It is most
essential, however, that the food requirements, as calculated
by physiological experiment, should be subjected to revision
and analysed by further experiment. There is a wide field
here for investigation requiring the closest collaboration of
the zoologist and the chemist.
The main feature of the work of Putter is bound up with
this last line of investigation and, whatever modifications this
part of his work may receive in the future, it has proved
stimulating to research. His figures are to a large extent
still awaiting a refutation based on experiment from those
who have most severely criticised his theory. Putter based
his estimations of the amount of food required by aquatic
animals on the results of experiments to determine the oxygen
consumption during a period of one to twenty-four hours.
In general, the animal was enclosed in a volume of water in
a more or less elaborate container, and the amount of oxygen
present was determined at the beginning and at the end of
the experiment. Now the oxygen which has been consumed
has been utilised for the oxidation of the organic substances
of the animal body. If the oxidation were complete and we
knew exactly the relative proportions of the substances
oxidised, it would be possible to calculate directly the amount
of substance used up in twenty-four hours, and consequently
the amount of food necessary to make good the loss. In
practice, the results will only be approximate and should
indeed give minimal figures. If proteid only were oxidised
one part would require 1.26 parts oxygen, one part carbohydrate in the same way would require 1.23 parts oxygen,
and one part fat would require 2.88 parts oxygen. The
oxidation is incomplete, however, and we obtain only an
approximate measure of the actual metabolism.
The following table from Putter's work gives the results
of some of these estimations, and it will be seen that
the smaller organisms consume a relatively extraordinary
amount of oxygen per unit of dry weight, and that there
is apparently no direct relation between dry weight and
metabolism.
VOL. II.—NO. 3.
297
T2
W. J. Dakin and Catherine M. G. Dakin
Oxygen Consumption
Name of Animal.
Suberites
Cucumaria
Ceriactis.
Rhizostoma
Cestus
Pterotrachea
Tethys .
Aequorea
Murex
Aplysia* .
Galathea.
Palaemon
Calanus .
Heliastes 18 c
,.
74 c.
Scorpjena
>)
Maenula .
Saccharomyce s
Mould .
Collozoum
Weight of
Specimen
in gins.
600
150
200
232-0
73O
570
1520
80-0
9-0
6-3
21-5
0-95
000073
O-2
13-2
180
Percentage
of Organic
Substance
(Dry).
7-6
16-4
9-53
O-5
0-25
0-6
i-3
abt. 0-4
15-0
150
150
15-0
15-0
16-2
20-1
Temp,
of
Experiment.
°C.
15-0
18-7
15-5
26-0
160
160
16-0
13-2
II-2
i8-s
32-0
Per
kg. of
Organic
Substance
and per
hr. in
ings.
148
I'3
335
3-95
12-3
17-7
4400
1580
2050
1380
22-O
4-8
f2OO
230
720
22-5
26-8
23-S
17-7
23-7
22-1
790
345-O
518-0
3570-O
6io-o
358-0
418-0
480
53O
3i-5
5700
37-S
2O-O
200
2O-O
...
...
abt 15-0
abt. 15-0
29-0
0-4
l6-O
O-I
Per
kg. and
Er. in
nigs.
22-2
IOO-O
230
464-0
351300
26O
116000-0
IIIO-O
23OO
345O
238OO
377O
I77O
2O8O
5OO
232O
2360OO
772O0O
2770OO
Per
Animal
and per
hr. in
mgs.
0-67
0-276
0-64
5-10
0-288
0-70
269
038s
0-65
0-494
7-4
o-49
C-OO26
O-I22
4-7
7-5
64-0
17-4
*••
O-III
Now physiologists had recognised before Putter's time that
metabolism was not directly proportional to the weight or size
of the animal, and as far back as 1883 Rubner suggested40 a
relationship between surface area and basal metabolism. His
view was further advanced by his later investigations and by
those of other physiologists, until practically it became elevated
into a law—the so-called "body surface law." Putter insisted
that it was the active surface of the body, where oxygen and
other exchanges took place, which really determined the
basal metabolism. He suggested, for example, that by taking
the lungs into account one might explain some of the
discrepancies in the comparison of the basal metabolism of
the mammalia. It is singular how little reference has been
made by physiologists to this possibility in recent years.
Adopting this principle, Putter attempted to show that if the
metabolism were correlated with the active surface area there
was a remarkable agreement between diverse organisms, and
that knowing this area in any particular example one might
298
Oxygen and Aquatic Animals
legitimately attempt calculations of food requirements. A
calculation of this kind which might be questioned is
Putter's estimate of the food requirements of the Copepoda
(Vergleichettde Physiologie, Gustav Fischer, Jena, 1911, p. 266).
Practically all the modern work on basal metabolism which
bears any approach to accuracy has been carried out on
mammals. Benedict and Harris have shown2 that in one
and the same species—man—the so-called law of Rubner
is only approximately valid and that various factors, such as
sex, age, and athletic training, affect the basal metabolism.
Factors other than temperature influence the basal metabolism,
viz., condition of the alimentary canal in regard to food and
psychic conditions.
It must be evident then that it is impossible to predict
with accuracy the metabolism and the food requirements of
a series of different animals by the application of the law of
body surface or that of Putter's active surface. Recent
chemical work shows indeed that animals are not all of the
same flesh, and that the food requirements of any species
must be calculated from experiments on that species. Besides,
the actual area of the absorbing surface is only one factor
in the efficiency of a respiratory organ. The composition of
the membrane plays a great part, and there are other factors
which make area a secondary feature. But the possibility of
accuracy in theoretical calculations is more remote still, when
one realises the difficulty of recognising and of determining
the active surface area in most cases.
We bring forward this criticism because we regard the
figures put forward by Putter for the food requirements of
certain aquatic organisms as of peculiar interest. In fact it
is this part of his work that most requires further investigation.
Putter makes his actual experimental results agree with his
theoretical calculations on the basis of the theory of surface
area, and then draws on this agreement for support. We
consider that any theoretical calculation of the food requirements of an organism, made in this way, is interesting but
altogether unreliable, and to lump together all aquatic animals
as if their nutritional habits were likely to be the same is a
grave error. We know of cases already, like the Corals,
W. J. Dakin and Catherine M. G. Dakin
Convoluta, and the White Ants, where peculiar symbiotic
conditions prevail. The future will probably show that we
have not realised the extent of symbiosis in regard to nutrition.
There are likely to be numerous other causes of diversity.
We cannot dismiss Putter's figures, which are the
actual results of chemical investigations as summarily as
his theoretical calculations. When, therefore, Putter calculates
(in a criticism of a paper by W. J. Dakin *) that a Copepod
(Ca/anus sp.) would have to catch and devour 9,750,000
individuals of Thalassiosira nana daily in order to cover its
food requirements, we are faced with a problem. The Copepod
could not obtain sufficient food in paniculate form if it had
to meet, these demands suggested by the results of the oxygen
determinations. But could it possibly meet such demands
by the absorption of food in solution ? We suggest that
these high oxygen figures require further explanation.
It is impossible to obtain any real idea of the food
requirements of an animal by a few experiments at one season,
and grave errors readily creep into calculations based on the
results of experiments with five or six specimens.
The food requirements of a crab which were calculated from
experiments made on one or two individuals at the breeding
season could not be safely applied to the whole year. For
example, Johnstone has shown 9 how the constitution of various
marine fishes varies regularly throughout the year, and in
particular how in the herring there are " seasonal metabolic
phases." These are of the two categories (1) "those which
make up the annual reproductive cycle, and (2) those which
are to be related particularly to the annual wave of sea
temperature." These features are probably common to many
other aquatic animals. This being the case, it would be very
dangerous to compare even accurate estimations of the food
requirements of a species when metabolism was at its highest
with the food available in the sea water at another season and
perhaps at some other place. This seems a legitimate criticism
to bring forward against Putter's oxygen determinations.
We believe, however, that much of this experimental work
must be repeated. It is only too easy for aquarium experiments
to give abnormal results, and the interpretation of normal
300
Oxygen and Aquatic Animals
results is not always easy. The oxygen consumption of a
goldfish an hour after a meal is much higher than twelve hours
afterwards. Which of these figures approximates most closely
to the average in nature? If, throughout the year, the food
requirements of a number of aquatic organisms are as high
as the oxygen determinations would seem to indicate, there
is a factor present which still remains unknown.*
We may now turn to the experimental side, and to the
calculations of Putter based upon actual feeding and starvation
experiments which led to our own work with goldfish, axolotls,
and plaice eggs. The most elaborate series of investigations
of this kind were those made on fish.16 One of these will
serve as a type of the others, and the species is that which
we have also used.
A small number of goldfish were divided into three groups.
Group I. was used for analysis. Group II. lived without any
food in tap water renewed daily (later on a small amount of
various salts was added). Group III. were given organic
matter in solution—asparagine and glycerine. The goldfish
which received no food, Group II., lived forty-two days,
whilst those of Group III. lived up to fifty-six and seventyeight days. Furthermore, the oxygen analyses indicated that
the Group II. fishes diminished by 39 per cent, of their
weight during the experiment. The Group III. fishes not
only lived much longer, but were using 20 per cent, more
oxygen than the Group II. examples at the end of the
experiment; they had not lost so much weight, owing
apparently to the presence of asparagine and glycerine.
Putter assumed that the artificial solutions did not contain
all the necessary substances required as food, but that there
could be no question about the utilisation of dissolved carbon
compounds. He also added (Zeit. /. allgem. physiologie, Bd.
ix., p. 222): "There is no doubt that a nutrition without
* It should be pointed out that estimates of the paniculate food available
(organisms or detritus) in the sea or fresh water are probably also minimal figures.
Lohmann showed long ago that the plankton nets failed to capture the smaller
organisms and the recent work of Allen' has emphasised this. And of course a
complete calculation of the food so available during a whole year at any particular
place could only be used in connection with animals which do not migrate, following
up their food supplies. We are not specially concerned with this aspect of the
question in the present paper.
301
W. J. Dakin and Catherine M. G. Dakin
dissolved foodstuffs is possible, and it is not impossible that
cases of this kind are realised in nature. But the experiments
at Naples show that the fish in the Naples aquarium under
approximately natural conditions, obtain one-half to threequarters or more of their food requirements by the absorption
of dissolved food."
Now in any case the actual duration of life of the fish
in Plitter's goldfish experiments must not be taken into undue
consideration. We have kept goldfish of approximately the
same length and size alive for periods varying from a few
weeks up to three months when all food, particulate or in
solution, has been excluded. The duration of life depends
upon the condition of the fish at the beginning of the
experiment. Putter did not employ enough specimens to
cut out individual variation and a chance result. It will
also be seen that the oxygen consumption may diminish by
60 to 75 per cent, before the animals die.
After we had commenced our experiments we found that
Lipschiitzn had repeated some of Putter's experiments with
fish, and that his conclusions were not favourable to Putter.
This position seemed only to strengthen the necessity for
further investigation, especially since we had ourselves
experienced rather varied results at the beginning when we
were using only a few specimens.
SECTION A.
1. Experiments on the Oxygen Requirements of Goldfish.
We commenced our experiments with goldfish by keeping
half a dozen in glass jars holding several litres of water. Two
fish were fed chiefly with so-called ants' eggs (pupae); the fish
in the other jars were given no solid food, but were, in one
case, left without anything and in another were given the
same substances which Putter used—asparagine and glycerine,
and in the same strength. The water in the jars was emptied
out and renewed morning and evening to prevent the growth
of any organisms, and every morning the jars were cleaned
with hydrochloric acid. It was found that the addition of
a small quantity of NaCl to the water was of advantage.
302
Oxygen and Aquatic Animals
Chemically pure salt was used. No oxygen determinations
were made in the first experiment at all. The purpose was
to find out if there were an appreciable difference between
the length of life of fish kept in tap water and those in tap
water with added organic compounds, and in particular to
cut sections of the tissues from the dead fish in order to see
what changes, if any, had taken place. The first fish which
died was one of the couple kept in ordinary tap water. It
had lived without food for fifty-four days. The other fish
in this jar lived, however, as long as the two in the water
to which asparagine and glycerine had been added. After
sixty-five days (it was the beginning of the vacation) all three
were killed and sectioned. No difference whatever could be
made out between the tissues of the fish from the pure water
jar and those from the jar containing asparagine and glycerine.
There was a marked difference, however, between the tissues
of all these fish and those of the specimens which had been
fed on solid food. The fish not fed with solid food were
undoubtedly starved. The most marked difference was
obvious in the loss of fat and in the condition of the muscle
fibres. In sections these were clearly seen to have become
attenuated, whilst an increase of the surrounding connective
tissue had apparently taken place. In reality this was only
relative. Almost exactly the same histological condition was
observed in a goldfish which Professor Johnstone (Oceanography Department, University of Liverpool) examined about
this time. It bore a large sarcoma which had either preyed
on the other tissues or had resulted in the fish starving,
for the muscle fibres quite remote from the cancer region
were greatly attenuated, whilst the connective tissue surrounding them was relatively conspicuous. It may be stated
that the fish from the experimental jars were outwardly in
perfect condition when they were .killed, if judged solely on
the appearance of their scales and their fins. And this is a
good test of their condition. They were, however, inactive
and rested on the bottom for long periods. They had also
lost weight. We realised from this experiment that more
than six specimens would be required for a definite result
and that there seemed, so far as we had gone, to be absolutely
3°3
W. J. Dakin and Catherine M. G. Dakin
no difference between fish in water with organic compounds
and those in water alone.
The results of our first oxygen determinations served as a
warning against carrying out physiological work of this kind
on a small scale. We had been lucky in our first venture to
escape any outbreak of disease. For several months following
this we were so troubled that we encountered almost all the
possible goldfish diseases, the worst being Chilodoniasis due
to a Ciliate (Chilodon cyprini) and Gyrodactyliasis caused by
the trematode genus Gyrodactylus.
It was thought at first that possibly the handling and
the experimental conditions, whilst the oxygen determinations
were being made, was responsible for the outbreaks. An
experiment with starved fish that were not used for oxygen
determinations showed that this deduction was wrong. The
intense development of parasitic disease commenced after the
fish had been without particulate food for two or three weeks,
and it did not matter whether asparagine and glycerine had
been present on not. On the other hand the fish fed with ants'
eggs flourished no matter how much they were handled. Some
of these fish which had been discarded and removed to the
laboratory preparation room were in water which was often
left unchanged for days and was at times quite murky. They
survived. Nothing could have been more certain than the
indication that withdrawal of solid food left the fish open to
an intense attack of the parasites which had unfortunately
infected our stock.
Months elapsed before we had a safe
supply again, and in the meantime we had developed that
peculiar knowledge of the conditions favourable for the animals
which can only come after long practical experience. We were
also able to spot a fish which was unwell at an early stage.
At this point we were rather in favour of the view that
the goldfish in water with asparagine and glycerine consumed
more oxygen than the others and really did utilise the dissolved organic matter. It cannot be said, therefore, that we
were experimenting with set views against Putter's theory.
Technique.—The methods adopted for the later experiments
were as follows : The goldfish were kept separately in litre
glass cylinders. These were arranged in two series and all the
3°4
Oxygen and Aquatic Animals
goldfish were paired—that is to say, whenever one that had
been given food in solution was tested for the oxygen consumption, its fellow of the pair which had been in plain water
was also examined. A great effort was made to choose
goldfish of the same size, and indeed to choose those with
approximately the same oxygen consumption. The jars were
all well aerated with a special pump made for us by the
Aerograph Co., London. Oxygen determinations were made
by placing the fish in large stoppered bottles (3000 c.c.) which
were kept under water in a great slate tank which had a
capacity of 41.5 gallons. In this way we had a means of
keeping the temperature constant for all the bottles during
a long experiment. The oxygen content of the water was
estimated by the Winkler method before the experiment
commenced, by collecting with an indiarubber siphon three
or four samples from the jars in the tank. For greater
accuracy we increased the size of the samples for the Winkler
test until they were 365 c.c. After taking off three or four
samples in this way, the fish to be used were carefully placed
into the submerged bottles which were then closed, the
stoppers being inserted under water. At the end of varying
periods of two, three, six hours, etc., the bottles were taken
out of the large slate tank and opened carefully on a table. A
siphon was rapidly inserted and a quantity of water drawn
off for the oxygen test. The difference between the oxygen
contents of the bottle before and after the experiment represented the amount used by the fish.*
It is necessary to state that we found the simple method
just described quite as accurate as the more complicated
methods which have been tested. Several details are, however,
of importance in this respect. The experiments were only
of such short duration (three hours in the best series) as to
ensure that only a small proportion of the total oxygen available
had been consumed by the fish, and only a small quantity of
waste matter added to the water. Numerous controls of all kinds
convinced us that these results approached a normal more closely
than experiments of twenty-four hours' duration without change
of water, but where efforts had been made to supply oxygen.
• It is quite unnecessary to detail the chemical methods involved in Winkler's
process for the quantitative determination of dissolved oxygen in water.
305
W. J. Dakin and Catherine M. G. Dakin
In any case it must not be forgotten that the figures are
not used as absolute. In no case has an oxygen determination
been made on a fish from water containing asparagine and
glycerine without its corresponding plain tap water control
going through exactly the same procedure. The whole series
of experiments have been comparative, and any errors affecting
one side have also affected the other.
The Concluding Experiments.—Twenty-four goldfish were
taken from a large supply, kept under observation until they
appeared free from parasites and in good condition. Twentyfour jars were arranged quite arbitrarily in two rows of twelve
on a long bench. Then to cut out any personal choice, the
laboratory attendant "tossed" a coin to decide which series
should receive glycerine and asparagine and which series
ordinary water. The fish were labelled A-L and A'-L', and
A and A' formed a pair, B and B' another pair, and so on.
Six fish on each side were utilised for oxygen determinations,
and the other six fish on each side left as further controls on
those which were being more frequently handled.
The first oxygen determinations were made after all the
fish had been starved for two days, an important point,
because we found that the oxygen consumption of fish that
were being well fed varied very considerably. Absence of
food for forty-eight hours brought down the oxygen consumption quite considerably. Here again is a point worth
noting. There is more likelihood of the oxygen requirements of the first few days of starvation being a normal
requirement than that of well - fed laboratory specimens.
The same probably applies to other experiments of this
nature. What evidence is there that under natural conditions
the animals employed in experiments ever capture as much
food as they have been consuming in the aquaria ?
Following the first oxygen determinations made in this
case on 12th October, the tests were repeated on various
dates until 5th December, a period of fifty-four days (compared with the forty-two days of Putter's experiment).
Fortunately the first deaths from starvation occurred amongst
the controls which were not being used for the oxygen determinations, and so we were able to run our comparisons for
a fairly long time. The incidence of the deaths was, how306
Oxygen and Aquatic Animals
ever, rather interesting, and showed how easily misconceptions
may arise when working with small numbers or over a short
period. Four fish died from the series in plain tap water
before one died from the series in water with glycerine
and asparagine. We should undoubtedly have taken this
as a proof that the fish in the water with the reagents
mentioned were utilising these substances in some way, had
it not been that the oxygen determinations lent no support
to this. Even so we were altogether puzzled, and it was
not until deaths occurred in the same condition in the other
series that we began to realise that this distribution was
probably nothing but coincidence, and was due to original
variations amongst the fish. The termination of the
experiment was almost a "dead heat," for on 19th January
one fish was left in each series and these two died within
four days of each other—more than three months after the
experiment was commenced! There was no evidence from
this result that the presence of asparagine and glycerine had
supplied any essential quantity of food. It was, however,
the oxygen determinations upon which we mainly relied.
The results of these (leaving out working calculations and
giving the milligrammes of oxygen consumed per hour at a
common temperature of 15° C.) are shown in the following
tables. The corrections for temperature were made by
applying the equation Q10 = 2.5. This was obtained from
oxygen determinations made at different temperatures between
io° and 15° C.
Tables I., II., III., and IV. and the corresponding curves
Figs. 1, 2, and 3 show quite clearly that no difference can be
distinguished between the oxygen consumption of the fish in
ordinary tap water and those in the water to which glycerine
and asparagine had been added.
The series A, B, C and A', B', C (Fig. 1) are the best
for comparison because from the first the duration of the
experiment was three hours, the period which was found later
to be most satisfactory. The series D, E, F and D', E', F '
(Fig. 2) were one-hour experiments, and it was noted that
more variation occurred in the results. These variations
disappear somewhat if the total oxygen consumption of the
fish at each'date is compared (Fig. 3). Eventually the series
3°7
W. J. Dakin and Catherine M. G. Dakin
TABLE I.
Goldfish in Tap Water with Glycerin* and Asparagine. Results calculatedfor 15° C.
Date. Dec 12. Oct 15. Oct 18. Oct. 22. Oct 39. Nov. 1. Nov. 12. Nov. 23. Dec 5.
Fish.
A
B
C
1-917
1-280
0-989
I-I55
0-859
i-6o8
I-III
0803
0-756
O495
0-550
0-92
o-9M
0-84
072
0-52
0-452
0-302
0-46
1-30
I-2O
1-14
0-856
O-773
0-613
O-43'
0-487
r
Mg. of
Oxygen used
-1 per hour
during
I Experiment.
TABLE II.
Goldfish in Plain Tap Wattr. Results calculatedfor 150 C.
Date. Dec. 12. Oct 15. Oct. 18. Oct 22. Oct 29. Nov. 1. Nov. 12. Nov. 23. Dec 5.
Fish.
A'
B'
C
1-548
0-881
1-158
1-359
1030
O596
0-642
0-477
0-461
1-894
1-29
1-43
1-18
102
072
0-62
0-49
0-42
1-597
O-955
0-989
1-04
072
0-589
0-420
0-551
0-450
r
Mg. of
Oxygen used
per hour
during
. Experiment.
TABLE III.
Goldfish in Tap Water with Glycerin* and Asparagine. Results calculatedfor 15° C.
Date.
Oct 15.
Oct. 18.
Oct. 32.
Oct 30.
1-49
1-40
145
o-?9
109
Nov. 14.
Nov. 19.
Fish.
D
E
F
172
i-i3
1-15
I-2O
0865
2-03
i- 3 8
i-43
'•34
0-728
0-63
Mg. of
Oxygen used
per hour
during
. Experiment
Total
5-24
4-91
4-03
3-43
2-683
...
...
0-68
c
TABLE IV.
Goldfish in Plain Tap Water. Results calculated for 15* C.
Date.
Oct 15.
Oct 18.
Oct 22.
Oct 30.
Nov. 14.
Nov. 19.
0-50
Fish.
D'
E'
F
1-301
0-965
O-8I5
108
0-863
0-989
0-810
0-758
0-443
0-763
1-131
1-29
1-28
I-I2
0-728
O53
Mg. of
Oxygen used
per hour
during
. Experiment.
Total
3-421
3-O74
2-853
3-643
3-354
...
...
308
Oxygen and Aquatic Animals
Ott.tr
ir
at
FlC. I.—Oxygen consumption of the fish A, B, and C in tap water + asparagine and
glycerine ; and A', B', and C in ordinary tap water.
Oiis
FIG. a.—Oxygen consumption of the fish D, E, and F in tap water + asparagine and
glycerine ; and D7, E', and F' in ordinary tap water.
VOL. II.—NO. 3.
3°9
u
W. J. Dakin and Catherine M. G. Dakin
D, E, F, etc., was not tested as frequently as the series A, B,
C, etc., but it was used for other experiments.
It will be remembered that Putter found that his goldfish
in water with organic compounds lived over i£ times as long
as those in ordinary tap water and consumed i | times the
Octlff
19
23
SO No*. 19
FIG. 3.—Total oxygen consumption of the fish D, E, and F, and D', E', and F'.
amount of oxygen. There is no indication of any such effect
in our experiments, which are in agreement with those of
Lipschiitz.11
2. The Oxygen Consumption of developing Plaice Eggs and
its Relation to the Amount calculated from Analyses of
the Composition of Plaice Eggs at Different Stages" of
Development.
It should be clear from our discussion of Putter's theory
that we regard that author's calculations of the food of
certain aquatic animals from their oxygen consumption, and
from analyses, as the section of greatest importance and
most in need of further investigation. As part of that
inquiry Putter made several experiments to show that the
oxygen consumption of certain aquatic animals deprived of
solid food was greater than could be accounted for on the
loss of dry weight which took place. In other words,
absorption of food in solution must have taken place, although
it might not have been sufficient to prevent starvation. Thus
in the goldfish experiments of Putter the oxygen determinations gave 832 mgs. as the oxygen consumption, whilst only
365-375 mgs. oxygen were really necessary for the oxidation
310
Oxygen and Aquatic Animals
of the substances consumed, according to calculations from
the loss of weight which had occurred.
Unfortunately in all the experiments of this kind made up
to date one has had to analyse certain animals at the beginning
of the experiment, and that was naturally the end of them.
The final analyses were made on a different set of specimens
which had been utilised for the experiment. It was necessary,
therefore, for approximate accuracy that the animals should
have all been of the same composition at the beginning of
the experiment. This can seldom be realised in practice, for
the wet weight (if it can be obtained) is not a reliable guide
to dry weight or composition, and the same criticism might
be raised against other theoretical methods of computation.
We endeavoured, therefore, to take advantage of the
opportunity presented by a fish hatchery at the Biological
Station of Port Erin to make a test on plaice eggs. These
could with a little trouble be counted and, if thousands were
used (of the same age), any individual variation might be
considered as eliminated. One could take a sample of any
age from a hatching box of eggs and analyse them. Following
this the oxygen consumption of the eggs of the same box
could be determined every day, and after several days another
batch taken for analysis. If the embryos depended solely on
the food stores within the egg membranes, the oxygen consumption during the period of the experiment should agree
with that calculated from any loss of weight which might
occur in the substance of the eggs.
Technique.—Two samples of eggs were taken from a
hatching box which was supposed to contain eggs of about
the same age. Most were about the 8- or 16-cell stage. They
were measured out (with as little sea water as possible) by
means of an ordinary measuring glass, previous tests having
shown that one could approximate to within a few hundred
in this way, and that it required about 2500 eggs to give a
wet weight of roughly 10 gms., an amount deemed necessary
for accuracy in analysis. The eggs were then turned out
on to an ample supply of clean filter paper and carefully dried
of sea water by rolling them over and over very gently. At
this stage they were counted, and to obviate any delay before
weighing several helpers took a hand so that only a few
3"
W. J. Dakin and Catherine M. G. Dakin
minutes were involved. Each sample was next placed in a
weighed Soxhlet thimble and weighed accurately on a chemical
balance. This gave the wet weight. The thimbles were next
dried at about 85° C. for the dry weight. In the meantime
the oxygen consumption was being determined, using other
eggs from the same hatching box.* These oxygen determinations were repeated day after day and always on samples
from the same box until fourteen days had elapsed. Two
more samples of eggs were then taken for analysis and counted
and weighed in the same way as the first samples. The counts
and analyses are as follows:—
Experiment commenced 1 \th April:—
Sample of Eggs A.
Wet weight .
Dry substance
Water .
Fat
Residue
Nitrogen in 0.3 gm. Residue
Sample of Eggs AA.
Wet weight .
Dry substance
Water .
Fat
.
Residue
Nitrogen in 0.3 gm. Residue
No. of eggs—2982.
10.240 gms.
0.721
9-5*9
„
..
0.009
0.71a
it
11
0.0436 „
No. of eggs—2945.
9.676 gms.
0.701
„
8.965 „
0.008
„
0.694
„
0.0429
„
Experiment ended i$th April:—
No. of eggs—2470.
Sample of Eggs B.
8.11 T gms.
Wet weight .
•
0.529
..
Dry substance
Water .
•
7-58*
,,
0.034
„
Fat
.
0.497
„
Residue
0.0413 „
Nitrogen in 0.3 gm. Residue
Sample of Eggs BB. No. of eggs—2250.
7.549 gms.
Wet weight .
„
Dry substance • 0.458
Water .
•
7-091
.»
0.015
..
Fat
.
0.444
„
Residue
0.0431 „
Nitrogen in 0.3 gm. Residue
• Numerous determinations of oxygen consumption of plaice eggs had previously
been made so that the experimental errors were reasonably well known, and such
details as the number to take and the best duration of experiment had been
discovered. (Details will be published in another paper.)
3"
Oxygen and Aquatic Animals
Since we are dealing with large numbers of similar eggs
these results may be reduced to a common denominator,
namely, the composition of 2000 eggs, and we may take the
average of the two samples.
Composition of 2000 eggs at beginning and end of experiment,
of experiment:—
At beginning
2000 eggs.
Wet weight .
Dry substance
Water .
Fat
.
Residue
Nitrogen in 0.3 gm. Residue
Proteid calculated from Nitrogen
6.72
0.479
gms.
6.251
0.0057
°-4733
0.0432
°-4»59
At end of experiment:—
2000 eggs,
Wet weight .
Dry substance
Water .
Fat
.
Residue
Nitrogen in 0.3 gm. Residue
Proteid calculated from Nitrogen
6.638 gms.
6.2203
0.0204
O-3973
0.0422
0.3476
From these figures it may be estimated that 2000
eggs have lost 78.3 mgs. of proteid during fourteen days'
development. Now the oxygen determinations which were
made during the fourteen days of the experiment show that
2000 eggs in the hatching boxes used approximately something
between 82.086 and 98.0 mgs. of oxygen. Assuming that this
was used entirely in the oxidation of proteids, it corresponds
roughly to a loss of between 65.67 and 77.8 mgs. of the latter,
a figure which does not exceed (does not reach) the amount
of proteid actually lost.
The interpretation of the result along the lines of Putter
is that the developing fish eggs have not imbibed substances
in solution in the sea water, their own contained stores have
sufficed.
The consumption of oxygen during development as
calculated from the loss of organic matter in the eggs during
the experiment balances roughly the actual consumption as
directly measured, that is to say, it is not found to be
VOL. II.—NO. 3.
313
U2
W. J. Dakin and Catherine M. G. Dakin
only half or a third of the latter (as in the case of Putter's
experiments on other animals where food in solution was
supposed to be absorbed). The method of experiment seems,
therefore, to be sound, but the result is only an approximation.
Where an animal starved of paniculate food is absorbing
any significant quantity of food by way of solution, this type
of experiment would indicate the fact. Where, however, only
a few per cent, of the total supply comes from solution it
would not be safe. Nor would it be safe to say that a small
discrepancy between, the actual oxygen determination and the
theoretically calculated amount from analyses indicated an
absorption of food from solution. The oxygen consumption
may be accurately determined by actual experiment, but the
food requirements suggested by the amount must be regarded
as approximate only. In the case of the plaice eggs, for
example, a very unexpected result has been found, for there
is three times as much fat at the end of the period of
development as at the beginning. The total amounts, 6 and
20 mgms. are, however, so small that we must not stress this
result. This fat must have come from the proteids present,
for there is no evidence so far that plaice eggs contain any
carbohydrates to speak of. But as yet we have no evidence
to show what complex reactions have taken place in these
eggs. The discovery apart from our present investigation
is an interesting one. The fact is that the metabolism of
the invertebrata and lower chordata is a vast field that is
not to be covered with one general and simple formula.
Aquatic organisms are not to be dumped together in a class,
as if the mere fact that they are aquatic compels them to
function in the same manner. The problems of human and
mammalian metabolism are indeed complicated enough. There
is no reason to believe that the different groups of the lower
animals present conditions which are less complex. Thus,
F. A. Potts l t in an interesting paper on Teredo has shown
that this aquatic mollusc feeds upon wood and may die if
the wood is deficient in amount. It has only very recently
been shown ta that the white ants are unable to feed on
wood—their apparent normal diet—unless large numbers of
certain protozoa live within their alimentary canals. There
Oxygen and Aquatic Animals
are probably numerous other cases of a similar nature in
addition to those now recognised (like Convoluta, etc.)
as symbiotic. Again few physiologists and extremely few
zoologists seem to realise that the elasmobranch fishes and
the teleost fishes are physiologically far apart. They appear
to have much in common, yet the blood of the former has
the freezing point of the external medium and varies with
it, whilst that of the latter is more or less constant.6
It would surely be rash to calculate the food requirements
of an elasmobranch from those of a bony fish, granting
a knowledge of the gill areas in the two cases. There
is no point in adding further examples. Where a food
requirement is calculated for an aquatic organism, actual
oxygen determinations and analyses of its composition must
be carried out, and the metabolism and seasonal habits of the
particular organisms must be thoroughly well studied before
far-reaching deductions are made.
SECTION B.
3. The Absorption of Oxygen in the Course of Experiments made
to determine the Oxygen Consumption of Certain Aquatic
Animals.
Several investigators who have attempted the determinations of oxygen consumption of aquatic animals have remarked
upon a difference between the rate during the successive hours
of an experiment. The matter is an important one, and it is
necessary to know something of the cause and amount of
this variation in order to interpret the results. It would be
very easy to overestimate oxygen requirements if the consumption during twenty-four hours were taken to be twentyfour times that of the first hour of experiment, and this
happened to be abnormally high. An extreme example of
the kind to which we refer is the following from the
investigations of the late Professor B. Moore. A sponge
used 1.43 mg. of oxygen in eight and a half hours, but only
1.57 in forty-eight hours. The crab Cancer used 17.79 mgs.
in four hours, and only 19.49 m g s - when the experiment was
continued up to nineteen hours. (Other results of Moore
are, however, too varied altogether to illustrate the point.)
31S
W. # J. Dakin and Catherine M. G. Dakin
Brunow 8 found that the oxygen consumed by the crayfish
Astacus fluviatilis diminished as his experiment continued.
Thus in one case for sixty minutes the rate of oxygen consumption per hour and animal was 0.786 mg., whilst during
a 120-minutes experiment the rate fell to 0.667 mg-> a n d for
a 3-hour experiment the oxygen consumption was only
0.506 mg. per animal per hour. The cause of this very
considerable reduction was not worked out. In some experiments, however, in which the crayfish were out of water, the
production of COS seemed to exert a retarding influence on
oxygen consumption.
Lipschtltz u found the same kind of
retardation in his work with goldfish. The following figures
are from his paper :—
Duration of experiment- 59 minutes ( O x ^ e n c ° n s u m P t i o n 1 O . 8 2o mg.
I. per animal per hour )
»
i>
3
7^
„
,,
,,
0.626
,,
59
.,
•>
.,
°-645 .»
350
»
..
,.
o-593 „
60
340
„
,,
„
,,
„
„
1.45
1.03
58
344
,,
>i
,,
..
»
11
1-33 „
0-854 ,,
Lipschutz was indeed rather concerned about the error
which might arise by computing the oxygen consumption in
twenty-four hours from experiments of short duration. But
it must be noted in this connection that he regarded the
amount used in the first hour of experiment as too high,
and due to the increased excitement of the fish owing to
handling.
It is a probable theory, but was not proved by
experiment.
There are several possible ways of accounting for the
diminution in the oxygen consumption per hour (if it actually
does occur) during a prolonged experiment. They are as
follows: (1) Handling the specimens and so causing an
abnormally high rate of consumption at the commencement
of the experiment. (2) The gradual absorption of oxygen
316
Oxygen and Aquatic Animals
and the consequent diminution in the amount available in
the closed vessel resulting in retarded oxygen consumption
during the latter part of an experiment. (3) Accumulation
of COS or of other waste compounds acting as a retarding
influence on oxygen consumption and metabolism during the
latter part of the experiment. (4) The animals may have fed
immediately preceding the experiment.
All these are interesting possibilities with important
bearings upon the experiments made in connection with
Putter's theory, whilst Nos. 2, 3, and 4 are of wider
interest.
So far as our knowledge at present extends, the invertebrate groups do not present any unity in their reactions to
changes in the partial presence of oxygen. Mammalia are
not affected by pure oxygen at 760 mm. barometric pressure,
whilst this is apparently deadly for the leech. Konopacki1(>
has shown that the oxygen consumption of the earthworm
varies with the pressure of the oxygen, and may be calculated
from the formula
a = kj~d
where a = oxygen consumption, k = a constant, and d = the
partial pressure of the oxygen. The same thing apparently
holds good for Litnax.
On the other hand, Henze found that whilst the oxygen
consumption of Actinia and Anemonia varies in this way
with the reduction of the normal oxygen content of the water,
other invertebrates are very independent. Amongst these
were Echinoderms, certain pelagic molluscs, Crustacea {Carcinus mcenas and Scyllarus latus), the molluscs Aplysia and
Eledone, and certain fishes.
In order to test the matter we first carried out experiments
on plaice eggs. These were taken because they seemed
excellent examples on which to rule out the action of handling.
The eggs were constantly in motion in the hatching boxes and
their insertion into the experimental bottles did not even
necessitate removing them from this water. They were
simply run in with the water and the counts made at the
end of the experiments. There could not be any psychic
influences here!
317
W. J. Dakin and Catherine M. G. Dakin
The results of several experiments are as follows :—
Experiment 1.—1000 eggs use 0.071 mg. oxygen in 1 hour
„
„
0.113
„
,,
2 hours
Available oxygen reduced by 4.6 per cent in first hour.
Experiment 2.—1000 eggs use 1.052 mg. oxygen in 6 hours
o.838
5
„
0.647
4
2
„
„
0.366
6
1
hour
>,
>• ° - * 3
1
Experiment 3.—1000 eggs use 0.088
»
o-io5
Experiment 4.— rooo eggs use 0.120
0.210
2 hours
The results are in remarkable agreement in that the oxygen
consumption of plaice eggs in a closed volume of sea water
(the amount of oxygen available was considerable) was always
greater during the first half-hour or hour than during the
second and third. The effect of handling is ruled out here,
and consequently we regard the results as indicating in the
case of plaice eggs a retardation of oxygen absorption due
to reduction of the amount of oxygen available, or to the
increase in the water of COt and excreted substances, or of
both. Since the total amount of oxygen available in the
volume of water containing the eggs has been reduced by a
very small percentage, the eggs are either very susceptible
to oxygen pressure, or else it is the second possibility referred
to above which is the most important factor; and this may
be accentuated by the relatively more quiescent condition of
eggs during the experiment, although the jars were frequently
turned over.
Similar experiments were made using the goldfish, axolotls,
and the freshwater mollusc Anodon. A goldfish (specimen C)
was placed for two hours in a very small jar of water, capacity
808 c.c, containing only 7.97 mgs. of oxygen available in
solution. It used up oxygen at the rate of 0.3804 mg. per
hour. The same fish was then tested in a large jar containing
7050 c.c. of water with 69.20 mgs. of oxygen available in
solution. The amount of oxygen consumed was practically
the same, viz., 0.3884 mg. per hour. The experiment was
repeated with another fish, and again the difference was within
the experimental error and the usual amount of variation.
318
Oxygen and Aquatic Animals
In this case the fish actually used more oxygen per hour in
the jar containing only 7.9 mgs. available oxygen than in the
vessel containing 70.4 mgs. of available oxygen.
More extensive experiments were made with two axolotls,
varying the duration of the experiment from two to twenty-four
hours. No appreciable reduction was noticed in the amount
of oxygen consumed per hour. If any diminution took place,
it was less than the experimental error. Finally, we examined
two Anodons which had been under observation for some
time. The following are the results for mussel A :—
Date.
Feb. 19 .
) 2°
, 22
, 28
Mar. 4
, 5
, 6
i,
7
1
: l
,
12
.
13
Duration of
Experiment.
Temp.
I2-O
12-5
I2-O
13-5
12-5
13-0
13-5
13-5
13-0
16-0
16-0
1 hour
18 hours
2
,
2 ,.
5 >
5 ,
5 .
10 m m s .
15 .,
Oxygen available
in Experimental
Vessel
at beginning.
Mg.
3I-295
28-27
32-162
45-361*
i6-27t
3276
15-53+
26-07
32-29
13-65+
22-Ojt
5
5 >
5 >
S .
Oxygen used
per hour.
Mg.
O-7837
0-9303
0-8761
0-762
O-354
0-795
0-609
0-843
0-815
0-305
0-865
* More oxygen available because large jar used Actually less oxygen per c.c
t Oxygen contents reduced by addition to tap water of a certain amount of tap water raised tcboiling-point and then cooled.
In the case of both mussels an increase in the duration
of the experiment from one hour to eighteen hours did not cause
any appreciable diminution in the rate of oxygen consumption.
It will be noticed that in this experiment the reduction in the
amount of oxygen available at the end of eighteen hours was
roughly 50 per cent. Where the amount of available oxygen
was reduced by this amount at the beginning of the experiment
by the addition of non-aerated water a very appreciable
difference in the oxygen consumption was apparent. The
last experimental test of 13th March shows, however, that a
reduction by about 33 per cent, does not make a big difference.
We may conclude, therefore, that the axolotls, the goldfish, and
the freshwater mussels are all somewhat independent of the
oxygen contents of the water under normal conditions, and
that it is only when the amount of oxygen available falls
319
W. J. Dakin and Catherine M. G. Dakin
below a certain minimum that oxygen consumption is
profoundly affected. We should say also that handling did
not produce any effect in the experiments quoted above,
because the specimens were so accustomed to the treatment
received.
Moore's results can be explained upon these lines, in that
the amount of oxygen available was too small for the duration
of his experiments and fell below the minimum.
It is quite possible that handling was the cause of the
variation in oxygen consumption in Lipschiitz's fish experiments, but his suggestion that the greater variation when
large specimens were used supported this view is not altogether
satisfactory. Larger specimens would also use more oxygen
and produce more COj. We intend carrying out further
experiments on the fish eggs in order to test their sensitivity
to CO,
4. Summary.
1. The original theory of the food supply of aquatic
animals put forward by Putter, and based upon the results
of certain experiments and analyses, claimed that the chief
source of food of such animals was organic matter dissolved
in the sea, in lakes, rivers, etc., and that this was absorbed
directly and indeed often by the gills, if present. The
position he took up may be emphasised by his statement
regarding fishes : " There is no doubt that a nutrition without
dissolved foodstuffs is possible, and it is not impossible that
cases of this kind are realised in nature. But the experiments at Naples show that the fish in the Naples aquarium
under approximately natural conditions obtain one-half to threequarters or more of their food requirements by the absorption
of dissolved food."
2. It is quite possible that small quantities of organic
matter in solution in water are absorbed by aquatic animals,
and in some cases (particularly amongst protozoa living under
special conditions) this may be an important, perhaps the
most important, source of food. It is also possible that very
small quantities of organic matter in solution may eventually
be found to exercise a very profound influence (acting like
vitamines, for example) on the life of aquatic animals.
320
Oxygen and Aquatic Animals
Evidence for such is not evidence for the main thesis set up
by Putter on the results of his own experiments.
3. The food requirements of many aquatic animals as
calculated by Putter on the basis of oxygen consumption are
often remarkably high and need further investigation. It is
quite possible that there is an unknown factor at work here.
In connection with these calculations we consider that purely
theoretical computations based upon the measurement or
estimation of the active surface area of the body are not
permissible, and that the application of the law of surface
area in connection with metabolism must not be allowed to
supplant experiment
4. Experiments on goldfish similar to those made by
Putter, show that specimens kept in tap water without any
particulate food live for varying periods (which are often of
considerable duration) dependent upon the original condition
of the fish, and the freedom of the experimental tanks from
parasites. The addition of the organic compounds, glycerine
and asparagine, makes no difference to the duration of life,
and the consumption of oxygen by the fish living in tap
water with these compounds does not exceed that of the
control fish in tap water only. Gradual starvation takes
place, and sections show that the mass of muscle tissue
becomes gradually reduced.
5. The cessation of feeding on particulate food makes
the fish particularly susceptible to the attacks of parasites
[Chilodon cyprini and Gyrodactylus, sp.), if there is any chance
of such infection.
6. The consumption of oxygen by plaice eggs during
their development agrees fairly well with the amount computed
from analyses of the composition of young eggs and eggs
shortly before hatching, but the results are only approximate,
although they fit in with the assumption that such floating
eggs have their own food stores and absorb nothing from
the sea water.
7. Aquatic organisms are not to be grouped in one class
in so far as nutrition and metabolism are concerned.
8. It had frequently been noted that when the oxygen
consumption of aquatic animals is measured, and the deter3"
W. J. Dakin and Catherine M. G. Dakin
mination extends over several hours there is a gradual falling
off during the experiment. It is necessary to look for this
in every case before estimating the normal oxygen consumption over long periods. The variation may be due to
handling the specimens at the beginning of the experiment,
to the gradual reduction of the oxygen available, to the
accumulation of waste products, or to time of feeding.
We have found that goldfish, axolotls, and Anodon used
in our experiments are to a certain extent independent of the
oxygen pressure, which may fall considerably (until a certain
minimum is reached) before the oxygen consumption of the
animals per hour is affected.
5. References.
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