ABSTRACT
The range and distribution of the beach isopod Alloniscus perconvexus
was found to change over time. Compression and expansion of the
range was observed to coincide with spring and neap tides, respectively.
The upper limit of the distribution seemed to be set by moisture
content of the sand, whereas extreme tidal height appeared to
delimit the lower limits of the isopod's range. Range expansion
and compression takes place by nighttime activity patterns. While
the organism appears to be highly sensitive to the moisture content
of the sand, and moves in direct relationship to moisture gradients,
its activity can be modified by the presence of light, which
inhibits all movement, and by a thigmotactic response which is
often enhanced by the presence of potential food. Wave action
and physical disturbance may reinforce those factors delineating
its lower limits on the beach.
DISTRIBUTION OF THE TERRESTRIAL ISOPOD ALLONISCUS PEROONVEXUS ON A
SANDY BEACH AND FACTORS AFFECTING ITS RANGE AND POPULATION DENSITI
INTRODUCTION
The isopod Alloniscus perconvexus (Dana, 1854) has been noted
as a common inhabitant of sandy beaches by several authors (Van Name,
1936; Light et al, 1954; and Richardson, 1905). However, little else
is known of its biology other than its tolerances to salinity which
was examined by Brusca, 1965. The present study was undertaken to
determine the distribution of this isopod on a typical sandy beach,
and to examine various physical parameters that might affect its
range, activity, and population density. Special attention was
payed to the influence of moisture content of the sand, tidal height,
and the availability of food. Furthermore, laboratory investigations
were conducted to determine responses to light, moisture gradients,
and to investigate possible thigmotaxis and food preference.
MATERIALS AND METHODS
Study Area
The distribution of Alloniscus was studied on a pocket beach
on the southeast side of Point Cabrillo in Pacific Grove, California.
A 15-meter wide section of this beach that was relatively uniform in
appearance was intensively studied. The beach was semi-exposed, with
a slope of approximately .21 meters tidal height. The approximate
sand size throughout this section showed a Mdo of -.1. There was
a belt of beach grass immediately above the berm, and a large bank
of iceplant at the head of the beach. No small rocks were present
on the surface and few were buried beneath the sand.
Distribution
Ten transects approximately 1.5 meters apart were studied under
differing conditions of low and high tide, during both night and day.
Investigations were made between April 19, 1972 and May 23, 1972.
Cores were taken every .75 meters along each transect. The corer
(Fig. 1A) was an aluminum box, 25.08 cm x 25.08 cm x 28.57 cm, with
two open ends. Four metal strips 24.45 cm x 5.08 cm were attached
to the top of the corer to allow the box to be easily withdrawn
from the sand. The corer was hammered into the sand, which was
subsequently scooped out with a trowel into a sieve with a screen
size of 3 mmx 3 mm. At each station, for convenience in handling,
the core was divided into two sections. The top 10 cm of sand were
sifted first, and the number of A. perconvexus and quantity of algae
and large pieces of detritus recorded. The remaining 15 cm of the
core was subsequently treated similarly. The sampling along a transect
was ended when no Alloniscus had been obtained from two consecutive
cores.
The distribution of isopods moving on the surface of the sand
during the night was likewise studied. Traps were set along a single
transect on each of two nights. In one study the traps were set
.75 meters apart, and in another, 1.5 meters apart. The traps were
glass bowls 5.0 cm in diameter and 10.2 cm deep, positioned in the
sand so that the top of the bowl was level with the surface. Organisms
walking on the sand fell into the bowls and were unable to climb up
the glass sides.
Physical Factors
To find those factors that might most readily affect the range
and population density of Alloniscus, environmental parameters were
likewise measured in the sample area. Moisture content of the sand,
temperature, presence of food on the surface, presence of objects
in the sand, and sand size were monitored for many of the transects
studied.
When a given tide was at its highest or lowest point, samples
of sand were taken every five feet along a single transect, both
at the surface and 10 cm below. The moisture content of the sand
was calculated by weight loss after drying to constant weight.
Temperature was also recorded with standard Centigrade ther-
mometers at the sand surface and 10 cm below, every 1.5 meters
along the transect. Finally, the presence of grass or detritus on
the surface and the quantity of objects in the sand were generally
observed and recorded. The sand size was determined by dry sieving
through a series of standard Tyler screens according to the methods
of Morgans, 1956.
Experimental
Experiments were conducted on the isopods to test the effects
of moisture, light, and the presence of objects and food in the sand.
A gradient box made of plexiglas (Fig. 1B), devised by Mr. Samuel
Johnson at Hopkins Marine Station, was utilized. Four pieces
(42.55 cm x 47.63 cm, 42.55 cm x 47.63 cm, 10.16 cm x 45.08 cm,
and 10.16 cm x 42.55 cm) were fastened together to form an open¬
ended box. The two smaller side-pieces were attached so that one
was at the same level as the top and the other 2.5 cm below the top.
Grooves were cut in the 42.55 cm x 47.63 cm sides, so slats serving
as top and bottom could be slid into place (dimensions 45.72 cmx
10.79 cm). Grooves were also cut, perpendicular to the top and
bottom, dividing the large sides into four. Three 41.59 cm x 10.48 cm
slats could then be slid into these grooves, creating four separate
compartments.
Moisture Preference
The isopods were first tested for moisture preferences. Sand
from the study area was completely dried in an oven at 80° C. for
approximately 24 hours, and then mixed with sea water to arrive at
the desired moisture level. The weight of 1 liters of sand was
approximately 733 grams, thus 43.98 ml sea water were added for
every 2% increase in moisture content. A gradient of O, 28, 48,
and 10 moisture content was set up, using 1 liters of sand per
compartment. 10 large Alloniscus (11 mm - 16 mm) were placed in
each section and allowed to burrow. The partitions were removed,
the sand smoothed, and the top slat slid into place to prevent
evaporation. A control box was similarly set up with 2% moisture
content throughout. The two boxes were then covered with black
plastic sheeting and left overnight at room temperature for 15 hours.
At the termination of the experiment, partitions were hammered into
the sand. Each compartment was then sifted separately and the isopods
recovered. After completion of the experiment, the moisture content
of each compartment was determined once again as noted above.
Two additional sets of moisture gradients were set up, with O,
48, &, 12, and 10, 12, 1%, 16% moisture content, respectively.
Thigmotaxis
To measure thigmotactic responses, a gradient of O, 13, 8.
and 12% moisture content was again set up, with 4 Macrocystis floats
half buried in the sand in both the O and compartments. A moisture
gradient containing no algae was used as a control.
Salinity - Light
Two variations of the moisture gradient O, 28, 48, and 10% were
conducted, to test the effects of salinity and light. In one experiment
distilled water rather than sea water was utilized to moisten the sand
in both experimental and control boxes. In another experiment, using
the same gradient, the two boxes were kept under constant illumination
from an overhead light. 20 large isopods (11 mm - 16 mm) and 20 small
isopods (6 mm - 8 mm) were used per box.
Observations were next conducted to test the effects of light
and dark on large and small Alloniscus. Four plexiglas boxes were
filled with sand of 2% moisture content. 40 isopods were placed
in each, two containing large and two with small organisms. Two of
the boxes were placed under a bright lamp, and the others outside in
the dark. The numbers of isopods remaining on the surface were
recorded every hour.
Food
A test was made to verify the suspected food of A. perconvexus.
Dead Orchestoidea californiana and pieces of the algae Macrocystis
were soaked in the vital dye Rose Bengal. They were then drained,
blotted, and put on the surface of moist sand in separate glass
bowls. 10 large isopods which had been kept in the laboratory for
two days were introduced in each bowl and placed in the dark. The
bodies of the isopods were examined 24 hours later for the presence
of dye.
A final experiment was conducted to identify possible food
preferences. Three plexiglas boxes were filled with 14 liters of
sand with 4% water content. In one box, one fourth of the surface
of each compartment was covered by a different type of dry wrack:
Cystoseira, Gigartina corymbifera, Macrocystis, and Phyllospadix.
The second box was similarly filled with damp wrack. The third
served as a control, containing no algae.
RESULTS
Distribution
The range and population density changed appreciably during
the study as indicated in Figure 2. The transects on April 25
and April 28 were studied after a period of high tides and strong
winds; waves had reached tidal heights of 4.8 m during this time.
The range of A. perconvexus during these two studies was relatively
short, with large numbers of individuals concentrated over a small
area. On the other hand, the distributions on May 9, May 17, and
May 22 indicated a greater range and a shift in relative numbers.
The waves at this time reached tidal heights of only 1.2 to 3.5 m.
The weather had been much calmer and the waves had not come so far
up the beach previous to these dates. However, on May 27 water
came to the 4.86 m tidal height. The range in this transect was
once again highly restricted.
In contrast, the four distributions shown in Figure 3 indicate
that the range of Alloniscus does not change greatly with the four
tides occurring during any given 24-hour period.
Activity
The activity pattern of Alloniscus was also observed on this
beach. During the day, the isopod was found burrowed in the sand,
a large majority (758) within the top 10 cm of the surface. On
dark nights, many came to the top but their behavior could not be
observed directly. The isopods stopped movement immediately when
illuminated with a beam of white light from a flashlight. Red light
produced by taping red cellophane over the flashlight brought about
the same response.
Traps were found to be the best way to determine nighttime
activity. Results shown in Figure 4A were obtained April 22,
during the period of higher water described earlier. Note that
activity was confined to the upper beach with 32% of the isopods
caught between tidal heights 5.7 m to 6.0 m. However, there was
a striking contrast between these results and those shown in Figure 4B,
taken on May 22 after a period of lower water. No isopods at all
were collected between tidal heights 5.7 mto 6.0 m and the isopods
were seen to be active down to a tidal height of 3.5 m.
An indication of their aversion to light can be obtained from
Figure AB. During this study period, the moon was shining brightly
until 11:45 PM. Clouds then covered the moon and the surroundings
became much darker. Only six isopods were caught before 12:15 AM,
whereas 167 in total were obtained from 12:15 AM to 5:15 AM.
Similarly, only 3 Alloniscus were caught from 5:15 AM to 6:15 AM,
when the sky was gradually lightening.
Physical Parameters
Sand moisture was next studied along each transect. The
surface tended to be consistently dry (below .5% moisture) whereas
distinct differences were noted along the transects 10 cm below the
surface. In Figure 5, moisture content 10 cm below was plotted against
the four distributions taken on May 1, May 17, and the two taken on
May 22. The range in which Alloniscus occurs had a water content of
.65% to 7.3%. A marked difference was once again evident between the
distribution of May 1 and the later four study periods. The range
and population density shifted down the beach as the moisture content
dropped. On the transect taken on May 1, the percent water at tidal
height 4.8 m was 4.34%, whereas the average at this point for the later
distributions was only 2.218. Thus, the distribution of Alloniscus
appeared to change with fluctuations in moisture content. Note
also, that at low tides, Alloniscus appeared in greatest densities
in sand of approximately 3% moisture content.
Field observations indicated that Alloniscus seemed to
congregate in areas in which much algae was buried beneath the
surface. As many as 42 organisms were found at a single such
location. In Figure 3, the first transect contained a large amount
of buried material at 4.5 mtidal height. The next three transects
had a scanty amount at this level, and correspondingly fewer Alloniscus
were collected.
The isopods were found in sand with temperatures 10 cm below
ranging from 21.2° C. to 13.2° C. On any single study period,
however, there were no drastic fluctuations along the transect,
but rather gradual changes (Figure 6). The temperature at the
surface showed more drastic changes, with ranges of 27.50 C. to
9.100.
Sand size on the surface and 10 cm below did not change
drastically along a transect in this section of the beach. Along
one transect, from 5.8 m to 3.5 m tidal height, Md o 10 cm below
ranged from.4 to.6.
Experimental
Typical results of gradient experiments designed to determine
preferred moisture content showed that Alloniscus congregated to a
large extent in the wettest portion (Table 1). Using the chi square
test, such a distribution was significant to the 98 confidence level.
The average moisture content in the four compartments after 15 hours
was .218, 2.93, 5.00%, and 7.42%, while the control was 1.90%, 1.768,
1.90, and 1.878.
Similar trends of orientation toward the wettest sand were also
found with large isopods tested in moisture gradients of G, 18, 88,
and 124 (Table 2). Again the results were significant to the 905
confidence level. The former experiment was duplicated by using
40 small A. perconvexus (6 mm - 8 mm). These smaller animals also
moved toward the wettest portion (Table 3). Chi square tests showed
this distribution not to be significant, due to an extreme bias in
the control. In the controls, the isopods showed a tendency to cluster
in one or both of the end compartments. When this data was compared
with an idealized control of 10 isopods per compartment, significance
to the 95% confidence level was obtained. The control shown in
Table 2 also exhibited a large degree of clustering. This was
considered to be another indication of thigmotaxis. All of the
controls except those run in the light appear to be biased by the
tendency of animals to cluster in the corners.
Small isopods placed in a gradient of 105, 12%, 14%, and 16%
showed little tendency to move (Table 4). While the observation
was made that Alloniscus can survive for several hours submerged in
sea water, it is suggested that these moisture contents may be well
above those normally encountered by the species and activity may be
reduced in saturated or near saturated sand.
Results from the experiment designed to test for a thigmotactic
response (Table 5) showed that more isopods congregated in the 4%
sand containing Macrocystis floats than in the control. This
distribution was significant to the 99.9 confidence level.
Results were drastically different between gradients run in the
dark and light. Those run in the dark showed the greatest number
of organisms occurred in the sand of highest moisture content (10)
and thus was significant to the 99% confidence level. A similar
experiment run in the Might, however, showed no significant change
in distribution (Table 6). The isopods showed considerable movement
in the dark, but much less tendency to move in the light. The boxes
in the dark had approximately 60 holes on the surface, indicating
that many of the isopods had come to the surface. Those boxes under
constant illumination, however, had only 5 holes. Some organisms
may have emerged from sand with O moisture content, but the sand
was so dry that no trace of any holes was left. In the control run
in the light, the typical movement toward one of the end compartments
did not occur, while the control in the dark once again showed a
a tendency of the animals to aggregate at the corners.
The observations made of activity patterns in organisms moving
over the surface of the sand were scored every hour in boxes placed
in the light and dark are recorded in Figure 7. It is significant
that no isopods were seen on the surface in the box kept under
constant light, while in the dark, approximately t of those originally
placed in the box were observed on the surface every hour.
- 12
Food
Two types of food of A. perconvexus were found: dead O. californiana
and pieces of Macrocystis. The isopods ingested these dyed materials,
turning their intestines bright red.
The experiment to find food preferences was not successful.
The isopods did not tend to cluster in the compartment that held
their "preferred" food. Large holes were seen in the pieces of
Macrocystis, however.
DISCUSSION
The distribution of Alloniscus was found to shift in relation
to tidal height. The upper limit of the population was quite stable
at a tidal height of about 5.6 m, but the lower limit changed
drastically with time. The range appears to decrease greatly
during spring tides when waves reach the upper portions of the beach.
Higher water caused a restriction of the population to the upper
beach, while a period of low calm water enabled the range to be
expanded to a tidal height as low as 3.6 m during the period of study,
It is not known, however, whether this represents the lowest possible
range of this species. While distribution of Alloniscus appears to be
very much influenced by long term cycles of spring and neap tides,
the populations do not move greatly during a single 24 hour period.
Moisture content of sand was also believed to play an important
role in setting an upper limit for the distribution of these isopods.
They were not found at a moisture level lower than .65%. No isopods
occurred above 5.6 m tidal height, which was found on one occasion
-1
to have a water content as low as.1% and never above.8.
Correspondingly, in laboratory experiments, definite movement was
observed away from sand with little or no moisture content. Sand
moisture content at the isopod's lower limit was less constant,
varying from 2.75% to 7.3%. However, the majority of Alloniscus
were almost always found in sand with a 28 to 3% moisture content,
The presence of detritus and objects in the sand had a great
effect on population density, both in the field and in laboratory
experiments. Large numbers congregated where many pieces of large
damp algae were found buried within the top 10 centimeters. This
seemed to indicate either thigmotactic behavior or attraction to
a food source. However, it was not possible to determine if the
organisms could eat while burrowed in the sand.
Sand size was not considered to be a significant factor in
determining distribution.
Experiments in the laboratory confirmed trends noted in the
field but also raised several questions. In moisture gradients, the
isopods consistently moved out of the  (.218) compartment, in
accordance with field data. On the other hand, in the gradient
boxes the isopods congregated at a greater moisture level than they
were normally associated with on the beach. Apparently other parameters
in the field inhibit this movement toward higher water contents.
Wave action, physical disturbance, and the presence of detritus
might be important variables in this regard.
Evidence was obtained to indicate that the isopods were positively
thigmotactic. No only did they cluster in areas containing large
pieces of algae buried in the sand, but in addition, in many controls
run in the laboratory experiments, there was a tendency for the
animals to group in the end compartments. While food studies
indicated that A. perconvexus was a scavenger, eating surface detritus
including wrack and dead Orchestoidea, it was not possible to
distinguish between a positive thigmotactic response and attraction
to food.
Changes in salinity had no effect on the distribution and
movements seen in the experiments conducted, and it did not appear
as an important factor in the field studies either.
Light had a great influence on the isopod's behavior. In the
illuminated gradient test, very few individuals came to the surface,
and comparatively little movement occurred. Field studies also
indicated that Alloniscus were negatively phototactic. They came
to the surface only on dark nights; moonlight was sufficient to
inhibit their emergence.
In summary, the distribution of A. perconvexus was found to
change greatly with time. The upper limit of their range was determined
by moisture content of the sand, whereas the lower limit seemed to
be set by tidal height and the height of the high tides of the
spring and neap tides. Population density resulted from movements
along the beach. Spring tides caused a compression of the range
and a consequent increase in population density in the upper beach.
The lower neap tides allowed the range to expand down the beach,
in general reducing the density at any given point. Several factors
appear to modify this general pattern of distribution. The isopods
cluster in areas containing large quantities of buried algae. This
is possibly due to a positive thigmotactic response, but may also
involve reinforcement through the presence of potential food material.
Vertical movement was also observed to be modified, since Alloniscus
came to the top of the sand only during the dark. The isopods were
very negatively phototactic, and even sensitive to moonlight.
ACKNOWLEDGMENTS
The author was very grateful to Dr. Welton Lee of Hopkins Marine
Station for invaluable assistance during the course of the study
and for critical reading of the manuscript.
8 Moisture 06 26 12 104
O 5 6 29
Experimental
Control
12 9 7 12
TABLE 1: Moisture gradient run with large Alloniscus.
Controls contained sand at 2 moisture content.
10 animals were placed in each compartment. The
number remaining was scored after 15 hours in
the dark.
2 Moisture
OB 16 85 12%
Experimental
O 3 10 27
Control
11 4 5 20
TABLE 2: Moisture gradient run with large Alloniscus.
Controls contained sand at 4% moisture content.
10 animals were placed in each compartment. The
number remaining was scored after 15 hours in
the dark.
8 Moisture 06 2% 12 105
O 1 8 31
Experimental
3 3 6 28
Control
Moisture gradient run with small Alloniscus.
TABLE 3:
Controls contained sand at 2 moisture content.
The
10 animals were placed in each compartment.
number remaining was scored after 15 hours in
the dark.
2 Moisture
106 12% 118 166
Experimental
9 10
6 9
2 10 9 16
Control
TABLE 4: Moisture gradient run with small Alloniscus.
Controls contained sand at 12% moisture content.
10 animals were placed in each compartment. The
number remaining was scored after 15 hours in
the dark.
2 Moisture 05  algae 1% algae
86
124
22
Experimental
18
12
Control
TABLE 5: Possible thigmotaxis. Pieces of algae were mixed in
the O and 4% compartments in a gradient. Controls
contained sand with a , 4%, 8%, and 12% moisture
content but no algae. 10 animals were placed in each
compartment. The number remaining was scored after
15 hours in the dark.
2 Moisture
O 27 16 106
2 11 10 17
Experimental
A Light
10 10 10 10
Control
O 4 10 26
Experimental
B Dark
Control
13 10 9 8
TABLE 6: Moisture gradients run in the light and dark.
Controls contained sand at 2% moisture content.
10 animals, half large and half small, were
placed in each compartment. In 6A, the boxes
were placed in the light, whereas they were
in constant darkness for 6B. The number remaining
was scored after 15 hours.
CAPTIONS TO FIGURES
Figure lA: Corer used for studies of distribution
1B: Gradient box. Grooves on one side indicated by dotted lines.
Distribution of A. perconvexus in numbers per.0156 cubic
Figure 2:
meters along 6 transects studied on April 25, April 28,
May 1, May 9, May 17, and May 28. The intertidal height
of the highest waves recorded during each study period
is indicated by a horizontal line. The arrow on the last
transect (May 28) indicates the height of waves the previous
night. Numbers were determined by coring every .75 meters
along each transect.
The distribution of A. perconvexus in numbers per.0156
Figure 3:
cubic meters as observed over a 24-hour period on May 22
to May 23. The intertidal height of the highest waves
recorded during each study period is indicated by a
horizontal line. Numbers were determined by coring
every.75 meters along each transect.
Numbers of Alloniscus caught in traps set on May 18 and
Figure 4:
May 22 from 8:00 PM - 7:00 AM. Collections were taken
every hour.
4A: Traps were set every .75 meters along the transect to a
tidal height of 5.3 m.
AB: Traps were set every 1.5 meters along the transect to
a tidal height of 3.2 m. The moon was shining until
11:45 PM, when it was covered by clouds.
Figure 5: Moisture content 10 cm below surface and the distribution
of Alloniscus determined on May 1, May 17, and May 22.
Figure 6: Temperature taken 10 cm below the surface of the sand
on the four transects studied on May 22 and May 23.
The dotted lines show low tide. The solid lines
indicate high tide. Circles show sites where the temperature
was taken.
Numbers of Alloniscus on the surface of the sand in
Figure 7:
artificial containers kept in constant light and constant
dark. Visual counts were made every hour from 8:30 PM to
6:30 AM.
5 08 cm
—
28 57 cm
125 08 cm
25 08cm
10 16 cm
L
4763cm
---

42 55 cm
APRIL 25
LOW TIDE
2:24 PM
APRIL28
MAY I
HIGHTIDE
LOW TIDE
J0:04 PM 7:16 AM
KEY
— - 10 Individuals
MAY 9
HIGHTIDE
8:28 PM
MAY17
HIGH TIDE
120PM
MAY 28
HIGH TIDE
11O5 PM
V
—
54
MAY 22
LOW TIDE
1:14 PM
KEY
— = 5 Individuols
MAY 22
HIGHTIDE
8:03 PM
MAY 2
LOW TIDE
2:43 AM
MAY 2
HIGH TIDE
9:00 AM
9
APRIL18
800 PM -
12:00 AM¬
7:OO AM
12:00 AM
KEY:
— =10 hdividuois
MAY 22
8:0O PM-
12:0O AM¬
7:O0 AM
12:00 AM
MOONLIGHT NO MOONLIGHT
41
3
6.
6
taatattta-

12 3 4 5 6 7 8
atataaaata-
L
5
% MOISTURE CONTENT OF SAND
MAYI
LOW TIDE
7:16 AM
MAY 17
HIGH TIDE
4:20 PM
MAY 22
LOWTIDE
kI4 PM
MAY 22
HIGHTIDE
8:03 PM

KEY
— 25 Individuals
5
MAY 23 MAY 23
MAY 22
MAY 22
8:03 PM
9:OO AM 2:40 AM
1:14 PM



1
10 11 12 13 14 15 16 17 18 19 20 21 22 23
TEMPERATURE (°C.)
20
10
80
70
60
50
40
30
20
LIGHT
8:30 PM 9:30PM 10:30PM II:3OPM 12:30AM 1:30AM 2:30AM 3:30AM 4:30AM 5:30AM 6:30AN
DARK
L
8:30 PM 9:30 PM 10-30 PM II:3O PM 12:30AM 1:30 AN 2:30 AM 3:30AM 4:3OAM 5:3OAM 6:30AM
REFERENCES CITED
Brusca, Gary. 1965. Studies on salinity and humidity tolerences
of 5 species of isopods in transition from marine to terrestrial
life. Bull. So. Calif. Acad. Sci. 65: 146-154.
Light et al. 1964. Intertidal Invertebrates of the Central
California Coast. University of California Press, Berkeley,
pp. 151, 155, 363.
Morgans, J.F.C. 1956. Notes on the analysis of shallow-water soft
substrata. J. Anim. Ecol. 25: 367-387.
Richardson, H. 1905. A monograph of isopods of North America.
Bull. U3 Natl. Mus. 54: 596-597.
Van Name, Willard G. 1936. The American land and fresh-water isopod
Crustacea. Bull Am Mus. Nat. Hist. 71: 215-217.