Substrate Selection in 1. resecate
Introduction
Idothea resecata, an isopod occurring in two substrate matching
colour variations, inhabits Macrocystis kelp beds and Zostera. This animal
was the subject of some initial investigations by Jones (1971).
He and Limbaugh (1955) constitute the primary contributors to a meager
supply of information about this crustacean. The focus of Limbaugh's
work was on predation of 1. resecata in California kelp beds by a
number of fish: Paralabrax clathratus, Oxyjulius californica,
Heterostichys rostratus, Sebastodes atrovirens, S. rastrelliger, and
a number of the Embiotocidae. Jones concentrated on more general
aspects of the biology of this isopod in relation to his interest in
the Macrocystis beds of southern California. Unfortunately, little or no
published work has appeared on the green colour variety of this
species, which occurs in association with Zostera marina Linnaeus.
As no specimens of the green colour type were available at the
study site, Monterey Bay, California, this authour focused her
investigations on the brown type, found abundantly in kelpbeds
composed primarily of Macrocystis pyrifera Agardh. The experiments
herein described were conducted to determine the substrate preferences
and other factors influencing the aggregation and distribution
within the kelp beds.
Results
Field Investigations
An initial study of distribution was carried out to determine whether
these animals were randomly or otherwise distributed, within the confines
of a single Macrocystis plant in the field. Animals were counted on two
separate occasions between 1200 and 1600 hours. In addition the number
thes
Substrate Selection in 1..resecata
of aggregates of one or more isopods was noted. Distribution was
found to be patchy with aggregates occurring at distances frequently
separated by several hundred yards. SCUBA techniques were employed
in actual counting with several dives made down to the bottom.
searching the lower regions of the thalli for animals. Procedures
employed in counting were as follows: when the first isopod was
observed, all visible isopods on that plant and adjoining plants were
counted and recorded on a standard underwater diving slate. Data

was grouped according to the part of the plant on which the aggregate
occurred, and the size of that aggregate. Also the state of
deterioration of the algae on which the animals occurred was noted.
Algal deterioration was classified as follows: I - no deterioration:
II - slightly grazed with little deterioration; III - well grazed with
little deterioration; IV - heavily grazed with some decomposition;
V - thorough deterioration often with blade loss.
Examination of material returned to the laboratory revealed
that animals less than two mm. in length were present but not detected
in field observations.
The isopods were never observed on any part of the plant more
than ten feet from the surface. They occur on the apical portion
from which are formed new blades, the floating stipe, and submerged
stipe to a depth of ten feet, and on the pneumatocysts. Occurrence
on mature blades was never observed during daylight hours but was
observed at night. However, it was not possible to quantify distribution
at night. These isopods show a high degree of proficiency in
swimming, and have been observed to swim towards a kelp plant when
Substrate Selection by 1. resecata
separated from it. The results of these field studies are presented
in Figures I and2. Analysis of variance and standard deviation of
these samples showed a significantly non-random distribution.
Figure 3 presents the percent of total animals found as a function of
algal condition. No significant correlation was detected.
Laboratory Investigations
As 1. resecata is known to occur, in another colour variety,
on Zostera marina, a feeding choice experiment was conducted in which
twenty uniform disks,cut from a blade of Macrocystis with a cork borer,
and two uniform lengths of Zostera were placed in each of two containers
with running sea water. Twenty isopods were added to one. Twelve
hours later, all of the Macrocystis had been eaten, and none of the
Zostera as determined. Algae in the control chamber was unchanged.
A similar experiment in which twenty disks of Macrocystis and of
Nereocystis Luetkeana Postels and Ruprecht were used, gave rather unusual
results. Control algae, and experimental Macrocystis were unchanged,
but the experimental Nereocystis had all been grazed down to the inner,
colourless cortex, none of which had been eaten.
A third similar experiment was performed, using equal sized pieces
of fresh Nereocystis and Nereocystis which had been artificially
scraped to remove as much as possible of the outer layers. The
isopods removed what the experimenter had been unable to remove of
the surface layers, and grazed the fresh piece down to the cortex.
No change was observed in the control.
Another series of experiments, using the apparatus portrayed
in Figure 4, was performed. Extracts were made of Macrocystis and
Substrate selection in 1. resecat.
Nereocystis by osterizing five large blades in sea water, and diluting
to 1000 ml., and then filtering three times through bolting cloth.
An isopod solution was made in a similar fashion, using 100 1. resecata.
The animals in aquaria readily attached to rubber tubing when their usual
substratum was absent. The rubber tubing was clamped in the center
to prevent mixing of the solutions, and holes made in the tubing with
a dissecting probe to allow the fluid to leak out. The apparatus was
flushed out with sea water between runs. The behaviour of ten isopods
placed into the center of the chamber was observed. At one minute intervals
five minutes, the location of the isopods on the tubing was recorded.
Values for each run were averaged, and the nine resultant pairs of
data were subjected to the Wilcoxon rank sum test (Snedecor, 1967).
A data pair was composed of the number of isopods on the control side
and the number on the side receiving the experimental fluid.
Discussion
North (1971) indicates that punctures from the feet of these
animals, and areas of grazing may become centers of growth of
bryozoa (especially Membranipora membranacea Linnaeus) and other
epiphytes, which may in turn lead to bacterial or fungal infections,
and especially, black rot. Black rot is a disease or condition
cound among the Phaeophyta following prolonged exposure to warm
summer temperatures. In fact, the isopods are usually present in
much diminished numbers at that time of year, although they may be
the originators of the chain of events.
The site which the authour observed to harbour the greatest
numbers of isopods, the juncture between the pneumatocyst and the
Substrate Selection by 1. resecata
stipe, was shown by Nicholson (1968) to be the area of greatest
transport of photosynthate.
A study of these isopods over a long time period is very much needed.
The location of these isopods during the winter months when the kelp breaks
up remains to be determined. It is most definitely an interesting
question for further research.
Jones (1971) describes a series of experiments which he describes
as "feeding choice". This authour stipulates that his experiments measured not
feeding choice but substrate choice. His particular series of experiments
were carried out without regard to which alga was actually consumed.
Instead his measurements are of choice of substratum for settling.
The experiments reported in the current study were directed toward
examination of food preference.
The use of Nereocystis in these studies resulted in unexpected results.
This authour's hypothesis would be that there is a substance "distaste¬
ful" to the isopod which is contained in the cortex of the Nereo¬
cystis. Such an hypothesis remains as yet untested.
The importance of these isopods in the kelp bed community has
been pointed out above. The effect of these animals on this par¬
ticular marine habitat must receive additional attention.
Summary
Substrate selection and distribution of the isopod Idothea resecata
was studied in relation to Macrocystis kelp beds in Monterey Bay,
California.
1. Field studies showed that this isopod prefers floating
stipe to other portions of the plant.
Substrate Selection by 1. resecata
2. No animals were found on stipe more than ten feet below
the surface, nor on the blades.
3. No correlation was observed between numbers of isopods and the condition ot
the alga on which they occurred.
4. 1. resecata prefers the outer layers of Nereocystis to the
cortex, and to Macrocystis.
5. This isopod prefers Macrocystis to Zostera as a food substrate.
6. The animals may be attracted to Macrocystis and members of
their own species by soluble factors.
Acknowledgements
would like to express my sincerest gratitude to Dr. John H.
Phillips for invaluable assistance and advice in conducting the experi¬
ments and writing the paper. Also I would like to thank Dr. Donald
P. Abbott, Dr. Robin D. Burnett, and Mr. Nathan Howe, for assistance throughout
the project.
Literature cited
Jones. Laurence G. 1971. Studies on selected small herbivorous
invertebrates inhabiting Macrocystis canopies and holdfasts
in Southern California kelp beds. in North, W.J., ed. The
Biology of Giant Kelp Beds (Macrocystis) in California.
Verlag Von J. Cramer. Lehre.
Limbaugh, C. 1955. Fish life in the kelp beds and the effects of
kelp harvesting. Univ. Cal. Inst. Mar. Res. IMR Ref. 55-9. 158 pp.
Nicholson, N.L. 1968. The Role of Photosynthate Translocation in
Nereocystis Luetkeana. PhD. thesis, unpublished.
North, Wheeler J. 1971. The Biology of Giant Kelp Beds (Macrocystis
Ar

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Snedec

r, G. W.
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ehre.
al Methods. ed. 6



7
Substrate Selection by 1. resecata
Captions to Figures
Fig. I. Number of aggregates vs. aggregate size for all areas of
the plant, and algal conditions.
Fig. 2. Percent of total animals ys. area on the plant.
Fig. 3. Percent of total animals ys. algal condition.
Fig. 4. Apparatus used in choice experiments.
Fig.. Significance of choice experiments.
TOTAL:
stipe 0'-10'
floating stipe pneumatocysts apical portions
hn
h

34 56 78 9 151
567
aggregate size
TOTAL:
I1
Hhn


H
hI

I
12345
aggregate size
APICAL:


—
aggregate size
FLOATING STIFE:
IV
H r
2 24 —
aggregate size
STIPE O'-10'

H
915 1

aggregate size
significance
leve!
STIPE 0'-10'
FLOATING
STIPE
95 %
PNEUMATOCYSTS APICAL PORTIONS
95%
90%
100.
50
AIGAL CONDITION
IV
2


a

a

—
a


a
L
L

++

4

a2
L
—

3

6
00%
CONIRO
significance
level
MACR OCYSTIS
97.5 %
NE REOCYSTIS
185.
ISOPOD EXTRACI
95%