Energetics of Idothea resecata
Introduction
Determination of an energy budget for any organism is
important in understanding its relationship to the environ-
ment. Most studies of this type have concentrated on ani-
mals of current nutritional importance to humans (Kleiber
1961). However, as research into food web relationships
deepens, more emphasis is being placed on the importance of
lower organisms in the system. Idothea resecata can be found
as a primary grazer on Macrocystis pyrifera, the common kelp
which is actively harvested by man for alginic acid. (Jones
1971). Thus it can be assumed that this isopod is able to
subsist chiefly on a diet of Macrocystis. For this reason,
I. resecata is an important animal on which to do an ener-
getics study.
In order to determine an energy budget for any animal,
several basic parameters must be characterized:
1. Ingestion
2. Excretion
3. Metabolic energy expenditure
4. Growth and reproduction
This study has attempted to find values for the first three
of these parameters for the isopod Idothea resecata.
Materials and Methods
Idothea resecata were collected from kelp beds to the
east of Hopkins Marine Station during the months of April
and May. Fresh algal blades were also collected at this
location. In order to avoid the errors involved in weighing
wet material and to eliminate variability between blades of
2.
Energetics of Idothea resecata
algae, strips of Macrocystis were cut in triplicate at the
same level from a single blade for each experimental container.
One strip was immediately labelled and dried in an oven at
80°C. The second strip was placed in a plastic container
with five male isopods of size range 2.5 to 3.0 centimeters
in length as measured from mid-eye to tip of telson. The
third strip was placed in a separate container to control
for weight changes in the algae independent of grazing.
Containers for the experiment consisted of 18x30 cm. plastic
boxes with plankton netting at eitherend to facilitate cir-
culation when suspended in an outdoor pond. At the end of
the specified time period, the animals were removed and
remeasured. Water remaining in the container was centri-
fuged for 20 minutes at 1500 rpm to collect fecal material.
A modified procedure was adopted beginning with test 47 in
which the water was partially drained through silk bolting
cloth. Another change in procedure at this time involved
resuspension of the fecal material in distilled water in order
to cut down on salt residue in drying.
Since lengths of animals were controlled during this
experiment, a length - dry weight standard curve was con-
structed (fig. 1). In addition, it later became necessary
to construct a blotted-wet weight - dry weight standard
curve in order to convert values obtained in respirometry
data (fig. 2). Regression lines were computed using a com-
puter program for regression (Sokal and Rohlf 1969).
Dried samples of algae, isopods, and fecal material
were ground to pass through a 60 mesh screen in a Wiley mill.
Energeties of Idothea resecata
The resulting powder was then pressed into pellets of approx-
imately 0.2 gram in weight. Caloric values were obtained from
runs in a Parr 1411 semi-micro calorimeter according to direc-
tions in Parr Manual f144 (table 1). Due to its high ash con-
tent, fecal material would not combust adequately on prelimi-
nary trials. Therefore, Nujol mineraloil (11026 cal./gm.)
was used as an additive (Paine 1971). Duplicate samples were
ashed in a muffle furnace at 500°C. for 5 hours in order to
convert dry weights and caloric values to ash-free equivalents.
Finally, average oxygen consumption of the isopods was
measured by the Warburg direct method of respirometry (Umbreit
1949). Each animal was covered with filtered sea water. Two
of the five vessels used were large enough to allow limited
movement by the animals. Eighteen animals were tested at 15°C.
Results
Data on consumption of the algae are listed in table 2.
Values for the total consumption by groups of animals were
obtained by difference between control and partially eaten
algal strips. A paired T-test performed on the algal strips
kept in the control box versus those dried immediately upon
cutting showed no significant difference in weights, supporting
the assumption that no appreciable growth was taking place
in the algae during tests. The same statistical test applied
to control strips and corresponding partially consumed strips
gave a significant difference (p.001). This indicated that
there was measurable grazing taking place. In calculating
Energetics of Idothea resecata
assimilation efficiencies for each of the boxes, caloric
equivalents for Macrocystis consumption and fecal production
were substituted into the equation
E - I
where E - assimilation efficiency, I = Ingestion, and
F - fecal production. Excretory waste of I. resecata has
been found to consist of gaseous ammonia and is thus negli-
gible in energetic considerations (Dresel and Moyle 1950).
When consumption and fecal production data are totalled for
the sixteen test runs an assimilation efficeincy of 0.71
can be calculated.
A regression line for oxygen consumption data is pre-
sented in figure 3. Calculation of total respiration for
the experiments was made of the basis of the mean length of
the isopods in each group. Because of greater statistical
significance in the wet weight - oxygen uptake regression
line than in the line for dry weight, lengths were converted
to wet weight estimates using data from figures 2 and 3. A
conversion factor of 4.82 cal./ml. 0, was applied to respi-
ration data. This is a reasonable figure when it is assumed
that I. resecata metabolize proteins and fat in addition to
carbohydrates (Dr. Frederick Fuhrman, personal communication).
Even if it were found that the animal metabolizes only carbo-
hydrates, the discrepency would be only 3.6%. Total energy
for respiration was found to be 371.9 calories. The total
energy values obtained in the experiments as well as values
reduced to calories per gram ash-free dry weight (AFDW) are
presented in figure 4.
Energetics of Idothea resecata
Discussion
The assimilation efficiency of 0.71 obtained through
this study seems plausible when compared to the value of
0.66 found for the Macrocystis-fed isopod Tylos punctata
(Hayes 1969) and to that of 0.88 found for the omnivorous
isopod Cirolana harfordi (William Johnson, personal commu-
nication). However, the large variance found between the
sixteen test efficiencies suggests that experimental error
is too large to consider the results conclusive. As was
mentioned earlier, two procedures were used to collect fecal
material. This seems to be reflected in the data as boxes
1-6 showed strong evidence of high salt contamination and
have correspondingly low values for assimilation. On the
other hand, boxes 7-16 give extremely high values for assi-
milation, giving strength to the suspicion that fecal mate-
rial was lost in filtration. Another possible error might
be found in the caloric value used for fecal material.
Although the two calorimeter runs on this material gave
very similar values for its caloric content (1361 and 1420
cal/gm.AFDW), the use of an additive in such a determination
is considered a potential source of error (Paine 1971).
Hayes (1969) assumed that non-ash material in fecal pellets
would contain the same number of calories as Macrocystis
and calculated his values accordingly. Such an estimation
might be considered to give an extreme lower limit to assimi-
lation. In this study, an assimilation efficiency of 0.26
would constitute such a lower limit.
Energetics of Idothea resecata
The relatively low caloric expenditure accounted for
by respiration in the energy budget defined in this study is
further evidence for suspecting a lower actual assimilation
efficiency for Idothea resecata. Although data from a few of
the respiration vessels in which the animals were able to move
somewhat might be indicative of a slightly higher active meta-
bolism (fig. 3), these values are only about 50% higher than
their regression values. Data available for swimming and
resting larvae of Mytilus edulis show that the energy require-
ment of a swimming animal is less than twice that of a resting
animal (Zeuthen 1970). Such an activity factor would do rela-
tively little to help balance the energy budget of this study.
Considering the error terms involved, it must be concluded
that the estimation of assimilation efficiency derived from
the data in this study is rather high. A value closer to
0.5 might be more realistic for Idothea resecata on the
basis of the possible experimental variables mentioned above.
Summary
An energeties study was carried out on the marine kelp
bed isopod Idothea resecata. An assimilation efficiency of
0.71 was found for animals living on a diet of Macrocystis
pyrifera.
Acknowledgments
I would like to thank Dr. Frederick Fuhrman for all the
help and guidance he has given me for this project. I would
also like to thank Dr. William Johnson for his help with the
calorimeter.
Energetics of Idothea resecata
Literature cited
Dresel, E. and V. Moyle. 1950. Nitrogenous excretion of
Amphipods and Isopods. J. Exper. Biol. 27:210-225.
Hayes, W.B. 1969. Ecological Studies on the High-Beach
Isopod Tylos punctata Holmes and Gay. PhD. Thesis
Univ. of Calif. San Diego.
Jones, L.C. 1971. Studies on selected small herbivorous
invertebrates inhabiting Macrocystis canopies and hold-
fasts in southern California. The Biology of Kelp Beds.
W.J. North, ed. Cramer. 600pp.
Kleiber, M. 1961. The fire of life and introduction to animal
energetics. Wiley, New York. 454 pp.
Paine, R. 1971. The measurement and application of the calorie
to ecological problems. Ann. Rev. of Ecol. and Sys. 2:145-163.
Sokal, R.R., and F.J. Rohlf, 1969. Biometry. Freeman, San
Francisco. 776 pp.
Umbreit, W.W., R.H. Burris, and J.F. Stauffer. 1949. Manometric
techniques and tissue metabolism. Burgess, Minneapolis. 227 pp.
125-
100
50
10
15
210
Length (cm.)
Flg-
130
100
60
40
100
300
200

Wet weight (mg.)
Fle. 2
400—
500
1.00-
60
.40
J.30
20
N.15
30
o
dry weight
wet weight
5...
4
------
-----
00 %0
o8
70 10 150 200 360
Welght (mg.)
Flg. 3
500
Ingestion: (1)
2966.7 cal.
(O.18cal./gm)
-

-
- O.715
Respiration:
371.9cal.
(0.02 cal gm
Growth
—Reproduction


Assimilation:(A)
Vole

2121.02 cal.
2

(0.13calgm.)

2


Feces: 845.7cal. ()
(0.05 cal/gm.)
Fig.4
Macrocystis
lresecata
Feces
Ash
33.85%
34.0%
72.7%
Table 1.
cal./gm.AFDW
3607
3620
1392
Table 2
Algae
Fecal
Average
consumed
material
Assim.
No.
length
Time
(gm.AFDW)
(gm.AFDW)
Efficiency
Isopods
(cm.)
(hr.)
0.102
0.165
0.36
2.87
24
0.078
0.129
0.36
2.79
24
0.022
0.39
0.035
2.80
24
0.140
-0.06
2.80
0.008
0.008
0.62
2.80
28
0.067
0.007
0.96
2.88
28
0.044
0.013
0.88
2.8
25.5
0.068
0.024
0.86
2.82
25.5
0.030
0.012
0.82
2.80
0.046
0.008
0.93
2.82
0.005
0.008
0.29
2.95
32.5
0.089
0.022
0.90
2.87
0.086
0.014
0.94
2.77
20
0.047
0.014
0.89
2.71
20
0.029
0.0004
0.99
2.78
0.051
0.008
0.94
2.79
25
Total
0.607
0.82:
0.71
-2.80
423.5
(x =0.69)
x-0.32
values for fecal material are corrected for residue collected in controls
Box
igure Captions
Fig. 1. Relationship between length (measured from mid-eye
to tip of telson) and dry weight of Idothea resecata.
Regression equation: Log I = -2.33 + O.39 X
Fig. 2. Relationship between wet and dry weights of I. resecata
Regression equation: Y = -5.03 + 0.22 X
Fig. 3. Relationship between body weight and oxygen consumption
per mg. per hour in I. resecata. indicates values in
which animal had the option to be active.
Regression equations:
Log Y = 0.86 - 0.62 X
Dry weight:
Log Y - 0.78 - 0.69 X
Wet weight:
energy flow in I. resecata.
Fig. 4. Schematic diagram showing
Lower values are computed for ash-free dry weight.
Table 1. Ash content and caloric values for Macrocystis pyrifera,
Idothea resecata, and fecal material of I. resecata.
Table 2. Data for algae consumption, fecal material production
and assimilation efficiencies for 16 test runs.
Fecal material is corrected in each case for residue
collected in the control box.