OBSERVATIONS ON THE FATTY CONSTITUENTS
OF MARINE PLANKTON
I. BIOLOGY OF THE PLANKTON
BY E. R. GUNTHER.
("Discovery" Investigations, c/o British Museum (Natural History).)
(Received 1st November, 1933.)
(With Three Text-figures.)
I. INTRODUCTION.
BRANDT (1898) carried out chemical analyses of plankton collected with a quantitative plankton net in Kiel Bay with a view to ascertaining the possible yield of
plant and animal products from a unit volume of sea water. His collections were
small and investigations of the oils were not attempted. In a series of subsequent
papers he, Apstein, Hensen and others have examined the conditions of organic
production in the sea, and Johnstonc (1908), quoting estimates by Biebahn and
Rodewald, has compared the productivity of the sea with agricultural standards.
Since the classioal work of the Kiel planktologists, the chemistry of marine
plankton has received comparatively little attention until recently when, as a result
of advances in the knowledge of accessory food factors, biochemists looked to
plankton for the origin of vitamin D which was then known to abound in cod-liver
oil. These researches were published in a series of historically interesting papers
by Hjort (1922), Jameson (1922), Zilva (1922) and others: but when in 1926 the
separate identity of vitamins A and D was accepted, this aspect of the subject was
in need of revision.
The search for vitamin D in dried diatoms obtained from cultures of Nitzschia
grown in the laboratory (Leigh-Clare, 1927) proved unsuccessful; but since the
inception of the present work1, Ahmad (1930) has proved that the oil extracted
from similar cultures of Nitzschia is a good source of vitamin A. In no case prior
to 1928 are we aware of vitamins A and D having been found in native plankton,
and, since it is the ultimate source of food of most marine animals, plankton may
also be of importance in a study both of the part played by vitamins in nutrition
and of the influence of plankton oils on the fats present in fish, seals, whales and
other animals of economic importance.
Little is understood of the r&le of vitamin D in the promotion of calcium and
phosphate metabolism in fish; we do not know whether the cartilaginous Elasmobranchii have the same nutritional requirements as Teleostei, or whether the delicate
1
Drummond (1930).
174
E. R. GUNTHER
and sometimes fibrous nature of the bones in deep-sea fish is attributable to life in
the greater depths of the ocean where they and their food are screened from solar
irradiation. Leaving for future investigation the physiological significance of vitamins
in these organisms, there remains the problem of origin of the factors themselves:
whether they are absorbed with the food or whether synthesised within the animal
by irradiation or other means. Bills (1927) has claimed that there is no increase in
vitamin activity in the oils extracted from fish that had been subjected to ultraviolet light: and at the same time he notes no diminution of vitamin activity after
the fish had been kept in the dark for three months. From this he argues against
irradiation. It was therefore deemed advisable, as a first step, to examine the food
eaten by the larvae of most fish and by the adult of so many. A study of littoral
plankton from British seas was begun in 1928.
With the progress of the work it became evident that the numerical method of
representing plankton analyses that has been used by the majority of planktologists,
gives an incomplete account of the relative importance of the different species.
In dealing with the chemical composition of a mixed assembly of organisms or
with its value as a source of food, or with its economic aspects, the relative weight
of the various species may be of more significance than their relative strength in
numbers, although weight alone, unqualified by chemical analyses, might give
equally misleading results if applied indiscriminately to jelly-fish and other organisms
of exceptional composition. A gravimetric method is obviously applicable only to
given organisms of approximately similar constitution, as the more prominent
species in the present collections are believed to be. The specific gravities of floating
marine organisms are sufficiently similar to allow of good comparisons of their mass
by a volumetric method. Tables have been compiled to show the relation between
the length and volume of certain selected genera. The application of these data to
the ordinary numerical analyses of the plankton, converts them from figures representing numbers per c.c. of settled plankton intofiguresrepresenting volumes per c.c.
settled plankton. Preliminary results obtained by these methods are advanced in the
following sections.
II. COLLECTION AND PRESERVATION.
The phytoplankton and zooplankton were collected respectively in May and
July, 1928, from the environs of the Isle of Man. It is impossible to separate on
a large scale animal plankton from phytoplankton, but fortunately the maximum
abundance of the main mass of diatoms occurs in May while the maximum abundance of animal life occurs in July, and although the diatom efflorescence in May
is never without a quota of animal larvae nor the animals in July without a little
phytoplankton, the catches of May and July are sufficiently characteristic to present
a green and a red colour in striking contrast.
The Isle of Man, bathed in oceanic water free of land detritus, is moderately
well suited as a base for the plankton collector, while the valuable work of the
Port Erin Investigations enables prediction with a fair degree of certainty of
Observations on the Fatty Constituents of Marine Plankton
175
the actual week when the phyto- and zooplankton will reach their maximum
abundance.
The phytoplankton collected during four days (May 22nd-25th) was taken in
two silk tow-nets of mesh 70 and 150 per linear inch, towed from a 12 ft. fishing
boat rigged with mainsail, jib and centreboard. The nets were specially designed
to take bulky catches. The net with mesh 70 proved to be the more efficient, since
the larger mesh allowed better filtration and caught twice the bulk of vegetable
matter. Collection was restricted to the plankton at surface depths during daylight
hours (11.00-20.30). The weather was usually calm under varying conditions of
light from bright sunlight to overcast. Variable winds never exceeded the force of
a light breeze.
Preservation of the catch was beset by the problem of concentrating settled
diatoms to a reasonably small volume, and the following methods employed to
remove surplus water took three or four hours after each day's fishing. The settled
plankton was first squeezed lightly through silk of the same mesh as the net and
its volume reduced to a third. The volume of the strained plankton was again
reduced to a third by filtering on a Buchner funnel. It was found necessary to
interpose two layers of silk netting between the filter paper and the funnel, and
even then filtration was slow and filter papers had constantly to be changed. Two
methods of preservation were employed: to some samples was added an equal
volume of absolute alcohol and to others enough sodium chloride to ensure a
saturated solution, e.g. 400 grams NaCl per litre of concentrated diatoms.
Zooplankton was taken in July (i6th-2Oth). Two of the larger 70-mesh nets
were towed sometimes by a 26 ft. motor boat, the Redwing, fitted with mainsail
and jib, and sometimes by a 12 ft. rowing boat equipped with a lugsail and centreboard. Wind mostly from the west varied in strength and sometimes made the
surface lively. The weather was mostly sunny except on two days when the breeze
was at its strongest. As with the phytoplankton, the catches were preserved both
with salt and with alcohol. Both the pink appearance of the plankton and the microscopical analysis showed that diatoms were much in the minority and the fresh
material was easily strained off from surplus water over silk.
III. BIOLOGICAL ANALYSIS.
It is well known (Johnstone, 1924) that even in a very small sea area such as that
in Port Erin Bay there may be differences in the nature and abundance of the
plankton at places only a few yards distant from each other or at the same place
after only a few minutes' interval of time. Catches on a large scale tend to have
a variable composition, and unless they are very thoroughly mixed before sampling
it is possible that the samples subjected to biological examination may not be truly
representative of the masses of plankton used in oil extraction. Difficulties of another
kind enter into numerical estimations; plankton catches are frequently measured
volumetrically in terms of settled organisms, and as Russell (1927) points out,
the settled volume occupied by certain species may differ largely from that of
others, this has been overcome as far as possible by treating separately the plankton
176
E. R. GUNTHER
taken on different days. Errors of a statistical nature are inevitable in an estimate
dependent upon random sampling and upon the raising of small counts; we reduced
to some extent such errors in estimations of the larger, more important, or rarer
species by counting larger fractions; and in the identification of closely allied species
we have frequently used a generic or other categorical heading. Stress is laid on the
fact that neither the collections of phytoplankton nor of the zooplankton can claim
to be representative of the plankton community as it occurs naturally in the sea,
but they represent those species most easily selected by our nets. It is obvious
that the majority of the nannoplankton will at first pass through the meshes, and
that as these are clogged during the course of towing, the filtration coefficient and
so the catching power of the nets will vary. These considerations affecting much
less the treatment of plankton collected for quantitative work (the nets being fished
for relatively short periods), may account for the divergence of these lists from
the average taken over a number of years and published by Johnstone.
PHYTOPLANKTON.
Collections were made during four days from May 22nd to 25th, and five
samples each of about 100 c.c. settled diatoms were reserved for biological examination. Two sub-samples of each, (a) and (b), were examined. The first of about 5 c.c.
was examined in its entirety for its animal content and after dilution, in a fraction
of 1/20 for the larger and rarer species of diatom such as Coscinodiscus and
Biddulphia (see Table I, estimations (b)). The smaller and more abundant species
were also noted but were better estimated from the second sub-sample of about 1 c.c.
(Table I, estimations (a)). The latter was made up to a 1 per cent, dilution from
which fractions were extracted by the Hensen method. The fraction was then placed
on a glass slide ruled in squares and the organisms counted under a microscope.
The results of these analyses, listed in Table I, show that the five samples are
materially similar, consisting principally of the species Chaetoceros densus, Chaetoceros
debile and Lauderia borealis, with lesser numbers of Chaetoceros decipiens and
Chaetoceros compressus.
The catches of phytoplankton included a quota of animal life which consisted
of larval forms and small Copepoda. Their numbers were few, and although their
presence cannot be ignored they occurred with the diatoms, approximately in the
ratio of 1:100,000,000. In general appearance the plankton samples were like thick
pea soup in which, here and there, the animal organisms could be detected as
minute white dots. The comparative bulk of plant and animal matter has not
been estimated.
ZOOPLANKTON.
Numerical estimation, methods.
Collections were made during five days from July 16th to 20th, and eight
samples of about 100 c.c. of settled plankton were reserved for biological examination. Each sample was examined in the following way: The sample was diluted
with water, well mixed and shaken up, and enough poured into two 5 c.c. measuring
Observations on the Fatty Constituents of Marine Plankton
177
cylinders to give, in each, a sub-sample of about 5 c.c. settled plankton. Each
sub-sample, (a) and (b), was examined in the usual way by the Hensen method
of counting under a microscope the organisms contained in 1/30 of 5 c.c. Calanus
finmarchicus, which proved itself to be the largest copepod present1, and the most
important constituent of the plankton, was analysed in greater detail. Copepodid
stages IV to adult were all picked out of one of the 5 c.c. samples of each pair and
stages I—III were picked out of 1/20 of the residue. Analysis of the younger stages
of Calanusfinmarchicushas importance from two points of view. First, to determine
the percentage of Calanus that is effective towards oil yield, second, because the
ratio of the different stages in a sample may be of help in distinguishing between
one type of plankton and another. Data from the less common zooplankton constituents were obtained by examining a large sample of 100 c.c. from one of the
catches taken in the middle of the period.
Numerical estimation, results.
The results of these analyses are given as numbers of organisms per 1 c.c. of
settled plankton in Table II and as percentages of the total numbers present in
each sample in Table III. The analyses of each pair of sub-samples (a) and (b)
show a close measure of agreement and therefore each analysis may be taken as
an indication of the general characteristics of the sample. The eight samples (ref.
Nos. 40-47) are seen to fall more or less naturally into four groups indicated
under groups I, II, III and IV in Table III. The three samples (40-42) constituting group I, collected on July 16th and 17th, are very similar but differ
from the four samples of groups II and III (43-46), collected in the intermediate
period July 18th and 19th, which also show certain features in common. The
plankton of the last day (sample 47, July 20th) in group IV more nearly resembles
group I, but there are differences; for example, in the percentage of oithonids and
in the ratio of Calanusfinmarchicusstage V to adult. The features of significance
in the grouping of these samples have been emphasised by heavy type, and on this
basis the results of biological analysis have been similarly grouped (pp. 187-190).
In Table III the oil yield from each group has also been incorporated together with
detailed analyses of the stages of Calanusfinmarchicus.The correlation of oil yield
with abundance of adult and sub-adult stages of Calanusfinmarchicusis the striking
feature of the table and will be alluded to presently.
Other interesting correlations between groups I and IV on the one hand and
II and III on the other are the association of naupliar larvae with adult Calanus
and also the respective ratios of 40:60 and 60:40 of Calanusfinmarchicus,stages V
to adult. This and other evidence points to the conclusion that we are dealing with
two independent plankton communities, and the probability seems to be that they
were from different water masses. It is convenient to note here that the samples of
group I included much mucilage which may be correlated with the presence of the
diatom Rhizosolenia in rather larger quantities than in the other samples. This
seems to afford an explanation of the larger volume of settled plankton for the
1
With the exception of Anomalocera pattersoni.
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E. R. GUNTHER
i8z
samples of this group, and in consequence of the numerical poverty in the total
number of organisms per unit volume. The mucilage may have entangled the
Ceratium and Radiolaria and prevented their passage through the net, which
would account for the high percentage in group I. Groups I and IV, however, have
a further similarity in the higher percentage of Acartia and of Calanus finmarchicus,
stages III and IV. The material collected on July 19th, amounting to 3300 c.c, was
included in group III for oil extraction, but analysis of the plankton suggests that
it is intermediate in composition between groups I and IV on the one hand and
groups II and III on the other.
The analyses in Tables II and III are essentially little more than a generalised
statement of the abundance of the more easily recognised species, and those such
as Acartia may even be underestimated, some having been included under'' Copepoda
alia."
Table IV. Analysis of plankton organisms in sample No. 43, 18. vii. 28.
The genera and specie* are arranged in natural groups listed in order of abundance.
Nos. per
Noe. per
Genera and species
Copepoda
Calanus finmarchicus
Paracalanus parvui
Oithonids
Acartia clauxi
Temora longtcornis
Puudocalamit elongatus
Isi as clavipex
Centropages hamatus
Calanus sp. juv.
Centropages typxeus
Paraponttlla brevicorms
Harp act icoidea
Arwmalocera pattrrsont
Naupln
Tunicata: Oikopleura dioica
Peridinialefl: Ceratium tripos
Chaetognatha
Cladocera
Evadne nordtnanni
Potion xnUrmtdiut
Decapoda
Cancer pagurut zoeae
Ebalia sp.
Upogebia sp.
Urachyuran larva
CnHumassa sp.
IOO
C.C.
Genera and specie*
settled
plankton
37.000
32,000
30,000
29,000
15,000
6,500
2,600
2,600
2,400
320
280
80
30
24,000
8,500
3.900
2,200
1,200
20
270
40
30
20
20
Decapoda (cont.)
Hippoiyte varians
Pandalus sp.
Portunus megalopa
Gaiathea ip.
Eupagurus tp.
Pinnotheres sp.
Porcellana sp.
Euphausiacea
Nyctiphanes calyptopts
N. cyrtopia
IOO
C.C.
settled
plankton
20
20
8
2
1
i
r
IOO
N. furaha
Moll us ca
Gastropoda post-larva
I^amell 1 branch post-larva
Echinodermata: Ophiuroid post-larva
Polvchaeta
Tomopteris sp.
Folycnaeta post-larvae
Spionid
n
11
J
80
21
So
20
2O
5
5
r>yllia
Amphipoda: micromscus
Pisces: larvae
Ova
Medusae: Aequorea albida
Ctenophora
Total organisms
20
20
2O
3
1
108,461
NOTE. The list is compiled from an examination of four different sized fractions. The largest and rareat animals were
noted from the whole loo c.c, the others respecti\ely from 20. 5 and } c.c. The large number of about 1085 organisms
per c.c. Bhows that this sub-sample was more closely packed than other sub-samples of the same plankton analysed in
Table II.
The fauna has been treated in greater detail in Table IV, which gives a list of
the organisms noted in ioo c.c. of a sample (ref. No. 43) taken in the middle of the
period.
Observations on the Fatty Constituents of Marine Plankton
183
Volumetric estimation: methods.
The sorting out from every sample of enough animals for experimental determination of their volumes by direct displacement, would have involved undue
labour, and we have resorted to an indirect method. The method requires a knowledge of the numerical estimation of the different species in each sample, a knowledge
of their length, and a graph to show the relation between the length and volume
of the various genera. The lengths of the respective species are easily measured
by means of a micrometer eyepiece; and hence by means of the appropriate curve
it is possible by interpolation to read off their volume and so to convert the numerical estimate into a volumetric estimate.
Table V. Displacement measurements of Chaetognatha of different lengths.
Nos.
examined
Mean length
mm.
Displacement
Nos. per c.c.
40
4
6
57
557
2
55'5
29
2-2
5
4
54-2
53
3-6
3'3
9
48-88
3-2
46
69
6
4
45-2
3 -o
425
33
9
42-3
4-0
5
6
38-8
38-66
18
14
21
2
4
18
37
11
33
88
103
27
79
5-9
37
50
117
35-3
33'5
28-5
8-9
6-7
30-8
27-1
24-4
22-72
18-8
17-5
17-1
143
8-8
257
49-4
78-0
94'3
1955
172-0
270-0
1580-0
To construct these curves, it was necessary in the first place to measure the
volumes of animals of widely different length which had been sorted out into length
groups. For the larger organisms like Chaetognatha and the larger calanoids a displacement method was used: and Table V shows the relation between the mean
length of each group and the numbers of Chaetognatha per c.c. For the smaller
organisms such as nauplii, oithonids and young calanoids, a packing method was
adopted. The organisms of a like length to be measured, are laid out on a glass
slide ruled in millimetre squares, and the individual organisms are so packed together
that, one animal deep, they cover as near as possible a square measuring along
one side 1, 2, 3 or 4 mm., etc., according to convenience and the length of the
animal. Assuming that the shape of the animal is fusiform, the number of organisms
JEB-Xlii
13
E. R. GUNTHER
184
that would occupy a cube built up on a square is calculated by multiplying the
number of packed animals by the number lying side by side along one side of
the square.
From theoretical considerations a graph of the reciprocal of the length plotted
against the cube root of the numbers of organisms per c.c, should give a straight
line provided that there is no change in the shape of the animal with growth nor
with increasing experimental error (Fisher, 1933). Fig. 1 shows such a curve from
the figures obtained from estimates of Chaetognatha. To a lesser degree the same
relation is illustrated by displacement measurements of the larger calanoids
(Table VI and Fig. 2, curve (ii)). The packing method gives rather greater values
o-i 11—
0-10
0O9
0-08
0-07
C
o
I O05
a
'§ (MM
5
0-03
0-02
0-01
1
1
i
1
s
(>
7
a
9
10
11
12
Cube root of numbers of Chaetognatha per c.c.
Fig. 1. (See Table V.)
for the volume of each animal because of the spaces which are included between
the animals, and this divergence from the true volume probably increases as the
size of the animal diminishes. The volumes of calanoids, oithonids and nauplii
obtained by this method are listed in Table VI, and the calanoids when plotted
(Fig. 2, curve (i)) lie on a curved path. The nature of this curve is due partly to
the packing method of measurement, partly to the different shapes of the younger
individuals and especially of the nauplii which are on the whole more globular in
shape than adult Copepoda. The more attenuated slender shaped oithonids lie
outside the curve altogether. The volume of the large diatom Coscinodiscus has
been obtained by calculation on the assumption that it occupies a cubic volume of
0-0068 c.mm. which is probably a very liberal estimate. In view of the preliminary
Observations on the Fatty Constituents of Marine Plankton
185
Table VI. Volumetric measurements of Copepoda, NauplU and Coscinodiscus.
Mean
length
mm.
Genus
examined
0-19
0-42
0-756
0-87
Coscinodiscus
Nauplii
Oithona
Calanus juv.
Estimated numbers per c.c.
By
calculation
278,000
—
—
—
—
—
—
—
—
—
—
—
—
—
071
0-67
1 24
165
2-O
2-19
2-645
2-085
3-525
4'4
C. acutus
Packing
method
Weight gm.
control
Nos.
Displace- measure- examined
ment
ment
2,34O
1,260
—
—
—
—
—
—
—
—
5000
1923
994
1667
"5,5OO
157,500
72,000
66,000
50,000
9,37°
6,400
3,5°°
400
270
667
444
_
—
—
—
—
—
—
0-033
0-0535
0-296
0-1385
O-75
—
—
—
—
—
—
175
125
500
100
333
5-Or
•
•
-Curve (ii)
- Oithonid
= Coicinodiscus
12
16 20 24 28 32 36 40 44 48
Cube root of numbers of organisms per c.c.
Fig. 2. (See Table VI.)
nature of this inquiry it was decided that the amount by which the packing method
overstates the volume might not be enough to matter and the curve (i) in Fig. 2
was adopted for interpolation not only of Calanus but for other genera of Copepoda
also. The results obtained by this curve will therefore overstate the volume of the
smaller and slenderer Copepoda.
13-2
186
E. R. GUNTHER
Volumetric estimation: results.
Volumes of each of the species listed in Table II have been deduced by one
or other of the methods just described and are tabulated as numbers per c.c. in
Table VII. The mean length of each species is calculated from the frequencies
given as an appendix in Table XIII. One mean only is calculated for a species
whose length frequencies lie within a small range, i.e. not exceeding 0-6 mm. But
for those species whose length frequencies extend over a wider range than o-6 mm.,
and especially if they lie on a multimodal curve, two or more means are calculated.
The volumes found for each of these means are then adjusted according to the
proportion of organisms furnishing the lesser or the greater mean, and a compound
figure for the volume of the species results.
Table VII. Specific volumes of the organisms listed in Table II.
Mean length
mm.
Nos.
examined
Species
64
94
23 132 % 1
49 | 6 8 % \
37
Calanui finmarchicus •s
?
V
V
IV
III
II
I
26
24
27
21
19
58
20
78
78
125% \
(75% 1
I 20 "1
(8o%
i
1
)
196 I 7 4 " o
69 \ 26 " 0
1 10-4 ", D
25
37
5
1
1
1
)
Nos. per c.c.
830
3
3-i
658
2-83
1,125
248
1,816
2-1
3.243
7,078
I-6 5
1-25
094
Oithunn
Copepoda alia
1-6
09
14,880
24,900
70,960
33,080
15,260
30,380
7,763
30,950*
o-88
32,768
,,
i-39
22
11,240
2,863
166,375
2,197
3<4°°
Nauplii
Acartia clausi
O-585
0-87
Temora longicornit
0905
123
M
Appendicularia
Chaetognatha
Evadne nordmatmi
Podon intermcdius
Radiolaria
Ceratium
2
8-5
o-88
—
O'l
o-i
3.4OO
1,000,000
1,000,000
• Estimated on the basis of the calanoid curve, Fig. 2.
For the conversion of numbers to volume the numbers of organisms per c.c.
may be regarded as a factor to be applied to the numerical estimations that have
already been made. These numerical estimations (Tables II and III) showed that
the plankton samples fall naturally into four groups and it will be preferable to
treat them as groups instead of separate estimations. A mean estimation representing the numbers of each of the species present in i c.c. of settled plankton is
computed for each group and is given in Table VIII A. Dividing by the appropriate volume factors in Table VII, the numbers in Table VIII A are converted to
the volumes of each species. These are expressed in c.mm. in Table VIII B. In
Tables IX A and B the same data are reduced to percentages of the total present
Observations on the Fatty Constituents of Marine Plankton
187
Table VIII.
A. Mean number of animals per c c . settled plankton in groups I-IV.
Group No.
Reference No. of plankton
included in each group
Calanus firtmarchicut o"
V
Copepodid
V
IV
III
II
I
Nauplii
I
II
III
IV
4 0 , 4 1 . 42
43
44. 45. 46
47
0-266
2-533
12-4
77
56-8
4-0
2-533
4833
4-733
14-666
11-4
126
34-2
57-6
102
Acartia clausi
Temora longicormt
Oithona
Copepoda alia
Appendicularia
Chaetognatha
Evadne nordmarmi
Podon intermedius
Radiolaria
Ceratium
166
81
148
234
129
132
186
319
333
i-o
no
96
33-o8
10-71
682
7-2
38-25
8
8
4*4
28-20
207-2
18
i77
228
174
186
234
528
795
45
3
15
43i
49
24
9
6
7
0
4
0
2
3
17
2
H5
9
24
IIOI
1387
68
8
Total
410
4634
57
6
11
1279
33
2101
NOTE. T h e means in groups I, II and IV are computed from the estimations given in Tables
II and I I I . T h e means given under group III are compound figures and are constituted as follows:
57'7 % represents the meaned estimation of samples 44 and 45 (the volume of settled plankton of
these samples is 4500 c c . ) ; 4 2 3 % represents the meaned estimations of sample 46 (the volume
of settled plankton of this sample is 3300 c c ) .
B. T h e mean volume expressed as c.mm. per c c . settled plankton in each group I - I V .
Group No. ...
Calanus finmarcfricus cJ
Copepodid
II
III
IV
I4-45
4 - 94
7O-5
21-84
16-7
6-43
I
V
IV
III
II
I
Nauplii
Acartia clausi
Temora longicornis
Oithona
Copepoda alia
Appendicularia
Chaetognatha
Evadne nordmanm
Podon intermedius
Radiolaria
Ceratium
032
3-85
2648
078
0-683
0-318
0-59
0-144
944
8-908
4785
14-84
0-408
3-54
265
Total
117
37-4
3-52
1-78
2-3
2318
3'3
7-3I7
11-456
6
I5-52
O-343
6-84
1-76
3'3
0-965
2-57
1-13
2-92
1-041
19-17
6
20-23
0-295
10-9
2-06
1-205
2-218
0-621
0-588
O-32I
2-5
12-92
25-65
17-1
36-89
0-27
i-37
0
1-176
0
o-59
0-884
0-017
O-OO2
O-I45
0-009
0-024
0-006
0-033
55242
231-337
168-464
o-on
125-706
J
E. R. GUNTHER
i88
Table IX.
A. Mean number of animals per c.c. expressed as a percentage of total number
of organisms present in each group.
I
Group No.
Calanus finmarclacus $
'+
Copepodid
Nauplii
Acartia clausi
Temora longicornis
Oithona
Copepoda alia
Appendicularia
Chaetognatha
Evadne nordmanni
Podon intermedius
Radiolaria
Ceratium
V
IV
III
II
I
0-02
0-23
0-36
0-23
0-44
0-43
1-32
9-28
15-2
7-36
I3-48
29
5-69
o-73
0-83
036
1'54
132
II
I-I2
5-55
41
O-82
0-91
2- 4 6
4/16
16-9
9'3
9-53
13-41
24
4-n
1-08
0-43
0
0-65
i-73
III
0-32
3-62
259
0-84
O'S3
3-00
2-2O
16-20
I-4I
I3'6o
i4'S7
33'7°
3-84
i-88
0'54
0-15
0-15
o-86
IV
005
0-52
0-46
0'34
O-2I
0-38
0-38
H2
10-85
11-14
25-01
37-8
2-14
0-14
0
0-14
0-29
1-57
B. Volumes are expressed as percentage of total volume in each group.
I
II
III
IV
0-58
7-0
293
42-0
12-97
1 96
096
133
Group No.
Calanus finmarchicus
Acartia clausi
Temora lotigiconris
Oithona
17-1
Copepoda alia
Appendicularia
Chaetognatha
269
074
6-25
506
16-17
1-52
O77
o-95
1-02
1 43
316
495
2-6
6-73
0-15
't
0-76
Nauplii
Copepodid1 V
IV
III
II
I
4-8
141
1-24
0-58
1-07
026
161
8-65
6
206
o-57
1-53
067
1-74
062
11-39
3-56
I2-OO
0-17
648
1-24
Evadne nordmanni
Podon intermedius
4-8
Radiolaria
Ceratium
205
0
0-03
0-25
0-004
0-35
o-ooi
OOI
0-006
5-n
176
049
0-47
0-25
199
103
2O-2
13-6
294
0-21
I 09
O
07
OOO5
O-O2
in each group. The data are illustrated by a series of histograms in Fig. 3, and it
is at once apparent that the volumetric method conveys a materially different idea
of the plankton than is to be gained from a review of numbers alone.
Calanusfinmarchicusis invariably represented as of greater importance volumetrically than numerically, while the small organisms such as nauplii and Ceratium
show the reverse. Again it is seen that Copepoda alia which are always more
numerous than Calanusfinmarchicus,are in only two groups (I and IV) of larger
volume. The total yield of oil from each group has also been added and shows a
E. R. GUNTHER
190
The total volume of Calanusfinmarchicusthat has been extracted is calculated
from the foregoing data by multiplying together the estimated volume of Calanus
in 1 c.c. settled plankton, and the numbers of c.c. settled plankton extracted. The
volume of Calanus extracted and the total oil yield from each group is shown in
Table X and shows fair correlation. No such correlation exists between any of the
other species, which have therefore been omitted from this table. Nor do their
several volumes considered collectively under the heading "Animalia alia" show
any particular correlation. On the other hand in groups I and IV, which have the
least percentage of Calanus, the oil yield is, in proportion, slightly higher; this
suggests that some of the oil has been contributed by other species. Such contribution is probably small in consideration of the absence of any correlation between
the oil yield and either the total volume of animals extracted, or of the animals,
less Calanus., extracted. Owing to the very rough manner in which the volume of
settled plankton was measured and for reasons outlined at the beginning of this
section, the data here summarised do not allow of too precise interpretation, and
the relations between the oil yield and the various organisms in the plankton should
be regarded as an approximation.
Table X. Synopsis of data on oil yield and organisms extracted,
Group No
Oil yield, c.c.
Calanus finmarchicw. Volume extracted, c.c.
Animalia alia. Volume extracted, c.c.
I
II
16-553
"3
567
26-849
3°4
89
III
51715
822
494
IV
933
56
195
It has been shown that mixed with the phytoplankton is a quota of animal
organisms, and the question arises whether the yield of oil from those samples has
not been given by the animals and rendered green by chlorophyll dissolved from
the plant cells by the light petroleum used in the process of extraction. The data
collected above is insufficient to form a conclusion.
IV. DISCUSSION.
An attempt is being made in the present paper to translate by means of suitable
measurements, the figures representing the numbers of a species present in a given
quantity of plankton into figures representing the volume occupied by that species,
believing that it may be possible, by this method, to convey a more precise idea
of the relative importance of each species.
It is shown that in plankton giving a high oil yield, the copepod Calanus
finmarchicus takes a more prominent position than any other species; and that in
plankton poor in oil, this organism is present in very small quantity: other
Copepoda, though as numerous, are of smaller volume and consequently contribute
less to the oil yield. Fig. 3 shows that some organisms, like Ceratium and naupliar
larvae numerically abundant, occupy negligible volume as compared to less
numerous though larger organisms. The volume measurements are only tentative,
Observations on the Fatty Constituents of Marine Plankton
191
but it would seem that the fatty constituents from zooplankton that are noted in
Parts II and III, are derived mainly from Calanus finmarchicus.
The volumetric method, if it had been used, might have shown more clearly the
relation between species in the plankton and the results of its chemical analyses
obtained by previous investigators. Thus a plankton sample classed by Brandt (1898)
as Peridinian contained 4,000,000 diatoms, 50,000,000 Peridinians and 89,000 Copepoda. If, as might reasonably be expected, each of these Copepoda were a thousand
times the bulk of one Peridinian, the bulk of the Copepoda would exceed that
of the Peridinians and the origin of the oil from this sample would be in doubt.
Again, the important results obtained by Wimpenny (1929) promise to indicate
a new and significant orientation of planktology. The fat content of plankton is
compared with the distribution of herring catches off the east coast and shows
certain correlations. But we should first demand a correlation between the distribution of herring and of such plankton organisms as constitute its food. We are told
that the herring, before they spawn, feed upon the copepods of the plankton. We
have not been able to find a correlation between the fat yield of the sample and the
percentage of Copepoda in it from the numerical estimations given. It is of course
possible that the fat yield which is expressed as the weight of ether-soluble matter
per 1000 organisms, fails to give a true indication of the condition of the plankton,
since the size of the organisms is not taken into account. The part played by
Calanusfinmarchicus,for example, would be clearer if it were stated whether the
numbers in the tables represent young, adult, or sub-adult stages.
Schuette (1918) and Belloc (1930), in their investigations of the chemical composition of plankton, did not include numerical estimations of the various species.
This difficulty does not, of course, arise in work with monotypic plankton (a) by
Klem (1932) who collected Meganyctiphanes norvegica from the stomach of a Sei
whale, (b) by Ahmad (1930) and others who have grown cultures of Nitzschia in the
laboratory, and (c) by Becking (1927) who, with collaborators, obtained collections
of the diatom Aulacodiscus kittoni, in almost pure culture during an efflorescence
off the Californian coast.
If the application of a volumetric method were extended, more accurate factors
indicating the numbers of an organism per c.c. could probably be obtained. No
attempt has been made in the present paper to measure the volumes of the separate
phytoplankton species; the total oil yield amounting to 12 c.c. was obtained mainly
from Lauderia borealis (13-36 per cent.) and from several species of Chaetoceros
(59-85 per cent.). Included among the phytoplankton, however, was a distinct
quota of animal organisms, and although they were outnumbered by plant cells
to the extent of 1125,000,000-622,000,000, yet the possibility of their contributing
to the oil yield should not be overlooked. These oil samples were charged with
vitamin A (or its precursor) and other chlorophyll pigments in a quantity which
cannot have been supplied by the animal organisms, but the origin of the fats is
less easily decided; it appears from microscopic examination that, as in immature
fish (Bruce, 1924), the younger stages of plankton organisms contain comparatively
little oil in contrast to the adult stages, and it seems likely that the oil from the
E. R. GUNTHER
192
May plankton samples represents diatom oil. This view is strengthened by the
chemical constants which distinguish this oil from those of purely animal origin.
The oil content of the phytoplankton (approx. 6-89 per cent, dry weight) is
intermediate between the values obtained by Brandt, Becking and Schuette, who
report respectively that they found a percentage of 2<2i-4-24, 9-7, and g-SS-io^oS
of the dry weight. Mann (1916) states that oil in diatoms rarely falls below 5 per
cent, and that he has "samples of diatom material in which a careful measurement
of the contained oil shows a proportion of 50 per cent." The measurements or
the precautions taken are not given. We suggest that the oil content may vary with
the species and fluctuate during the life history.
The oil content of the zooplankton collected from the Isle of Man varies between 15-05 per cent, and 19-3 per cent., a higher figure than the majority of
determinations summarised below in tabular form1.
Table XI.
Investigator
Brandt* (1897)
Nature of plankton
Mainly Ceratium and other Peridiniae
Schuette (1918)
Copepoda, fresh water
Copepoda, mixed, marine
Mixed, fresh water (Daphma, Diaptomus, Cyclops)
Wimpennyf (1929)
Mainly Daphnia pulex
Marine animal plankton (species not given)
Klem (1932)
Copepoda, Calanoidea
Meganyctiphanes norvegica
Oil yield
% of dry
weight
1-77
2'20
3-21
4-71
6-01
7-4°
8-oi
I3-47
21-25
3-571
9268
T*l
rot
8-7St
2-5-4-iJ
• Analyses of other samples of mixed plankton have been omitted from the table.
f These results were obtained by extracting the plankton in a Soxhlet apparatus; other results
obtained by shaking the wet macerated plankton with ether cannot be considered comparable and
have been omitted.
X These percentages represent wet weight.
The wet weight percentage of oils from Copepoda and Meganyctiphanes, if
calculated properly from the data given by Klem (1932, p. 7), show an enormous
discrepancy which is hard to reconcile with the other findings.
1
I am indebted to Mr M. H. Hey for the following chemical analyses of settled plankton. 5 c.c.
each of samples Nos.m 36 (phytoplankton). 42 and 45 (zooplankton) gave dry weights of 27-3 mg.,
391 mg., and 154-4 g- The phytoplankton contained 33 per cent, silicate, whereas the two zooplankton samples contained -7 per cent, and -45 per cent., figures that are in agreement with the biological analyses in Tables I and II. The dry weight figures suggest No. 45 has four times the animal
matter as compared to No. 42, and this also accords with biological observation. Oil content when
based on these figures works out at 689 per cent, dry weight of phytoplankton and 1505 per cent,
and 19-3 per cent, of zooplankton. Thus the oil yields of these two zooplankton samples are in the
ratio 1 : 1 -28, whereas the oil yields per volume of total animal extracted as given in Table X, of
groups I and III, are in the ratio 1 : 1-59. The agreement is close. Determinations of calcium in
these three samples gave 4-6 per cent., 13 per cent, and -12 per cent. CaO of the dry weight.
Observations on the Fatty Constituents of Marine Plankton
193
As regards the chemical nature of both phyto- and zooplankton the quantity
of oil extracted was unfortunately too small for an analysis of the fatty acids by
the method of fractionating the methyl esters, and we have no data to compare
with the very excellent results presented by Guha, Hilditch and Lovern (1930) on
the composition of mixed fatty acids present in the glycerides of fish liver and
other marine oils. Determination of the constants of plankton oils showed that
they were generally similar to fish oils of the type characteristic of Clupeoids and
Gadid liver oils. Of special interest in the plankton oils is the presence of a high proportion of polyethylenic acids of the Cg,, and Cj, series and the presence of a hydrocarbon, probably squalene. We may then look to plankton as a possible origin of the
fatty compounds in such widely different oils as those of fish like Centrophorus and
Scymnorhinus with a high percentage of squalene or Raia where the highly unsaturated
acids predominate. It would appear probable that these highly unsaturated acids
contained in the fat of marine birds are also traceable ultimately to plankton.
Methanolysis of 123 gm. of the oil of the planktonic organism Meganyctiphanes
by Klem (1932) was found to yield esters with constants suggestive of the presence
of myristic, palmitic, hexadecenic and oleic acids. Higher unsaturated acids were
also indicated by a yield of 10 per cent, of octobromide. These are among the
commonest of the fatty acids found among marine oils, and without further quantitative data we are unable to ascribe them to any one class of marine oils or to
determine the extent to which the oils of Meganyctiphanes and of calanoid Copepoda
resemble one another. The iodine values of Meganyctiphanes oil is 167-5; °f three
oil samples of calanoid Copepoda, 157*9, I 57'5 anc^ 158*1 (Klem, 1932); the
consistency of these determinations is remarkable in view of the fact that the
plankton samples from which the oils were extracted were collected on the dates
May 1929, June 1929 and June 1930. The iodine values of our own calanoid oils
were as low as 125-128. It might be appropriate here to mention that a qualitative
test to find squalene in the unsaponifiable fraction of the calanoid oil examined by
Klem, gave a negative result.
Table XII.
Type of oil
Iodine
value
Saponification No.
% nonsaponifiable
Iodine
value of
non-saponifiable
Sp. gr.
fraction
Aulacodiscus (Becking)
Phytoplankton (Collin)
Daphma (Schuette)
Zooplankton, fresh (Schuette)
86-7*
138-144
172-88
87-58
IO2-8
Calanus (Klem)
>»
Meganyctiphanes (Klem)f
Zooplankton (Collin)
157-9
157-5
158-1
1675
125-128
65-7*
107-1
179
—
—
—
—
—
70
—
—
—
—
—
132
25
95
—
184-5
208-56
2486
—
134
119-129
23'3-32'4
56-70
—
—
—
—
—
—
—
0-9108
—
—
• Semi-solid oils remaining after decolorigation of extract and separation of unknown sulphur
compound.
t 123 gm. of oil yielded 40 gm. of methyl ester.
194
E. R.
GUNTHER
The principal constants of plankton oils from these and other sources are set
forth in Table XII.
The ether extracts from fresh-water Crustacea, and especially from Daphnia,
have been described as having the odours of fish oil (Schuette, 1918). Determination of the usual constants showed a great range in the iodine values from 87-58
to 172-88. On exposure to air crystals of glycerides were deposited.
Very few data are available on the composition of phytoplankton oils.
The quantity and physical nature of the ether extracts of Fragillaria, Microcystis
and of the blue-green algae Aphanizomenon and Anabaena from Lake Mendota,
did not permit of the determination of the usual constants (Schuette, 1918).
1-82 gm. of the oil of Aulacodtscus kittoni was found to contain an unknown sulphur
compound whose properties are given and which represents 11-3 per cent, of the
chlorophyll-free extract. The remaining semi-solid oils had a saponification value
of 86-7 and non-saponifiable matter amounting to 65-7 per cent. The authors
conclude: "It is quite apparent that the acids of the diatom oil are a mixture
of the higher, wax-like fatty acids and lower unsaturated fatty acids" (Becking,
1927). Our own results (Collin, 1934) would therefore appear to stand alone.
As regards the non-saponifiable fraction of our zooplankton oil, besides a hydrocarbon suggestive of squalene we have found cholesterol, cetyl alcohol, eicosyl
alcohol and possibly batyl alcohol. A Qo alcohol such as eicosyl alcohol noted in
the head oils of the Sperm whale constitutes, as far as we are aware, the only
previous record of its occurrence in marine oils. That traces of ergosterol were
present in addition to cholesterol may be inferred from the slight antirachitic
activity shown by these oils. This accords with the results of Belloc, Fabre and
Simonnet, who extracted detectable quantities of ergosterol from Porcellana larvae,
calanoids and Cydippe. These authors also extracted, among other sterols, ergosterol from plankton consisting exclusively of Cydippe, Beroe and Acephaline
Scyphomedusae.
It is now well established that phytoplankton can be a source of the carotinoid
precursor of vitamin A (Ahmad, 1930) and that zooplankton may be a source of
vitamin D (Belloc, 1930); our own work confirms this but assigns a lower order of
vitamin activity to both phyto- and zooplankton. That variation in the amount of
vitamin D occurs in zooplankton has been shown by Belloc. The sterols extracted
from spring plankton were inactive, while those from summer plankton were
active; a fact correlated by Russell with the depth of the plankton and the consequent amount of illumination (? irradiation) at those seasons. The precautions we
took to prevent oxidation of vitamins through overheating or access to air while
extracting the specimens collected from the Isle of Man should have been sufficient to
give a reliable indication of vitamin content; on the other hand the plankton samples
were stored for several months before extraction and may have undergone some
deterioration. Our own results gave negative values for vitamin A in zooplankton but
the absence of vitamin D or its precursor in phytoplankton we do not regard as settled.
The low order of vitamins consistently found in plankton compared with the
rich sources of A in the liver oil of cod, halibut and whale has considerable interest.
Observations on the Fatty Constituents of Marine Plankton
195
MacPherson (1933) finds that the intensity of the blue colour developed by codliver oil with antimony trichloride increases steadily with the age of the fish and
bears no direct relation to its size. He notes too that the red and yellow colour
of the oil increases with age. The increasingly rich stores of vitamin A and of
pigments in the older codfish and the rich stores in the halibut and certain other
fish and in the rorqual point to a retentive capacity of the liver for these substances
which has not yet been accounted for. MacPherson states that the vitamin A
content of the cod's food is low, and we know that the quantity in zooplankton is
either low or zero. It is worth considering whether the vitamin A present in these
liver oils is not derived from phytoplanktonic organisms ingested with other
nutriment. Opportunities for ingesting diatoms would be smaller among predatory
forms and greater among phytoplankton feeders like the herring, the basking shark
and the whale.
SUMMARY.
1. Collections of phyto- and zooplankton made off the Isle of Man were
examined for vitamin content, and for chemical and biological characteristics.
2. The important species entering into the composition of the plankton are
noted, and a new method of estimating plankton catches is described. Figures
which represent the numbers of a species present in a given quantity of plankton
are translated into figures representing the volume occupied by that species. Comparisons are drawn between the numerical and volumetric estimations of the various
zooplankton species.
3. The phytoplankton catches taken in May showed a fairly uniform composition throughout the collecting period, species of the genera Chaetoceros and
Lauderia forming more than 90 per cent, of the material. The zooplankton taken
in July consisted mainly of Copepoda. The catches were of two types: those having
a higher percentage of Acartia, Calanus copepodid stages IV and III, and the
Peridinian Ceratium; and those containing less of these but rich in the adult and
other stages of Calanus finmarchicus.
4. By the use of curves correlating length and volume, measurements of important species are translated into factors representing numbers per c.c.; application of these factors converts the numerical estimation of a species into an
estimation in terms of volume. The numerical and volumetric estimations of various
species in four separate groups of plankton are compared and the yield of oil from
each group is also recorded. The volumetric method conveys a significantly different
picture of the plankton than is to be gained from the numerical method alone.
Calanus finmarchicus is shown to constitute a relatively larger part of the plankton
than other more numerous but smaller species and likewise its adult or sub-adult
stages than its earliest copepodid stages. The correspondence of a good oil yield
with those groups having a high Calanus content is held to be suggestive of a
possible correlation between the two.
5. Chemical and biochemical analyses of planktonic organisms by other investigators are reviewed and their results compared with those obtained during
196
E. R. GUNTHER
the present work. The role of plankton organisms in the nutrition of various other
organisms is discussed.
The thanks of the author are due to the many individuals who have helped in
this investigation. At Port Erin, Miss Catherine Herdman very kindly placed her
yacht Redtuing at his disposal for the purpose of towing plankton nets. To Sir
F. G. Hopkins and to the staff (especially to Dr and Mrs Needham) at the Sir
William Dunn Institute of Biochemistry, he is indebted for the facilities granted
for the work of extraction of the plankton; a monetary grant was made from the
Thruston Fund by Gonville and Caius College. To Mr Robin Hill, to Dr F. H.
Carr and to the author's wife, Dr Mavis Gunther, who have been in close touch
with all stages of the work, the writer desires to record his special gratitude.
APPENDIX.
Table X I I I . Copepoda length frequencies. The measurements are from
tip of rostrum to tip of caudal furea.
1
1^
.3
2.
§
a
S
fc
8
ft
1
is
•i
§3!
I:
I! i
<3
0
5
0-5
o-6
0-7
o-8
09
-o
•1
•2
•6
1
21
17
12
14
24
11
3
3
20
28
19
4
11
4
6
IS
19
6
16
24
17
10
7
4
1
10
1
o
o
o
3
10
I
4
3
11
4
17
20
2-9
30
33
3-4
35
7-6
14
2
4
2
64
54
39
2
1-9
2-0
2-1
2-2
23
2-4
2-5
2-6
3
14
30
IS
7
7
15
16
5
10
5
2
7
IS
I
6
6
2
11
10
2
Observations on the Fatty Constituents of Marine Plankton
197
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