ADAPTATION TO CHANGES OF SALINITY IN THE
POLYCHAETES
I. CONTROL OF BODY VOLUME AND OF BODY FLUID
CONCENTRATION IN NEREIS DIVERSICOLOR
BY L. C. BEADLE
University of Durham College of Medicine; Newcastle-on-Tyne
{Received 18 July 1936)
(With Five Text-figures)
INTRODUCTION
are several possible means by which marine invertebrates could survive the
passage into brackish water. They may be summarized as follows:
A. The animal may be able to maintain its body fluids hypertonic to the external
medium, thus avoiding undue internal dilution and distension of the body resulting
from osmotic inflow of water. To this end the following factors might contribute:
(a) Excretion of an hypotonic urine. This has been demonstrated only for the
freshwater crabs Potomobius astacus and P. leptodactylus (Herrmann, 1931; Scholles,
1933). On the other hand the concentration of the blood of these crabs is little
altered by blockage of the excretory organs (Herrmann, 1931). The urine is isotonic
with the blood in the brackish water crab Carcinus moenas and in the fresh-water
forms Telphusa Jluviatile and Eriocheir sinensis (Schlieper & Herrmann, 1930;
Scholles, 1933).
(b) Low permeability of the body surface to water and salts. Bateman (1933)
argues from the blood dilution curve of Carcinus moenas that there is probably a
decrease in passive permeability to salts in dilute sea water. Bethe (1934) and Nagel
(1934) produce evidence that brackish water forms are less permeable to salts than
marine forms. Experiments on Gunda (Procerodes) ulvae suggest a decrease in
permeability to water as a result of treatment with dilute sea water (Beadle, 1934).
Reduction in permeability, unless it were reduced to zero, could not of course by
itself maintain hypertonic body fluids.
(c) Active resistance to inflow of water and/or outflow of salts by all or part of
the body surface involving the expenditure of energy. The fact that there is an increase
in the rate of respiration on transference to dilute sea water of forms such as Nereis
diversicolor, Gunda ulvae and Carcinus moenas which can survive these changes has
been adduced as evidence of this activity (Schlieper, 1929; Beadle, 1931). Further
support is given by the action of cyanide, which causes an increased accumulation
THERE
Adaptation to Changes of Salinity in the Polychaetes
57
of water in the body in dilute sea water (Beadle, 1931). But this increase in rate of
respiration is not observed in some resistant forms (e.g. Eriocheir sinensis, Schlieper,
1935), anc^ th e increase is not always maintained (e.g. Nereis diversicolor, Schlieper,
1929; Beadle, 1931). Pieh (Schlieper, 1935), from measurements of the water
content of various organs, concluded that the increase of respiration is due to
hydration of the tissues and is not directly connected with osmo-regulation.
(d) Active uptake of salts from the external medium. Nagel (1934) showed that
if Carcinus moenas after treatment with dilute sea water is transferred to water of
a higher concentration but still lower than that of the blood, the concentration of
the latter is raised. In view of the isotonicity of the urine he concluded that this
must be brought about by active uptake of salts from the water against a concentration gradient.
(e) The maintenance of a high internal hydrostatic pressure to balance the
osmotic pressure difference. The possibility of this playing a significant part has
apparently not been systematically investigated though Bethe (1934) has considered
it as a possibility.
B. There may be no mechanism for the maintenance of hypertonic body fluids.
But the latter might be diluted to a considerable extent without harm, provided that
the body is not unduly distended. This would entail either a high permeability to
salts or a rapid removal of bodyfluidsby the excretory organs. The degree of dilution
of the external medium which the animal could withstand would in this case
obviously be limited.
It will thus be seen that maintenance of body volume and of a favourable internal
concentration are two problems confronting brackish-water invertebrates which are
not necessarily completely interdependent. That in the polychaetes they are less
connected than might have been expected will emerge from these and other results
to be published later.
The object of the following work on Nereis diversicolor, which can survive large
changes of salinity, was to make parallel determinations of weight and of internal
concentration changes resulting from treatment with dilute sea water, to investigate
the effects upon the body fluid concentration of cyanide and calcium-deficiency,
both of which influence the weight curve (Beadle, 1931; Ellis, 1933), and to decide
the relationship, if any, between the rate of respiration and the maintenance of
hypertonic body fluids. Direct measurements of hydrostatic pressure and of the
amount and concentration of the fluid excreted by the nephridia were not possible,
but certain deductions are made concerning these from the results of the experiments.
METHODS
The animals were all obtained from the same position in the River Blythe estuary.
Northumberland.
Weight. After placing on dry filter paper for a few moments to remove excess
of water they were weighed in water to the nearest o-oi g.
Calcium-free sea water and cyanide. Calcium-free sea water and sea water
containing M/1000 NaCN were prepared as previously described (Beadle, 1934).
58
L. C. BEADLE
Body fluid concentration measurements. Baldes's modification of the Hill vapourpressure method was used (Baldes, 1934). Three pairs of constantan-maganan
thermocouple loops were mounted together so that three determinations could be
made simultaneously. The filter paper lining the surrounding moist chamber was
soaked with distilled water, and the two loops of each pair were supplied with a
drop of distilled water and of the unknown solution respectively. The apparatus was
standardized for each experiment with 100 per cent sea water. The galvanometer
used was a low resistance Cambridge A and M model with 4 m. between mirror and
scale. Reading to the nearest 0.5 mm. on the scale it would theoretically have been
possible to obtain results accurate to the equivalent of the nearest 0-5 per cent sea
water. In practice the errors occurring from unknown causes made the determination accurate to the nearest 1 -o per cent sea water. All concentrations were expressed
as the equivalent of a percentage of sea water.
I am much indebted to Prof. A. V. Hill and to Miss B. M. Garrard for advice
on the use of this method.
Extraction of body fluids. The animal was placed on dry filter paper and the
fluid was extracted by means of a fine glass pipette inserted into the body cavity.
With the same pipette the fluid was transferred direct to the thermocouple loop. The
whole operation was performed in a moist chamber.
Respiration measurements. These were made with Barcroft manometers. Increase
in rate of oxygen consumption was expressed as percentage of original rate per gram
original weight.
In all cases the animals were kept in the experimental solutions in a thermostat
at 150 C. Temperature changes were found to have considerable influence upon the
weight curve—a subject which will be dealt with in a subsequent paper.
WEIGHT AND BODY FLUID CONCENTRATION
Experiments on weight changes were made at different times of year with animals
collected from the same place in the Blythe estuary. It was found that during the
winter months the increase in weight in 25 per cent sea water was greater and more
prolonged than during the summer. The curves in Fig. 1 are each typical of four to
six obtained at the same time. Five animals were weighed together to obtain each
point. The curve 13. ii. 35 is typical of animals which survive the change well with
no diminution of activity. The weight rose to a maximum after 5-7 hours and then
fell to a relatively steady value at 24—30 hours.
Unfortunately body fluid concentrations were measured only for a few points on
the winter swelling curve simultaneously with Fig. 1, curve 9. xii. 35. Body fluid
concentrations of single animals expressed as percentage sea water are enclosed in
circles in Fig. 1 and will serve as a comparison with those given later. A determination made after 9 days gave 42 per cent and after 5 weeks 32 per cent. Hypertonicity
of the body fluids is therefore permanent, but is not very great.
It was found that in 100 per cent sea water the body fluids were isotonic ± i-o
per cent.
Adaptation to Changes of Salinity in the Polychaetes
59
More exhaustive parallel determinations of weight and body fluid concentration
in 25 per cent sea water were made in May 1936 when the weight curve was showing
the summer form. The results are shown in Fig. 2 (a and b). The points were
obtained by weighing a number of batches of worms (five in each) at the start and
at intervals after transfer to 25 per cent sea water and by estimating the body fluid
concentration of three of each batch after the second weighing. It can be seen that,
though the points are rather scattered in places, the concentration curve (b) follows
a logarithmic course towards a value above that of the external medium. On the
assumption that no significant loss of salts nor excretion of fluid had occurred
during the first half-hour it was possible to construct from the concentration curve
a weight curve which would theoretically have been followed if there had been no
18. ix. 34 sluggish
120
/
//
110
100
/
/
Summer
v
90
/
oc
5 80
sV
00
a
/
70
^
^
^—iS
-
60
/
- /^f^^!^^- 4 9 % 4
.4%
50
s§ 40
30
20
10
1 Winter
/ /
/
/
0
,
i
10
i
i
i
i
i
i
20
30
40
Time in 25 % sea water (hours)
i
1
50
i
I
60
Fig. 1.
salt loss nor excretion throughout the whole experiment (Fig. 2 c). The difference
between this and the actual curve obtained (a) might be due to continuous loss of
salts through the surface and/or to removal of fluid by the excretory organs.
The experiment was repeated in the same manner but using 50 per cent sea
water (Fig. 3). One of the thermocouple pairs was damaged and only two determinations of body fluid concentration were made at each time. The concentration curve
however is obviously of the same form as in the previous experiment and approaches
a value closer to the concentration of the external medium (Fig. 3 b). The weight
curve (a) shows a marked maximum and subsequent drop, and in general form
resembles the curves obtained in the winter using 25 per cent sea water. If this is
compared with the theoretical weight curve (Fig. 3 c) calculated as above from the
concentration curve it will be seen that the two do not diverge until after one hour.
If salts were being lost through the body surface it is reasonable to conclude that
6o
L.. C. BEADLE
they would have left at a maximum rate at the beginning of the experiment when
the difference between the internal and external concentrations was greatest. The
fact that the curves do not diverge at once suggests that the subsequent divergence
was not entirely due to passive salt loss through the surface.
This conclusion is further supported by the results of both these experiments
(Figs. 2 and 3) if the theoretical amount of salts lost through the surface at intervals
10
15
Time in 25 % sea water (hours)
20
25
Fig. 2.
during the first 5 hours is calculated on the assumption than none occurs during
the first half-hour and that salt loss is the sole cause of the lowering of the weight
curve.
V, the osmotically active weight expressed as percentage of the original weight,
can be found from the following equation:
Adaptation to Changes of Salinity in the Polychaetes
61
where \ = body fluid concentration at the start (= per cent sea water), A, = concentration of body fluids after time t, and Wt = weight increase (per cent) after
time t.
By substituting this figure for V in the above equation a series of decreasing
values can be found for \ corresponding to different moments of time. The figures
so obtained represent the theoretical initial concentrations of the body fluids if the
weight and internal concentration actually attained at each moment occurred with
no loss of salts or water, ioo —A,, will therefore give the quantity of salts theoretically lost after a given interval of time expressed as percentage of the original
concentration in the body fluids, on the assumption that the difference between the
two weight curves (a) and (c) is due entirely to outward diffusion of salts. When these
5
10
15
Time in 50 % sea water (hours)
20
Fig. 3-
figures derived from the two above experiments are plotted against time (Fig. 4)
most of the points fall upon a straight line. This result would not be expected on
the above assumptions since the rate of salt loss should progressively diminish as
the result of the decreasing difference between external and internal concentrations.
These considerations combined with the fact that there was apparently no loss of
salts during the second half-hour in 50 per cent sea water suggest that the main
cause of the divergence of the actual from the theoretical weight curve must be
sought elsewhere.
The most remarkable feature of the experiment shown in Fig. 3 was the fall in
weight after the maximum accompanied by very little reduction of body fluid concentration. If after 6 hours water was still being taken up, the drop in weight must
have been due to the removal of fluid hypertonic to the body fluids. If water inflow
had somehow been stopped, then the fluid removed must have been isotonic. In
62
L. C. BEADLE
either case forces other than diffusion or osmotic pressure must have been involved.
It seems unlikely that an isotonic or hypertonic fluid could be removed through
any other channel than the nephridia. These in the genus Nereis have open nephrostomes and thus afford a clear passage from the coelom to the exterior. The question
remains as to how the rate of flow through the nephridia could be sufficiently
increased to account for the observed lowering of the weight curve. A possibility
which suggests itself is that the rate of flow through the nephridia is partly determined by the hydrostatic pressure in the body cavity. This in turn might be controlled by the tonus of the body wall muscles. The swelling of the body cavity might
induce a response in these muscles which would increase the hydrostatic pressure.
If this were the main factor controlling the weight, the earlier divergence of the
actual from the theoretical weight curve in 25 per cent sea water (Fig. 2 a and c)
16
15
op 14
C
13
o
12
11
10
9
S-o 8
7
1
/
Its los
fluid
/
S-S
ore
1a
6
5
4
3
2
1
25% seawater/*
/ 5 0 % seawster
/
/
2
3
Time in hours
4
than in 50 per cent sea water (Fig. 3 a and c) could be explained. The rate of
swelling is greater in 25 per cent sea water and the muscular response might be
expected to occur earlier. On this hypothesis it would of course be necessary to
postulate that the fall in weight from the maximum (Fig. 3) was due to a relatively
violent reaction on the part of the muscles when a certain degree of swelling was
reached.
The expulsion of fluid as a result of a swollen body cavity could alone explain
the result of the experiment summarized in Table I. The concentration of the body
fluids of two sets of animals was brought to the equivalent of 54-55 per cent sea
water by different methods of treatment: (a) 4 hours in 25 per cent sea water, and
(b) 3 days in 50 per cent sea water. Both sets were then weighed and transferred
to isotonic sea water (55 per cent). Subsequent weighings done after 3 and 22 hours
showed that the more swollen ones, i.e. those previously treated for a short time
with 25 per cent sea water (a), decreased considerably in weight, whereas the weight
Adaptation to Changes of Salinity in the Polychaetes
63
of (b) remained relatively constant. Since the body fluids were in both cases at the
start isotonic with the external medium, the result must be ascribed to the higher
internal hydrostatic pressure in (a) causing the expulsion of fluid.
Table I
Time in 55% sea water (hours)
Previous treatment
0
(a) 3 days 100%
4 hours 25%
(6) 3 days 50%
22
86
86
86
97
Weight 100
100
100
Weight 100
100
100
75
75
96
103
103
103
i°5
Table II
Time in 55% sea water (hours)
Previous treatment
4-25
2"5
(a) 3 days 100%
4 hours 25%
(6) 3 days 50%
Body fluid 69
= % sea water 64
64
68
Body fluid 61
= % sea water 61
61
61
7i
64
66
67
58
68
60
61
21-25
72
79
77
69
64
70
62
60
The experiments illustrated by Figs. 2 and 3 show that there was little connexion
between the maintenance of hypertonic bodyfluidsand the regulation of body volume.
It was in fact found from experiments done on single animals that, though the
weight attained after a given interval varied considerably, the concentration of the
body fluids could be predicted fairly accurately. The weight and concentration
figures given in Table IV will serve to illustrate this point. The body fluid concentrations correspond fairly closely with those previously found after the same
period in 25 per cent sea water (Fig. 2), but there is great variation in the weights.
There is therefore no obvious connexion between the degree of swelling and the
internal concentration, the latter being determined only by the time elapsed since
transfer to dilute sea water.
In order to investigate this point further the experiment previously described
(Table I) was repeated, but in this case the object was to determine the effect, if
any, of isotonic sea water upon the body fluid concentration of swollen and nonswollen animals (Table II). It can be seen that in both sets there was a rise of internal
concentration during the first 2-5 hours which was greater in the more swollen set,
i.e. those previously subjected to 4 hours 25 per cent sea water (a). The subsequent
figures are not very satisfactory but there seems to have been a general tendency for
64
L. C. BEADLE
the internal concentration of the originally more swollen animals (a) to increase
still further during the next 20 hours, whereas the concentration of (b) remained
more constant.
The results of these two sets of experiments (Tables I and II) suggest that the
establishment of hypertonic body fluids can be brought about without significant
alteration in weight in isotonic sea water (Tables I and II b), but that the process
is speeded up when a fall of weight occurs as a result of a high internal hydrostatic
pressure (a). This might be explained if the nephridia were excreting an hypotonic
urine. The higher internal hydrostatic pressure in (a) would cause a faster flow of
fluid through the nephridia, whereas in (b) the lesser degree of concentration might
have been due to a balance between the action of the nephridia and the consequent
osmotic inflow of water without any extra stimulation of the former as a result of
a high internal hydrostatic pressure.
Histological examination of animals at intervals after transfer to dilute sea water
showed no obvious change other than a swollen body cavity. There were no tissues
which appeared swollen such as were found in the case of Gunda vlvae (Beadle, 1934).
CALCIUM DEFICIENCY AND CYANIDE
It has been shown previously that calcium deficiency and Af/1000 cyanide cause
a greater weight increase in dilute sea water than normally occurs, and that this
effect is reversible (Ellis, 1933; Beadle, 1931). The following experiments were done
Table III
Treatment
Body fluid
= % sea water
Condition
(a) Transferred direct
100—30% sea water
M/1000 N a C N :
(i) 25 hours
4i
41
Moderately active
44
(ii) 72 hours
42
29
3°
(iii) 72 hours plus 5 days
normal 30%
(6) Transferred direct
100-30% calcium-free sea
water:
(i) 48 hours
(ii) 72 hours
(iii) 72 hours plus 4 days
normal 30%
(c) Controls. Kept 42 hours
normal 30%
36
36
36
35
Swollen and quite motionless
but reacting to touch
Quite normal
33
37
Just moving
32
32
Quite motionless
36
38
47
47
Complete recovery in 22 hours
Adaptation to Changes of Salinity in the Polychaetes
65
to test the effects of these conditions on the body fluid concentration. The results
are shown in Table III. There is no doubt that both calcium deficiency and cyanide
bring about an osmotic equilibrium between internal and external media and that
this effect is reversible. This further confirms the results described in the last
section which showed that concentration of body fluids can occur in an isotonic
medium. It is remarkable that under both conditions the attainment of equilibrium
requires at least two days and that this prolonged treatment has no permanent
deleterious effects. Similar results were however obtained in experiments on Gunda
ulvae (Beadle, 1934).
RATE OF RESPIRATION AND OSMOTIC REGULATION
Table IV gives results of experiments done on single animals subjected to 25 per
cent sea water for varying periods of time, the increase in weight and in respiratory
rate, and the final body fluid concentration being determined in each case.
Table IV
Time in 25% sea
water (hours)
(.")
(*)
(c)
(J)
(«)
4-25
4-25
22-25
22-25
50-75
if) 50-75
% increase in
weight
% increase in
Body fluid
respiration rate = % sea water
47
54
87
32
26
135
0
74
35
42
136
26
55
54
37
38
38
42
It is evident that, though the body fluid concentration approximate to the values
previously found at the corresponding times (Fig. 2), both the weights and respiratory rates are extremely erratic. It should however be noted that (d), whose
weight had increased by 135 per cent, showed no increase in respiratory rate,
though its body fluid was of the same concentration as that of (c) subjected for the
same period to 25 per cent sea water, whose weight had increased by only 87 per
cent and whose respiratory rate was 42 per cent higher than the original.
If, as originally suggested by Schlieper (1929), the increased rate of respiration
in dilute sea water is the result of increased osmotic work, it should be of interest
to observe the effects upon the respiratory rate of transferring animals from dilute
sea water to a solution isotonic with the body fluids. This was tested in the following
experiment the results of which are plotted in Fig. 5. Two single animals (Fig. 5 a
and b), whose weight and respiratory rate were previously determined, were placed
in 25 per cent sea water for 4 hours, when it was assumed that their body fluid
concentrations were equivalent to 54-55 per cent sea water. The weights and
respiratory rates were then redetermined and they were transferred to 55 per cent
sea water. An immediate remeasurement of the rate of respiration showed that it
had risen in both cases by about 30 per cent (Fig. 5). Subsequent determinations
from 3 to 45 hours after transfer to 55 per cent showed that the respiratory rate
66
L. C. BEADLE
had fallen to a steady value approximating to the original in 25 per cent sea water
in (a) and about 15 per cent below it in (b), but in both cases this value was well
above the original in 100 per cent sea water. The weights of both animals fell
during this period practically to the original in 100 per cent sea water (Fig. 5, lower
curves).
Although it is not possible to draw any positive conclusions from this experiment, the immediate and temporary rise in respiratory rate cannot be ascribed to
a temporary intensification of the osmotic regulatory mechanism, since hypertonic
body fluids are not only established but also maintained under these conditions
(Table II a). It might perhaps have been due to the stoppage of water inflow on
10
20
30
40
Time in 55 % sea water (hours)
50
Fig- 5-
transfer to isotonic water and the consequent release of internal pressure which was
being resisted by the tonus of the body wall muscles. This sudden release might
result in the immediate contraction of these muscles, which might be reflected in
increased oxygen consumption. It is difficult however to understand why this initial
rise should have been of such short duration, since the weight continued to decrease
after it was over. Another possible explanation might be that the sudden change in
concentration of the external medium stimulated peripheral sensory structures and
caused a temporary rise of metabolic rate, which was not connected with muscular
or osmotic work.
Even though the weight had fallen after 45 hours to the original in 100 per cent
sea water an increased rate of respiration was still maintained. This might have been
Adaptation to Changes of Salinity in the Polychaetes
67
explained as the result of osmotic work were it not for the experiments previously
done (Table IV), which showed that hypertonic body fluids could be maintained
with no rise in oxygen consumption. It can at least be concluded from the results
described in this section that the majority of the extra oxygen consumption in dilute
sea water is not the result of osmotic work.
DISCUSSION
Two main conclusions can be drawn from the foregoing experiments: (i) that
the degree of osmotic regulation of the body fluids is relatively slight, and cannot
be considered as being of direct importance in enabling the animals to survive in
dilutions of sea water between 100 and 25 per cent, and (ii) that survival is possible
mainly because they can prevent excessive swelling of the body due to osmotic
inflow of water.
Schlieper (1929) made freezing point determinations on the body fluids of Nereis
diversicolor. After 3 days in 50 per cent sea water (i7°/oo salinity) he found the body
fluids equivalent in concentration to 68 per cent sea water and after 2 days in 25 per
cent they were equivalent to 49 per cent sea water. These figures are higher than
those found in the above experiments (Figs. 1 and 2), but Schlieper states that in
the localities from which he obtained his material he sometimes found specimens
living in a salinity as low as 4%o (11-8 per cent sea water). Salinity estimations on
the water at Blythe showed that the worms were probably never subjected to sea
water of lower concentration than 60 per cent. It is possible therefore that the
osmotic regulatory mechanism of the animals with which Schlieper worked was
better developed as a result of continual subjection to water of lower salinity. In the
case of the animals used in the present experiments it does not appear that their
capacity to maintain hypertonic body fluids has any survival value, since it was found
that the internal concentration could be reduced to the equivalent of 32 per cent sea
water without harmful results even after a period of several weeks.
No direct light is thrown upon the nature of the osmotic regulatory mechanism
by the present work. But since the concentration of the body fluids is increased
when an animal previously treated with dilute sea water is transferred to an isotonic
solution (Table II), the mechanism must entail an active process and one which is
not merely controlling the inflow of water. There seem to be two remaining possibilities : (i) addition of salts to the body fluids from the tissues or by active uptake
from the external medium (as was suggested by Nagel for Carcinus moenas),
(ii) excretion of hypotonic fluid by the nephridia. The fact that worms which are
more swollen and consequently have a higher internal hydrostatic pressure are able
to concentrate their body fluids more rapidly in an isotonic medium than less swollen
ones (Table II) might be explained if the nephridia were producing an hypotonic
urine. The amount of fluid passing through the nephridia might be greater under
the greater hydrostatic pressure. It certainly could not easily be explained if the
mechanism entailed the addition of salts to the body fluids, since an increase in
hydrostatic pressure is not likely to speed up this process. There appears therefore
5-3
68
L. C.
BEADLE
to be some circumstantial evidence that the nephridia are largely responsible for the
maintenance of hypertonic bodyfluidsand that the internal hydrostatic pressure may
play an indirect part in determining the amount of fluid dealt with by the nephridia.
Bethe (1934) concluded that the departure of the swelling curve of Nereis
diversicolor from that theoretically expected if the surface were impermeable to
salts is to be attributed to the loss of salts by diffusion through the body surface.
Though salt loss may occur in this way, it has been shown above that it cannot be
the major cause, since the rate of loss of salts, which would theoretically account for
the form of the weight curve, remains constant over a period during which the
difference between internal and external concentrations is diminishing (Fig. 4).
It would not in any case explain the fall in weight after the maximum. Bethe (1934)
has in fact shown that brackish water invertebrates are in general less permeable to
salts than marine forms. In that case salt loss cannot be suggested as the cause of
the lowering of the weight curve when the curve of typical marine forms such as
Nereis cultrifera rises higher and more nearly approaches a perfect osmotic curve
(Beadle, 1931).
When five animals were weighed together, weight curves of regular form were
obtained (Figs. 2 and 3), but it was found that there was considerable individual
variation in weight at any given moment (Table IV). In spite of this, the body
fluid concentrations determined on single specimens followed a fairly regular curve.
Since it cannot be suggested that the internal concentration is independent of the
amount of water taken up, the most reasonable conclusion is that the animal behaves
as a normal osmotic system as regards uptake of water and change of internal concentration, but that the volume of the body fluids is continually being reduced by
removal through the nephridia. The individual variation in weight might then be
due to variation in the amount of swelling which has to occur before the muscles
will react to cause increased expulsion of fluid.
Cyanide and calcium deficiency cause both an increase in weight and an ultimate
osmotic equilibrium between internal and external media, both of which are reversible. It cannot be said whether or not these two effects are interdependent,
since it was not possible to measure more than once the weight and internal concentration in the same animal, and histological examination revealed no abnormality
other than a swollen body cavity. These experiments were therefore not as illuminating as in the case of Gunda ulvae (Beadle, 1934), but they furnished a convenient
method of demonstrating the re-establishment of hypertonic body fluids in an
isotonic medium.
It has been concluded from the results described in the last section that the
majority of the extra respiration in dilute sea water is not concerned with osmotic
work. Pieh (Schlieper, 1935), from his experiments on Carcinus moenas and Eriocheir
stnensis, has suggested that the rate of respiration is dependent upon the water
content of the tissues. It does not seem, however, that this could apply to Nereis
diversicolor, since there is no obvious relation between the rate of respiration and
the concentration of the body fluids. It has previously been shown (Beadle, 1931)
that the respiration and weight curves of the same animal in dilute sea water are of
Adaptation to Changes of Salinity in the Polychaetes
69
similar form and that both have a roughly simultaneous maximum and subsequent
fall. This suggests a relationship between the degree of swelling and the respiratory
rate. It is therefore possible that the increase in the latter is the result of work done
by the body wall muscles in resisting an increase of volume. If this were true,
animals which swell to an unusual extent might be expected to show little increase
in the rate of oxygen consumption (e.g. Table IV d). From former experiments it
was concluded (Beadle, 1931) that the effect of cyanide in causing an increase of
weight in dilute sea water was evidence that the extra oxygen consumption is
connected with osmotic regulation. In view of the lack of connexion between the
weight and internal concentration this conclusion can no longer be upheld. The
osmotic regulatory mechanism may entail oxygen consumption, since it is inhibited
by cyanide, but it is possible that the effect of cyanide upon the weight is a separate
phenomenon due to inhibition of the body wall muscles.
SUMMARY
1. Nereis diversicolor collected from the same locality at different times showed
smaller weight increases in dilute sea water (25 per cent) during the winter than
during the summer months.
2. In spite of great variations in the weight curve, the body fluid concentration
curve was very constant.
3. The maintenance of hypertonic body fluids and the regulation of body volume
are largely unconnected.
4. The lowering of the weight curve below that theoretically expected from
the concentration curve cannot be attributed to passive salt loss through the body
surface. It is suggested that this is due to the removal of fluid through the nephridia
under the hydrostatic pressure produced by the contraction of the body wall
muscles.
5. Animals previously subjected to dilute sea water, when placed in water
isotonic with the body fluids, will increase the concentration of the latter. This
result is more marked when the internal hydrostatic pressure is high.
6. The results suggest that the osmotic regulatory mechanism involves the
removal by the nephridia of fluid hypotonic to the body fluids. But no direct
evidence for this is available.
7. Calcium deficiency and cyanide in dilute sea water cause an increase of weight
and ultimately inhibit the maintenance of hypertonic body fluids. Both these effects
are reversible.
8. The mechanism by which body fluids are maintained hypertonic to the
external medium is not sufficiently developed to be of survival value in the locality
in which the animals were found.
9. The control of body volume is probably of greater importance.
10. The majority of the extra oxygen consumption in dilute sea water is not
the result of osmotic work. It is suggested that it may be due to work done by the
body wall muscles in resisting swelling.
70
L. C.
BEADLE
The cost of the apparatus used in these experiments was born by a grant from
the Beaverbrook Fund of the University of Durham College of Medicine.
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