Introduction
Idot
Pent
otea ster
ops (Benedict) is an herb¬
ivorous isopod with a range from Coos Bay, Oregon to
Monterey,California (Menzies, 1950). It is found in
the lower littoral region, uncovered only by "minus'
sides. The vast majority of the population resides on
gregia, although occasional specimens may be found on
aminariaor
Yllospadix.
Preliminary studies of gut contents revealed a diet
consisting almost entirely of Egregia. Small amonts of
lospadix and Laminaria, and other forms of algae,
mostly epiphytes, have been found in the gut contents
of specimens examined.
The lower intertidal has a rich variety of flora
(Smith,1966), most of which I.S
enops is apparently
ignoring. There are considerable differences among
these plants in terms of cell wall composition, stor¬
age products, etc. perhaps important to their suitabil¬
ity as a food source (Kreeger, 1962) (Meeuse, 1962). The
research presented investigates how well I. Stenops is
able to utilize Egregia, and several other plants which
do not appear to be its normal food source.
Results
I Experiments on Newborn Animals
Two dozen ovigerous females were placed in an aquarium
with Egregia. After two days approximately two hundred
newborn animals were collected.
A random sample of 15 newborn animals was dryed at
60' C and weighed. Standard deviation in weighing was
O.11 mg.
Five aquaria in the form of plastic boxes had been
set up previously to hold these newborn isopods. These
plastic boxes, of slightly less than 1 gallon capacity,
were given running sea water and vigorous aeration. Four
boxes contained Egr
egia, Phyllospadix, Iridaea, and Lamin-
ria respectively as sole food source of the newborn. The
fth aquarium contained all four plants in equal amounts.
In each case an abundance of food was provided. The plant
species chosen served to contrast Egregia with another brown
alga, a red alga, and a flowering plant.
Croups of newborn I. sten
were counted and placed
in their respective aquaria.
Counts of the animals were made at intervals of not
more than seven days. At each count the groups and their
algae were placed in clean aquaria.
After thirty-four days the surviving animals were
arved for twenty-four hours to empty their guts, dryed
at 609 C, and weighed.
Mortality during the thirty-four days is presented
in figures 1-2.
Mortality per day was computed for each of eight
intervals between counts. The means of these rates and
their 95% confidence limits are presented in Table 1.
A students t-test for significance indicated that
mortality per day of the group fed on
gre
a was sig-
Ificantly different from all groups except the group
fed on a mixed diet. The highest mortality per day,
observed in the group fed exclusively on Phyllo
adix was
ificantly different from all groups except the group
si
fed on Iridaea.
Growth of the animals during the period of observation
is represented in Figure 3 and expressed as mean dry weight
with 95% confidence limits. The weight at the beginning
is included and labeled source group. A students t-test
for significance indicated that mean weight of the group
fed on Egre
ia was significantly different from all groups
except the group fed on a mixed diet. The group fed on
a mixed diet was significantly different from all remain¬
ing groups. The groups fed on Laminaria and Iridaea showed
a significant increase in weight over the source group.
However, no such significant gain in weight was observed
with the group fed on Phyllospadix.
1i Experiments on Adults
Twelve quart jars were set up as aquaria, with
vigorous aeration and maintained at ambient ocean temper¬
ature in a greenhouse. Weighed amounts of Egregia,Phyllo,Lridaea,
and Pelvetia were added to groups of three jars respective¬
ly. One jar in each group received four isopods weighing
between .04 and .16 grams. Similarly one jar in each
food group received one isopod between .71 and .78 grams.
and one jar received one isopod whose weight was between
1.3 and 1.9 grams.
The initial net weights of plants and isopods were
obtained by blotting them with paper towels, and preventing
further weight change during weighing by placing them
in tared, covered petri dishes.
The animals were allowed to feed on the plant
material and at intervals of 4 to 8 days both plant and
isopod were weighed again. Fresh weighed plant material
was introduced at thede times. This was continued for one month.
Deterioration of the plants was not found to be
a problem under the cond
tions of the experiment. Plant
growth was considerable during these intervals. Controls
for the assessment of plant growth in the absence of
isopods were established. A figure of mean per cent
weight gain per day was calculated for each plant species. This
figure was used to estimate the plant growth that should
have occured in a jar during the grazing interval.
Weight
decrease in the plant over a grazing interval plus
probable
growth gave the amount of food ingested. Variance in
plant growth necessarily contributed to variance in
values for food ingested. Dry weight to wet weight
sios were obtained for the plants. Food ingested was
converted to dry weight, and expressed in terms of grams
dry weight of the food per gram wet weight of the animals
per day. The results are presented in Figure 4.
These mean ingestion rates were tested for significant
difference by a non-parametric test of the sum of the
ranks of differences between data pairs (Snedecor, 1967).
The test is appropriate to the data because of the large
variances. The ingestion rate on Egred
gia was significantly
ifferent from the rates on Phyllos
spadix and Iridaea,
but not the rate for
Pelvetia. All other rates are sig¬
nificantly different from one another.
Fecal material was collected at intervals of two
to four days so as to avoid fouling of aquaria, and loss
of fecal material by decomposition. Fecal material was
measured as dry weight. The difference between dry weight
of ingested food and dry weight of fecal material was
considered as an estimation of assimilation, and expressed
as grams dry weight assimilated per gram isopod per day.
Standard deviation in weighing plants was approximately
25 milligram. Weighings were made to the hundredth of
a gram.
Standard deviation in weighing the animals was
approximately 13 milligrams. Weighings were made to the
nearest milligram.
The mean assimilation rates are presented in Figure 5
h95% con
wit
fidence limits. The same non-parametric test
was employed to determine significant differences between
pairs of means. The as
similation rate with Phyllospadix
is significantly dif
rent from the rate on all other diets.
No other significant differences were noted. It was
noted, however, that there was an apparant lack of vigor
in the
animals fed on either
llospadi
These animals showed a lessened escape respor
capture was at
empted. The different diets prese
no discernable d.
erence in quality as a substrat
attachme
t of the animals.
Molting of the
dul
animals was
the course of the exper
The num!
hthe diet of the anim
Table 2 presen
observations.
Discussion
Newoorn 1. stenops were good subjects for the study
of growth and mortality in relation to diet. They were
exceptionally homogeneous in weight as well as age.
They could be expected to show rapid growth if developing
properly. The dietary response of these young animals
night show differences from the response of mature adults.
and, therefore, be an important consideration.
The data on newborn growth indicates that the diets
can probably be ranked in decreasing quality as, Egregia,
Laminaria, Iridaea, Phyllospadix. This agrees with data
from the feeding of adults in terms of ingestion, assimil¬
ation, and molting. The data on ingestion is especially
interesting since statistically significant differences
were found between the brown algae, and Irid, and Phyllospadix.
Onfortunately the large variance makes statistical
comparisons more di
icult for assimilation.
The wide confidence limits on means for ingestion
and assimilation are a reflection of the fact that feeding
was intermittent on all diets. In particular very little
feeding was done during molting periods. Nevertheless
the range embraced by the confidence limits indicates that
aximum values are much higher on brown algae tested than on
the red alga, Iridaea, and on either of the former than on
llospadix.
The relatively small differences in growth and survival
between newborn fed on Laminaria and Egregia are interesting
since Laminaria is in fact found in the gut contents of
leld specimans, and I. stenops are occasionally found
grazing on it. The data indicatesthat it is only slightly
less suitable than Egregia as a food source.
All in all the data indicates that Egregia is a
significantly better food source than I
daea or Phyllospadix.
Such food specialization could certainly be a factor in
1. stenops distribution, and habitat choice. While the
data seems to indicate that brown algae has the most food
value to I. sten
ps the data on relative food values of
The brown algae is less conclusive. It does suggest th

ia is the most suitable food. The results fron
experiments on newborn animals seem to eliminate th
possibility that a mixture of the plants
to Egregia alone.
Summary
1) Newborn Idotea sten
s were raised on several diets
and their mortality and growth determined.
2) Quantitative feeding experiments were performed on
os using several weight classes. Quanti¬
ties
I. sten
of food ingested and assimilated were calculated.
3) I
yllospadix was found to have much less food valu
than the other marine plants tested. Egre
aae the
best results in terms of survival and growth of the
newborn. Other forms of brown algae approach
he food
value of Egregia. Other plants tested were t
algae Le
ed
aand Pelvetia, andth
Acknow
select
Literature Cited
y and Bio
emist
Kreeger, D. R. 1962. Cell Walls, Physiolog
of Algae, Edited R. A. Lewin. New-York: Academic Press
Meeuse, B. J. D. 1962. Storage Products,
Physiology
and Biochemistry of Algae, Edited R. A. Lewin.
New York;Academic Press
Menzies, R. J. 1950. The Taxonomy, Ecology, And Distrib¬
ution of Northern California Isopods of the Genus
Idotea, With a Description of a New Species, Wassman
Journal of Biology, 8: 155-195
Smith, G. M. 1969. Marine Algae oft
he Monterey Peninsula,
California. Stanford: Stanford University Press
Snedecor, G. W: and Cochran, W.G. 1967 Statistical Methods,
Ames, Iowa: Iowa State University Press pp. 128-130
Captions for Figures and Tables
Figure I Cumulative Percent Mortality with Time for
Newborn Idotea !
enops Maintained on Various Diets
=- Laminaria Diet - Initial Population Size=29
•- Egregia Diet - Initial Population Size=32
A- Mixture Diet - Initial Population Size=25
Figure 2 Cumulative Percent Mortality with Time for
Newborn Idotea stenops Maintained on Various Diets
E-Phyllos
ix Diet - Initial Population Size = 29
ad
A-Iridaea Diet
- Initial Population Size = 29
Figure 3 Mean Dry Weight and Confidence Limits of Newborn Source
Group, and of Survivors After 34 Days of Feeding on
Various Plant Speies
Figure 4 Ingestion Rates by Idotea stenops of Several Plant Species
ea stenops of Several Plant Specie
Figure 5 Assimilation Rates of Idot
Table 1 Mean Percent Mortality per Day for Isopods
Maintained on Various Diets
Table 2 Number of Molts in Thirty Days for Groups of
Twelve Isopods Maintained on Different Diets
Diet
Phyllospadi
ridaea
Lamina
regia
Mixture
Diet
Egregia
elveti
Irideae
Phyllos
TABLE I
Mean Mortality/Day
6.7%
4.8%
2.0%
.289
.52%
TABLE 2
5% Con:
idence Limits
12.3%
14.0%
1.5%
+.40%
+.9%
5 5
Centlate erent Mereli,
0
Cumulative Percent Mortality
8
CHAMPION LINE NO. 810-3
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Pelvet.
FIG.4
DIET
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FIG.5