Zonation in isopods. page 2
INTRODUCTION
Preliminary investigation of one end of a sand beach
at Hopkins Marine Station, Monterey Bay, California,
indicated that there were at least four species of ter-
restrial isopods living very close to each other in
apparently similar habitats. As the area did not appear
divisible into more than two zones, sand and iceplant,
I became interested in attempting to delineate the
environmental and behavioral factors that separated the
species. In the course of this investigation I looked
at tolerances to several stresses as well as the animalös
behavior in the field. Analysis of gut contents was
ttempted but proved largely inconclusive. As the study
was carried out over a limited period of time, replicate
sampling under similar conditions of moon, tide, and
weather were not possible.
The species studied were Alloniscus perconvexus
(Dana, 1054 ), A. cornutus (Budde-Lunde, 1885 ),
hiloscia richardsonae (Holmes and Gay, 1909),
Armadilloniscus lindahli (Richardson, 1905), A.
holmesi (Arcangeli, 1933 ), and Porcellio sp. The
later species was not studied extensively due to the
low numbers encountered. There remains some doubt as
to the identification of Armadilloniscus holmesi, as it
Zonation in isopods. page 3
apparently is very similar to A. coronacapitalis and
reference specimens were not available to allow conclu¬
sive identification.
There is almost no literature relevant to the
habitats of either these five species of isopod or
similar beach inhabiting forms. Hainsworth (1972)
has studied the behavior of A. perconvexus and Hayes
(1969 ) examined the ecology of the isopod Tylos
punctatus in Southern California. Miller (1930)
looked at comparative distributions of isopods relative
to water, but his results were rather general in nature.
Mulaik (1960 ) stated that Armadilloniscus and Philoscis
were commonly found together under wrack, although this
was not found in the present study.
THE HABITAT
The area studied was the rocky end of a sand beach
located on the SW corner of Cabrillo Pt., Monterey,
California. The beach was made up of unbroken sand to
a height of + 1.5 feet from O tide level; there followed
a narrow (about 3m) band of scattered rocks and finally
at the upper reaches of the beach, iceplant. (See fig.
1)
Wave action was moderately strong but waves reached
the iceplant only once during the study peiod. Such
Zonation
sopods. page
heavy surf was seen to change the level of sand b
almost ten inches in a single night.
Air temperature ranged from 7 to 27c and am
humidity from 35% R.H. to 100% R.H. during the stu
period; the results of one day's measurement of tei
erature and humidity under rocks are presented in
fig. 1. Humidity was measured with a Honeywell Re.
Humidity Readout Instrument which was found accura
3% when tested with saturated salt solu
measurements were used to indicate the ran
tolerance tests
Zonation in isopods. page 5
IYSIOLOGY
1) Temperature
Procedure: Ten to twenty animals of each species were
placed in petri dishes with a piece of wet towelling
to insure 100% R.H. The dishes were then left in
constant temperature boxes for 12 hours; mortality
at the end of this period was established by a lack of
response to probing. Tempenatures tested were 10, 20,
30, 35, and 10c.
Results: The results are shown in fig. 2a. Note that
the ranges of all species were very similar, with
Philoscia showing the least tolerance to temperature stress.
2) Relative Humidity
Procedure: Thirty animals of each species were placed
in individual cups to prevent clumping, which is possibly
a factor in preventing water loss (Cloudsley-Thompson,
1956 ). These were then placed on supports in covered
bowls of saturated salt solutions each giving a constant
R.H. The solutions and humidities utilized were:
98% R.H. - K,Cr,0
92% R.H. - KNO,
90% R.H. - Bacl,.2H20
Zonation in isopods. page 6
84% R.H. - KCl
100% R.H. - distilled H,O
The animals were examined after sufficient
time had passed for a sizeable number of animals to
die. The experiment was done at a room temperature of
20 - 25c.
Results: The results are presented in fig.2b. Note that
in order of increasing tolerance to dessication the ani-
mals are A. holmesi, A. lindahli, Philoscia, A. cornutus,
and A. perconvexus; also significant is that Philoscia
were in the 12 hour group. Although I did not test
Porcellio, they have been found by others to do quite well
for over a day at 80% R.H. (Fiedler, pers. comm.) which
is significantly longer than any of the other isopods
dealt with.
3) Submersion
Procedure: Fourty individuals of each species (note:
for the two species of Armadilloniscus only twenty ind-
ividuals were used ) were placed in open finger bowls
of sea water at room temperature. They were kept untill
almost all had died; death was again defined as a lack
of response to probing.
Results: The results are shown in fig.3. Note Philoscia's
low tolerance to submersion.
Zonation in isopods. page 7
FIELD STUDIES
1) Static Distribution
Procedure: A point was chosen which represented the lowest
rocks on the beach. This was at + 1.5 feet from the
0.O tidal level. Ten meter transects parrallel to the
water were then layed down at one meter intervals from
the base point to the upper beach, a horizontal distance
of 9m. All the rocks that were small enough to turn
over and were touching the line were examined and the
substrate sampled. The number of isopods present was
estimated in incremental units of 25 (ie. - 0, 25, 50...
etc. ). The total estimate for each level was then
corrected to represent a sample of ten rocks (actual
numbers of rocks ranged from 7 - 13 ). Finally, each
increment of 25 was assigned a value of 1; the final
result being a scale of relative abundance from 1 to 38.
At 2 - 5pm on the same day temperature and humidity
measurements were made under representative rocks along
the transects. These are illustrated in fig. 4.
Note that this distribution was measured several
days after an unnusually high tide, and it may not be
"typical".
Results: The results are illustrated in figure 45.
Armadilloniscus holmesi, Alloniscus cornutus, and Porcellio
sopods. page 8
Zonation in
appear restricted to specific zones, A. holmesi to th
lower beach and Alloniscus and Porcellio to the upper
On the other hand, Armadilloniscus lindahli and
Philoscia are found over a wider range, Philoscia
tending towards the upper beach while A. lindahli
appears most heavily in mid-beach. No Alloniscus
perconvexus were observed.
Note that with the exception of Philose
order in which species occur from low
corresponds exactl
ith the
dessicat
Zonation in isopods. page 9
2) Activity Patterns
Procedure: The activity patterns of the isopods were
studied using plastic cups ôcm accross sunk flush with
the surface of the sand, into which the isopods fell.
A total of 13 such traps were distributed at various
distances from the base point described earlier.
Several of these were in directional sets, in which
microscope slides set in the sand were used to block
approach from all but one direction. These were set in
pairs, one facing up the beach and the other toward the
water.
Traps were checked at two hour intervals and all the
isopods were identified and counted; the two species of
rmadilloniscus were usually treated together due to the
ficulty of accurate field identification.
di.
After counting the animals they were released about
Im away from the trap in order to minimize the effect of
trapping on the population. Experiments in which the
animals were marked with enamel paint showed that there
was no significant recapture of just - released individuals
using this method.
Observations were carried out during three nights
with a new moon, two with a full moon, and one with a
first quarter moon. On one of the full moon nights a
Zonation in isopods. page 10
heavy fog obscured the moon totally. Due to the lack
of a consistently reliable light meter, light intensity
was not measured. Temperature and humidity were moni¬
tored on most of the nights, but proved to be relatively
unimportant.
Results:
New Moon
May 3-4 (fig. 5 ) - Alloniscus and Philoscia both appear
to be strongly cued by the absence of light.
Armadilloniscus do not show such a response to light
and are active well into the morning; this is correlated
with the first direct sun on the beach and hence pos-
sibly dessication. None of the isopods appear to move
any significant distance from the areas in which they
are found during the day.
The data from May 29-31 (figs. 6 & 7) show the
effect of high tide on the animals. Rather than simply
shift their activity up the beach (Hainsworth, 1972),
the high water kept all species but the two Armadilloniscus
from appearing. The submergence of the zone 1 to om
above the base point on May 30-31 brought out a large
number of A. lindahli that did not show up the preceding
night.
First Quarter Moon
May 9-10 (fig. 8 ) - The most striking difference
Zonation in isopods. page 11
between this night and that of May 3-4, which has a
comparable tide but no moon, is the relative absence
of Alloniscus cornutus, while Philoscia and A.
erconvexus seem unaffected by the moon. The absence
of Armadilloniscus is probably related to the lower
high tide of that night.
Full Moon
May 16-18 (figs. 9 & 10 ) - The tides on these nights
were roughly comparable to that on the new moon of May
3-4, so presumably the major difference between the two
sets of nights was the amount of ambient light. In
this regard, note that on May 17-10 the moon was obscured
by fog, which seems to have made little difference.
The primary thing to note is the general similarity
between these two nights and the night of quarter moon,
as well as the close agreement between May 16-17 and 17-
18, in spite of the fact that the moon was not visible
on the second night. Note also that with a slightly
higher tide, Armadilloniscus have appeared in relatively
large numbers.
Directional Sampling
The results of the directional sampling are ambiguous,
and only selected data are presented (fig. 11 ). There
appears to be a tendency for Alloniscus cornutus to move
pods. page 12
Zonation in is
toward the water in the early night and to return up
the beach later on. Remember, though, that the population
as a whole did not seem to move (see figs. Ib and 5).
Philoscia was not caught in large enough numbers to make
any valid statements about, but any pattern in their
activity is fairly obscure.
The sample of Armadilloniscus is also quite small,
but does seem to indicate a pattern of movement sin
to that of A. cornutus. This is the opposite of w
nat
would be expected of a low-middle beach animal, for th
traps were situated near the upper boundary of the
range. They therefore would be expected to move
beach first, and later retur
ods. page 13
Zonation in
GUT CONTENTS
Procedure: Ten to thirty individuals of each species
were collected at night from an assortement of areas
(sand, rocks, wrack, grass, and iceplant as well as
traps ) and preserved in 80% alcohol. When possible
the animals collected were ones that had appeared to
be feeding. The gut was then dissected out, rehydrated,
fixed in Karo syrup and mounted on microscope slides.
They were then examined by myself and by Dr. I. A.
Abbott and the contents identified to whatever extent
possible (generally no farther than algae - vascular
plant - animal ). The relative proportions of the
material was estimated on a relative scale designation:
some, half, or most.
Results: A. perconvexus - Almost exclusively brown
algae, some other algae and vascular plant material.
This was true even for animals taken in the beachgrass
and iceplant.
A. cornutus - Evenly divided between algae,
vascular plant, and arthropod skeletons. The animal
matter was largely made up of insect pupae cases, and
made up most of the contents of several individuals.
Philoscia - Contained both insect pupae and
plant material, but very finely ground and difficult to
isopods. page 11
tion
Zor
see. Fair quantities of sand grains were also p
Armadilloniscus - There was no obvious
between the species. Both contained almost exclus
algae, much of it blue-green and simple green.
Porcellio - Almost exclusively ar-
exoskeletons, with some vascular plant.
Zonation in i
ds. page 15
CONC LUSION
Although there seems to be a relatively homogeneous
habitat, the isopods on the beach studied have all
adapted to particular physiological and behavioral niches.
Of the six species studied, only the two Armadilloniscus
have the same diet. Physically, the animals are distri¬
buted from low to high beach in accordance with their
tolerances to submersion and dessication, with one
exception (Philoscia ). Activity patterns are roughly
similar, all being primarily nocturnal, but within this
similarity there is great variation, with Alloniscus,
liloscia, and probably Porcellio responding negatively
to daylight itself while Armadilloniscus appears to be
responding to some other cue, possibly desication.
Moonlight affects the activity of Alloniscus cornutus and
possibly A. perconvexus (Hainsworth, 1972 ) but does
cia or Armadilloniscus. The
not alter that of Philos
activity of both these animals does appear to be related
to high surf, Philoscia responding negatively and
Armadilloniscus positively.
For simplicity, I will deal with the animals one
species at a time.
Zonation in isopods. pagel6
Alloniscus perconvexus
This animal is found between the high tide mark
and the iceplant, buried in sand near rocks. They
were not found in significant numbers here, and little
can be said about their activity. Other researchers
(Hainsworth, 1972 ) have found them to respond nega¬
tively to light, including moonlight. They are entirely
herbivorous, feeding mostly on brown algae (usually
Macrocystis).
A. cornutus
Also found on the upper beach, this isopod is more
closely associated with rocks, which may provide the more
constant high humidity that they recquire relative to
A. perconvexus. They are active only at night and appear
to remain inactive when the moon is out. This negative
response to light is found in both Alloniscus and possibly
Porcellio, to some extent in Philoscia (which ignores
the moon but responds to daylight ), and is allmost
absent in Armadilloniscus. A possible explanation for
this is that while Alloniscus are large, slow isopods,
Philoscia is very fast and Armadilloniscus are extremely
small. Small size or great speed are both good defenses
against most predation, especially that which is vision-
dependant. Therefore it seems possible that the negative
Zonation in isopods. page 17
photoresponse of Alloniscus is a means of protecting
the animals from predators which they could not escape
otherwise. Although no large predators were observed
on the beach at night, flocks of birds foraged on the
beach allmost every morning as soon as it was light,
A. cornutus also seems to respond negatively to
heavy surf, but I have no explanation for this, as they
were not active even in areas where there was no physical
threat to them of being washed out.
They travel to some extent, but probably go less than
1 meter in an average night. This is indicated by the
fact that while the populations as a whole do not move
noticeably, the directional traps found some evidence
for a definite cycle of movement during the night, down
the beach (to forage ?) and later back up.
Their diet is omnivorous but it is not known if they
are active predators or scavengers.
A. cornutus has evidently replaced the sandy beach
A. perconvexus in this rocky area, possibly because of
their greater tolerance for water (which is held in the
sand by the rocks) and/or their increased utilization
of vascular plants as food, represented by the beachgrass
common at the upper beach.
Zonation in isopods. page 18
Philoscia richardsonae
These animals are a puzzle. They are omnivores, as
are A.cornutus, but the appearance of the gut contents
is very different. They are active regardless of moon
phase but are highly inhibited by heavy surf. Although
tolerance tests found them the most delicate of the
animals studied, they have the widest physical range.
They are the fastest of the isopods studied.
A possible explanation for all this was first
proposed by Nat Howe (pers. comm. ). Simply, the
speed and mobility of Philoscia enables them to seek out
isolated microhabitats in an otherwise hostile environment.
This implies to some extent that they are existing on
this beach only marginally, and should be more common
elsewhere. A search of the neihboring coastline revealed
a very dense population of them underneath concrete rubble
on a beach near the Coast Guard pier in Monterey, about
one mile north of the study area. Although they were
not studied at this site, it should be noted that they
were in an extremely sheltered habitat, often beneath up
to two feet of loosely fitting rubble. From this evidence,
I believe that the study area on Cabrillo Pt. was not a
representative habitat for this animal.
Zonation in isopods. page 19
Armadilloniscus holmesi
This is a low beach herbivore, often submerged
by the tide. Their range does not overlap that of
any isopod but A. lindahli. They are active at night
or early morning when the sand has recently been sub¬
merged, which is a good time to graze on the microalgae
that makes up a good part of their diet (when the algae
is wet it will not be as tough or shrunken ). They
respond less to light than to humidity; this is probably
because their small size protects them from predation by
birds.
A. lindahl
These are found slightly higher on the beach than
A. holmesi, but feed on essentially the same things.
They are active only rarely, however, due to the infrequency
of very high tides. As herbivores need to eat fairly
regularly, this implies that they find a large part of
their food in the algae that is caught beneath racks.
Indeed, there were obvious deposits of dead algae under
almost every rock sheltering A. lindahli. Like A. holmesi
reatly sensitive to light. They range
they are notg
farther than A. holmesi when they are active; this may
be a mechanism of population dispersal, preventing pop¬
ulations under a given rock from stagnating.
isopods.
Zor
tior
ements
Acknowled
I would like to thank Dr. I. A. Abbott
assistance with the analysis of gut content:
Pratt for his field observations of A. perconvex
and Dr. Welton Lee for his advice and assistance
the writing of the paper.
Zonation in isopods.
Figure captions
Fig. 1
Beach profile from base point (0) to top of rise. The
horizontal axis represents the horizontal distance up the
beach; the vertical axis, the vertical rise in height
above O tide level.
Fig. 2a
Survival after 12 hours at 10, 20, 30, 35, and loc (100g
R. H. ). N for each species equals 10 - 20.
Fig. 2b -
Survival at various degrees of dessication. Times chosen
on the basis of casual observation of respective tolerances
to dessication. N started at 30 per point, but because
of escape from the holding cups and subsequent drowning.
n actually ranged from 4 - 28.
Fig. 3
Tolerance to submersion in sea water. The lines represent
number of animals (n - 10) left alive after a given
time interval.
Fig. la -
Temperature and R. H. measured under rocks every meter
up the beach from the base point. Temperature is rep¬
resented by the solid line, R.H. by the dotted line.
Fig. lb -
Relative numbers of animals at each meter above the base
point during midafternoon, when they are not active.
See text for explanation of vertical scale.
Figs. 5 - 10 - These figures show the number of animals (scale
in lower right) at any time (horizontal scale) and
distance above the base point (left vertical scale).
Zonation in isopods.
The dotted line represents the farthest a wave came up
the beach in a five minute interval. The dark bar on
the horizontal axis corresponds to night, and the small
bars on figs. 8 and 9 represent moon rise and set.
Fig. 11 -
Results of directional sampling. The vertical axis
represent numbers of animals; the horizontal, time of
night. Note that traps faceing up the beach are catching
animals comeing down, and vice versa.
Zonation in isopods.
Literature cited
Cloudsley-Thompson, J. L. (1956). Studies in diurnal rythms, VI:
Humidity responses and nocturnal activity in woodlice (Isopoda).
J. Expt. Biol. 33:576.
Hainsworth, B. (1972).
Hayes, W. B. (1969 ). Ecological studies on the high beach isopod
Tylos punctatus Holmes and Gay. Unpublished Ph. D. thesis.
University of California at San Diego.
Miller, M. A. (1938 ). Comparative ecological studies of Terrestrial
Isopod Crustacea of the San Francisco Bay region. Univ. Calif.
Pub. Zool. 13:113-112.
Mulaik, S. B. (1960 ). Contribucion al conocimiento de los Isopodos
terrestres de Mexico (Isopoda, Oniscoidea). Rev. Soc. Mex. Hist.
nat. 21 1:79-292.
1o)
FG

) AOSV
01
W
O RO
O
o
FIG 25
100
75
50
FG 2a
- Philoscia
7- A. perconvexus
o-A. lindahli
/- A. cornutus
A-A. holmesi

.

12 hrs.
24 hrs.
100 98 92 90
84
100 98 92 90 84
%o REL. HUMIDITY
A. perconvexus
100
+
OA. cornutus
* Philoscia
• A. lindahli
12 hrs.
10
30 35
40
20
TEMP. (°c)
(100 % R.H.)
1e
FIG. 3
/40
PD
+
3

—.

=

— —
e
=
:
e


2
NUMBER ALIVE

NUMBER
en








2
16




L
—



——
L

EG 28
24
e
a



STATICLDISTRIBUTION
8
2 3
4
DISTANCE FROMBASE
2/e
AIR

1
(M)
TANCE
FRON BASE
Aholn
lindahli
hiloscia
Aconnutus
orcelig
100%
R. H.
I
5

8991
758
627
506
495
324
2-3
22
ACTIVITY
5/3-4/13
NEW MOON
Philoscia
A. perconvexus
Armadilloniscus spp.
A.cornutus
Porcellio sp.
H
e
815
8
939


Dios
Ogs
039
1
LE
HH
L
a
936
932
L

WAVE HT.
Im
m
Ipm 9I
lam
TIME

15
105
Hot
7-8
6-1
5-6
4-5
L
+ 3-4
2-3
1-2
ACTIVITY
5/29—30/73 NEW MOON
Philoscia
A. perconvexus
Armadilloniscus spp.
A.cornutus
Porcellio sp.

1
E


3
234
LE L
I
1
La


WAVE
HT.
M

I
II
M
m
II
I
m
9pm
lam
TIME
10
17


758
67
50
455
354
2-3
22

ie
ACTIVITY
5/30—31/73 NEW MOON
A. perconvexus
Philoscia
A.cornutus
Armadilloniseus spp.
Porcellio sp.
026
1
L
I

AVE
HI.


II


M
5
TIME
9
11
105
1162

528
455
334
2-3
2

79
5/9-10/73 QUARTER MOON
ACTIVITV
Philescia
A. perconvexus
Armadilloniscus spp.
A.cornutus
Porcellio sp.
L


224

I
L
O2
030
L


E I
15
05
4
12
8
10 12
10
TIME
637
50
4
354
2-3
22
ACTIVITY 5/16-17/73 FULL MOON
Philoscia
A. perconvexus
Armadilloniscus spp.
A.cornutus
Porcellio sp.

A
H
938
342
28

H

48
E
E T I I
11
E

E
TIME
11
5
105
31
637
56
455
354
2-3

feggedse

ACTIVITY 5/17-18/73 FULL MOON (FOG)
Philoscia
4 A. perconvexus
A.cornutus
Armadilloniscus spp.
Porcellio sp.
05
ar

I
20
945
929
EE
E
9
935
923

I

E
E



21
2
TIME

05
6g1
8


8
1
19
U
o
O
V
1
1
0
0
O
111
8



9
1
0)
O
O
1
—
1
10