Page 2
ABSTRACT
Idotea resecata lives primarily on bay Zostera and Macro-
cystis and is considered to be a typical bay species (Menzies,
1950). Idotea montereyensis is adapted to open coast Phyllospadix
beds and is considered to be a typical open coast species.
Both species have been labelled as lower intertidal forms.
Tolerances to heat, salinity, and survival at low oxygen
tensions were measured, and morphology of the pereaopods was
also examined. It was found that I. montereyensis had
longer LT-50's for the higher temperature and salinity
stresses, enabling it to cope with both the lower
intertidal environment of the Phyllospadix beds and also the
higher intertidal tide pools into which it is sometimes
washed. I. resecata has slightly greater abilities to deal
with low oxygen situations, enabling it to cope with
the low oxygen concentrations often found in Zostera
beds. The pereaopods of I. montereyensis seem to be more
adapted than those of I. resecata to gripping the substrate
effectively in heavy wave action on the basis of morphology.
Because I. montereyensis migrates vertically as it develops
and has the physiological tolerances to cope with the range
of environmental parameters associated with such a migration,
it is felt that to consider it only a species of the lower intertidal
is misleading.
Page 3
INTRODUCTION
In 1950 Menzies described the distribution of the
California Idoteid isopods and while he found several species
inhabiting open coastal situations, he only described one,
Idotea resecata, as typically being a bay inhabitant. With
eight species being described from subtidal to intertidal
levels it seemed unusual that only one of these had the
appropriateadaptations to inhabit bay localities. It was my
intent to examine the bay inhabiting I. resecata, comparing
some of its morphological and physiological adaptations
to those of one of the isopods described by Menzies as a
typical open coastal form. Idotea montereyensis was chosen
as the open coast species to be studied because of its
availability and the ease with which it could be collected.
Menzies classifies both of these species as being lower
intertidal, so variations due to differences in vertical
distribution were expected to be minimal.
In the spring, plentiful number of I. resecata live in
the canopy of the Macrocystis pyrifera found off Hopkins Mar-
ine Station in Monterey Bay, California. While the Macrocystis
is not in the intertidal zone, I. resecata may also be found
on Zostera marina which occurs intertidally at about the
O.O tidal level. In these habitats, I. resecata is presumably
exposed to less wave action than is present in a typical
open coast rocky intertidal area.
In contrast, I. montereyensis lives on the flowering plant
Phyllospadix scouleri found along the open coast at approximately
Page 4
the same tidal level as the bay Zostera. Here, individuals
spend much of their time underwater but can be washed by
wave action into tide pools at higher levels. Work by Lee
(1966) seems to indicate that juveniles of this species
occur in the inshore tide pool areas but move back out into
the Phyllospadix beds as adults.
Given these two species, one a typical bay inhabitant and
the other a typical outer coastal species, it was decidedcto
focus on those physical parameters that probably repre¬
sent the major differences between bay and open coastal habitats.
These parameters were considered to be salinity, temperature,
oxygen level, and wave action. Accordingly, data was gathered
on salinity and temperature tolerances, respiration rates,
and tolerance to low oxygen conditions. The peraeopods were
also examined with an eye towards determing difference
in holding ability under conditions of wave stress.
RESULTS
Tolerance to Physical Parameters--Salinity.
Procedure: Concentrations of 25%, 50%, 75%, 100%, 125%, 150%,
175%, and 200% sea water were made from Instant Ocean (available
from The Dolphin, San Jose). Tap water which had been kept
in an open container overnight in a heat closet to drive
out the chlorine was used for fresh water. 1. resecata
and I. montereyensis were kept in holding aquaria in the
laboratory for several days at ambient sea water temperature
before tests were run. Subsequently samples of either 5 or 6
Page 5
animals were placed in finger bowls and covered by 200 ml
of each of the solutions. The animals were chosen randomly
and included males and females from approximately 6 mm to
25 mm in size. The bowls were placed in a 15° C constant tem¬
perature room and aerated continuously. At hourly intervals
the bowls were examined and the number of live animals
noted. Criteria for death were absence of pleopod movement,
pleopod extension from the body, open uropods, lack of per-
eaopod and antennal movement, and lack of response to mech-
anical stimulation or removal from the water. Tests were then
repeated four times, except in the case of 50% sea water
which was repeated only once. Data from all replicates
at each salinity were pooled.
Results: The data from these experiments is presented
in figure 1. At all salinities I. montereyensis survives at
least as well, if not better than, I. resecata. In fresh water
the stress is apparently too severe to allow much distinc-
tion between the two species. At 25% and 50% sea water,
however, the greater survival of I. montereyensis is marked.
Both species seem to survive salinities equally well from
75% to 175% sea water for at least 23 hours. I. montereyens
again appears to show greater survival in 200% sea water.
Several of the survival curves are approximately sigmoid,
suggesting that tolerance to salinity stress is normally dis-
tributed. Isolated cases of mortality are due primarily to
cannibalism.
LT-50's at the various salinities were interpolated from
the raw data and are summarized in figure 2. Both species are
Page 6
euryhaline, but I. montereyensis clearly survives better at
the more extreme salinities. The difference in LT-50 at
both 25% and 50% sea water is quite marked. No obvious
differential viability was noted relative to the size, sex,
or reproductive state of the females in either species tested.
Tolerance to Physical Parameters--Temperature.
Procedure: To test temperature tolerance to 15° C, 22° c, 30° c,
35° C, and 40° C, finger bowls with 200 ml of 100% sea water
were placed in heat closets in the dark at 30° C, 35° C, and 40° C,
in a dark room at 15° C, and in a darkened box at room temperature
2° C). All bowls were aerated continuously and allowed
to equilibrate to the experimental temperature for about
one hour. In a manner similar to that utilized in the
previous experiment, samples of 5 or 6 animals were introduced
into the bowls and checked hourly to determine the number
still alive. The criteria of death were the same as in the
preceeding experiment. The experiments at 15°, 30°, and 35
were replicated four times while the experiment at 40' C
was repeated only once. The data was pooled as in earlier
experiments.
Results: The pooled data is shown in figure 3. The severe
heat stress of 40° C killed all animals in too short a period
to show any distinction between the two species. At temperatures
of 30° C and 35° C I. montereyensis again shows greater survival
than I. resecata. Both species can survive temperatures of
15° C and 22° C for at least 23 hours with no significant
Page
mortality. Again, no differential mortality between sexes
or size classes was noted.
LT-50's at the various temperatures are summarized in
figure 4. For the sake of comparison this figure also
incorporates some of the data by Keever (1973) on another
Idoteid isopod, I. stenops, a lower intertidal open coast
isopod which does not migrate to higher tide levels as does
I. montereyensis. Note that the I. stenops has markedly
less tolerance to high temperature stress than either I.
resecata or I. montereyensis.
Tolerance to Physical Parameters--Ox
gen.
rocedure: Oxygen requirements of the isopods were investigated
in two ways. First, respiration rates in oxygen saturated,
filtered sea water were measured with a constant volume
respirometer. This information was needed to interpret the
experiments on low 0, tolerance. Eight male I. montereyensis
and nine male I. resecata were used in these experiments,
temperature being maintained constant at 15' C. Animals of
similar size for each species were used in the experiment,
ranging from about .Ol gm to about .23 gm. Second, tolerance
to low oxygen was determined. Ten individuals of each species
were used in each experiment. Nitrogen was bubbled through
a stock solution of filtered sea water for varying lengths
of time to lower the oxygen concentration. A sample was then
taken and a Winkler oxygen determination performed to measure
oxygen tension. Simultaniously twenty large test tubes
Page 8
with animals in them were filled with the anoxic sea water
and sealed. A second water sample for Winkler analysis was
taken after the tubes were filled in the first few exper-
iments to be sure that water of the same oxygen level was
being placed into each tube. In all cases where this
was done, the second sample was found to have essentially the
same oxygen level as the first sample (+.02 ml 0,/ 1 sea water).
The sealed tubes were checked every 10 minutes until only
one or two animals were still alive, and an LT-50 for the ten
animals of each species was subsequently interpolated for the
particular experimental oxygen concentration. The experiment
was then repeated nine times at different oxygen tensions.
Results: Figure 5 is a graph of log respiration rate vs.
log wet weight. While there is some indication tha a male
I. resecata of a given weight may have a slightly higher respiration
rate than a male I. montereyensis of the same weight, the
difference is statistically insignificant. Therefore, the
possible differences in oxygen levels between the bay and
open coast habitats are not reflected in the respiration
rates of the two species in fully oxygenated water.
An experiment was performed on I. resecata to determine
if body weight had any affect on time of death at a given
low oxygen level. The experiment was performed in the same
manner as the other low oxygen experiments except that the
animals were weighed before the experiment. Figure 6 is a
graph of log survival time vs. log wet weight at an oxygen
concentration of .42 ml 0,/ 1 sea water. Two females and
Page 9
eight males were used in this experiment and the females
show no significant differences from the males. The least
squares regression line has a slight positive slope, but does
not differ significantly from 0. It seems, then, that weight
has little affect on time of death, at least at this
particular oxygen concentration, and that individual variation
due to factors other than body size and sex may have greater
affect on the time of death.
The results of experiments on survival at low oxygen ten-
sions are shown in figure 7. While the unexplained variation
is not small for either species, the regression slope for I.
resecata is significantly different from that of I. mon-
tereyensis suggesting that if the oxygen stress is not extreme
(tensions below about .60 ml 0/ 1 sea water), I. resecata
may enjoy a better survival rate. When the oxygen stress
is quite extreme the animals die off too soon to show a
distinction between species, as was the case in the more
extreme salinity and temperature stresses tested in previous
experiments.
Care must be taken in interpretting this data for several
reasons. First, oxygen tensions above about 2.22 ml 0,/ 1
sea water were not investigated and an extrapolation of the
regression lines to higher oxygen tensions is probably not
valid. Further, the regression lines fitted to the data are
linear. More data might reveal an exponential relationship
between LT-50 and oxygen concentration although for both
species the linear correlation is significant at the 1% level.
Finally, it must be remembered that once the test tubes were
Page 10
sealed the oxygen concentrations decreased with time as the
animals respired. Consequently the data show LT-50's not at
a constant oxygen concentration but rather in water which had
an initially determined concentration. Differences in oxygen
depletion rates both between the species and between differ-
ent size classes probably accounts for some of the unex¬
plained variation, although the experiments shown in
figures 5 and 6 suggest that this variation is small. The
assumption is made that since the respiration rates of I.
resecata and I. montereyensis are not greatly different in
fully oxygenated water, as is shown in figure 5, the rates are
not significantly different in low oxygen water. A further
assumption is that time of death, shown not be affected much
by animal size in I. resecata at an oxygen concentration
of .42 ml 0/ 1 sea water, is not greatly affected by size at
higher oxygen tensions. Observations made during the course
of the experiment seem to bear out the assumption regarding the
independence of size and time of death although the
matter was not investigated directly at higher oxygen
concentrations. Furthermore, death did not seem to be significantly
correlated to sex or reproductive state of the females.
Morphology and Behavior.
Both isopod species studied are quite streamlined and are
strong swimmers. They can both hold on to their usual substrate
quite tightly, although some factors, such as removal of the
substrate from water appears to influence their holding
Page 11
ability. When a Phyllospadix blade is taken out of the water
most I. montereyensis either drop off the blade or crawl
back into the water. When Macrocystis is lifted out of the
water, I. resecata tend to slide to the underside of the stipe
or walk down it, although on occasion some may also drop off.
In general, I. resecata maintains a stronger grip on its substrate
when it is held out of water than I. montereyensis when it is
held out of water. How the animals behave in surf is a
different question, however. Holding ability in a natural
situation is quite difficult to investigate and so I therefore
directed my attention more towards examining the morph-
ology of the pereaopods hoping to find differences in
structure which would suggest differential holding ability and
could be correlated with differences in wave action
in the isopods' respective habitats.
Figure 8 shows drawings of the 7th pereaopods of I.
montereyensis and I. resecata drawn in poitions characteristic
for the animals in grasping their substrate. In I. montereyensis
the segments tend to be short and wide and there are more
bristles on the propodus, possibly to increase frictional
contact. In addition, the dactyl is more fully developed
than in I. resecata. I. resecata in contrast has rather long
and sparcely spaced bristles on the propodus and the
dactyl is weakly produced.
Characteristically, I. resecata keeps its pereaopods
more extended than I. montereyensis because it is on a broader
surface, either a Macrocystis blade or else the relatively large
diameter stipe. I. montereyensis, on the other hand,
Page 12
grips the thin, narrow Phyllospadix blades by holding them
against its ventral surface with the dactyls. While I,
esecata can pierce the surface of Macrocystis with its dactyl,
1. montereyensis seems to engage in a more secure stance
becuase it actually encircles its substrate. The fact the
1. montereyensis is moved into higher intertidal zones,
presumably by wave action (Lee, 1966), suggests that
its holding ability, particularly in juveniles, which
make up the predominant portion of those I. montereyensis
found at higher levels, is not perfect. Nevertheless, the
morphology of the pereaopods is consistant with the suggestion
of a holding ability greater than that of I. resecata.
This would have obvious advantages in light of its more
turbulent habitat.
DISCUSSION
Experimental evidence has shown that while both species
studied are euryhaline, I. montereyensis survives markedly
better than I. resecata at salinities of 25% and 50% sea water,
and somewhat better in both fresh water and 200% sea water.
I. montereyensis also seems more tolerant of temperature
stress, surviving better than I. resecata at both 30° and
35 C. While the evidence on oxygen stress is not clearcut,
this is a suggestion that I. resecata performs slightly better
than I. montereyensis at oxygen concentrations in the range
of .60 to 2.22 ml 0,/1 sea water.
Unfortunately a complete set of field data characterizing
Page 13
the actual environmental conditions faced by these isopods
is lacking. Some data in the literature, however, can
provide guidelines relative to the ranges in temperature.
salinity and O, tension experienced by both I. montereyensis
and I. resecata. Johnson (1965) examined the temperatures
in Tomales Bay, where I. resecata are commonly found living
on Zostera, and reported that whereas ocean temperatures
had an annual range of 12-15* C from winter to summer,
the cove in Tomales Bay which he studied varied from
8-23° C during the year, a range of 15° C as compared to
3 C on the open coast. Conceivably the temperature range
on the Zostera blades, where the I. resecata lives, might be
even greater due to factors such as increased heat absorption
due to its dark color.
Some physical parameters of the kelp beds off Hop-
kins Marine Station were monitored for 48 consecutive hours
in the summer of 1971 (Pearse, 1971), and the daily surface
temperature where populations of I. resecata are located
was found to vary between about 12.5°-15° C. Three measurements
on a spring day in 1973 showed the temperature during daylight
hours to vary between 12-16° C. There is even less data
on the habitat of I. montereyensis, but a bay Phyllospadix
bed where I. montereyensis does not occur was found to have
a late afternoon temperature of 15-16° C in the spring. Open
coast Phyllospadix beds might be presumed to have temperatures
approximately equivalent to those of the surface ocean
temperatures because of the large amount of wåter movement
and mixing between the intertidal Phyllospadix beds and the
Page 14
offshore ocean surface. This would give the Phyllospadix
beds an annual temperature variation of 12-15° C. Unfortunately
there is little information on the temperature conditions of
the tide pools into which I. montereyensis are periodically
washed.
Pearse's study showed the oxygen levels of the
Macrocystis to vary considerably, ranging from 11 ml 0/
I sea water in mid afternoon to 4 ml/1 shortly before sunrise.
Broekhuysen (1935) found oxygen levels in Zostera beds to ap-
proach O at night and observed I. resecata swimming free of
the beds under these conditions. A measurement of pre¬
sunrise oxygen levels in bay Phyllospadix showed them to
be around 1.5 ml/1, whereas daytime oxygen levels in the
same beds were in the meighborhood of 12 ml/1.
To summarize what is known about the comparitive
habitats of I. resedata and I. montereyensis, then, it seems
that in addition to marked differences in wave action, bays
tend to be more temperature variable than the open coast
llospadix beds. The intertidal tide pools into which I.
montereyensis is sometimes washed may in turn be more
temperature variable than the bays, however.
Oxygen levels in the kelp beds do not seem to get
very low, but Broekhuysen's work indicates that I. resecata
in Zostera may have to face quite low oxygen concentrations.
I. montereyensis, on the other hand, is limitted to the open
coast and probably does not see the same low levels of oxygen
that I. resecata does. Indeed, O, concentration may be a
factor limitting the horizontal distribution of I. monter-
eyensis. Presumably oxygen levels in open coast Phyllo¬
Page 15
spadix beds never reach levels quite as low as those in the
bay because of increased mixing due to heavy wave action.
Little is known regarding salinity variation in these
areas, but fresh water run-off could introduce salinity
variation where there is not adequate mixing with a large
salt water mass. Therefore bays might be expected to have
greater salinity variation than well-mixed open coast Phyllo-
padix beds. In intertidal tide pools, however, because of
both increased temperature causing high evaporation
rates and fresh water run-off, the tide-pool waters might be
quite variable in relation to salinity. On the basis of
this speculation, one might then characterize the habitat of
I. montereyensis as being more variable in terms of temperature
and salinity than that of I. resecata but more stable with
respect to ambiant oxygen levels.
Experimental results agree with this comparison of the
habitats. In short, I. montereyensis does better at high
salinity and temperature than I. resecata. However, I. resecata
appears to withstand lower O tensions than I. montereyensis.
This suggests that Menzies' original classification of the
vertical distribution of the two species is misleading.
He felt that both could be considered lower intertidal species,
suggesting they both could be expected to have the tolerances
of lower intertidal species. It has been shown, however,
that I. montereyensis has the temperature and salinity
tolerances to cope with the high variability of the intertidal
tide pools into which it is periodically washed. Because
Page 16
of this vertical migration, then, I. montereyensis should
perhaps not be considered an animal typical of the lower
intertidal zone.
Keever's (1973) work on I. stenops lends support to this
hypothesis. I. stenops is an Idoteid isopod which lives in
the lowest intertidal zone among Laminaria and lower
Egresia. In his study, Keever found the lowest oxygen concentration
in areas frequented by I. stenops to be 4.2 ml 0,/1 sea water,
and experiments showed that while this animal could sur-
vive in water slowly brought to.70 ml/1, it became noticably
stressed in levels below 2.8 ml/1. Both I. montereyensis and
1. resecata,however, swim normally in tubes with oxygen
concentrations down to about.90 ml/1 for close to an hour.
Evidently I. stenops is less tolerant of low oxygen concentra¬
tions than either I. resecata or I. montereyensis.
Keever never found spring temperatures in areas with
I. stenops to get above 12' C, and as might be expected,
figure 2 shows that I. stenops has a temperature toler-
ance less than that of either I. resecata or I. monter-
éyensis. At 20' C the LT-50 for I. stenops is under 9 hours,
while at a slightly higher temperature almost all I. res-
ecata and I. montereyensis studied could survive for over
23 hours.
In summary, the tolerance ranges of the animals studied
correspond to the ranges of environmental parameters for
each of their respective habitats. I. montereyensis,
living in a more temperature and salinity variable environment
Page 17
than either I. resecata or I. stenops, can tolerate higher
levels of temperature and salinity. I. resecata, living in
Zostera beds which approach low oxygen levels at night,
performs better in low oxygen conditions. I. stenops,
living in a more homogenious environment than that found
in either the bay or in Phyllospadix beds and tide pools, has
the least tolerance to extremes of all three species
studied. Until more information is gathered, however.
particularly field data, very little can be said conclusively
about what affect the tolerance differences have on the actual
distribution of I. montereyensis, I. resecata, or I. stenops.
ACKNOWLEDGEMEN
I would like to acknowledge Robert Keever for generously
letting me use his data and Chris Harrold for being kind
enough to wade into Phyllospadix beds for me at 4:30 am.
I want to thank Kathy Dereimer for making sure I got out of
bed for early morning collecting and Nat Howe for making
so many tiresome trips to the storeroom to get materials.
I am very grateful to Mrs. Connally for letting me use so many
envelopes and just generally being such a sweetie. I very
much want to thank Dr. Lee for providing encouragement,
advice, ideas, and understanding throughout the quarter in
addition to knowing an excellent Italian restaurant in
Occidental, California. Finally, I would like to acknowledge
everyone at Hopkins Marine Station in the spring of 1973
for making the place just what it was: a great spot to spend
some time.
LITERATURE CITED
Broekhuysen, G. J. (1935) The extremes in percentages of
dissolved oxygen to which the fauna of a Zostera
field in the tidal zone at Nieuwdiep can be exposed.
Arch. Neerl. Zool. 1(3), 339-346.
Johnson, Ralph. (1965) Temperature variation in the infaunal
environment of a sand flat. Limn. Ocean. 10, 114-120.
Keever, Robert. (1973) unpublished manuscript on file at
Hopkins Marine Station library.
Lee, Welton L. (1966) Color change and the ecology of the
marine isopod Idotea montereyensis (Maloney). Ecology.
47, 930-941.
Menzies, Robert J. (1950) The taxonomy, ecology, and
distribution ofnorthern California isopods of the genus
Idot
a with the description of a new species. Wasmann J.
3iol. 8, 155-195.
Pearse, et. al. (1971) A Kelp Bed as a Classroom: Results of
five week study of kelp beds in the Monterey Bay region.
Unpublished manuscript on file at Hopkins Marine Station
library.
FIGURE CAPTIONS
1. Survival curves for I. resecata and I. montereyensis
in solutions of different salinities. x- I. resecata,
o- I. montereyensis. Salinities are expressed as % sea water.
Experiments were terminated after 23 hours.
2. LT-50's for I. resecata and I. montereyensis in solu-
tions of different salinities. Salinities are expressed
as % sea water. Empty bars represent I. resecata, cross¬
hatched bars represent I. montereyensis. N=6 for 75% sea
water; N=10 for 125%, 150%, and 175% sea water; N=12 for 50%
sea water; N=22 for 100% and 200% sea water; and N-24 for 0%
and 25% sea water.
3. Survival curves for I. resecata and I. montereyensis
in different temperatures. x-I. resecata, o-I. montereyensis.
Temperatures are expressed in C. Experiments were terminated
after 23 hours.
4. LT-50's for I. resecata, I. montereyensis, and I. stenops
in different temperatures. Empty bars represent I. resecata,
cross-hatched bars represent I. montereyensis, and stippled
bars represent I. stenops. Temperatures are expressed in C'.
N=6 for 22°; N=10 for 40°; N=16 for 20°; and N=20 for 35
30, and 15° except for the I. stenops bar at 15% for which N=16.
5. Respiration rate as a function of wet weight in I.
resecata and I. montereyensis at 15° C. x- I. resecata, o-
yensis. Animals used ranged from.0186 gm to
I. monter
2325 gm and respiration rates ranged from 3.60 to 20.27
microliters O,/hr.
6. Survival of I. resecata under low 0, tension as a function
of wet weight. Log survival time in minutes is plotted
against log wet weight in grams. Animals ranged in weight
from .0195 gm to .3705 gm, and survival times ranged from
20 min to 65 min. The regression line is not significantly
different from 0. Eight males and two females (denoted "F")
were used in the experiment.
7. LT-50's for I.resecata and I. montereyensis at low oxygen
tensions. LT-50 in minutes is plotted against initial O,
concentration in ml 0/1 sea water. Each point represents an
LT-50 for 10 animals. x- I. resecata, o- I. montereyensis.
The regression coefficients are significantly different at
the 1% level.
8. The left 7th pereaopod of I. montereyensis and I. resecata
in characteristic grasping positions.
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