Vertical Distribution of L. scutulata
Merchant, Noroian
Introduction
The small checkered periwinkle, Littorina scutulata
(Gould, 1849), is prevalent in the upper intertidal zone of
the California coast. It is found to inhabit tidepools as
well as cracks and rock faces in this zone.
Previous work on movement and migration of L. scutulata
is not abundant. Hewatt (1937), demonstrated negative photo-
taxis in this species. North (1954), observed that differ-
ences in the degree of exposure to wave action appear to
influence distribution of the animal within intertidal zones.
Glynn (1965), further observed that vertical movements in
the Balanus-Endocladia zone are made with the rising and
falling tides, upward movement coinciding with the advancing
tide.
Preliminary field observations of L. scutulata in tide-
pools suggested the existence of rhythmicity in migration
into and out of these pools. The purpose of this study was
to obtain a clearer picture of vertical distribution in
tidepools over time and to look for possible factors influ¬
encing this aspect of behavior.
Study Areas
Observations in the field were carried out at Mussel
Point, Pacific Grove, California. Two areas of differing
Vertical Distribution of L. scutulata
Merchant, Noroian
geography providing different degrees of exposure to incoming
tides were studied. In each area two pools were observed.
See Figures 1 and 2.
Area I is located on the north side of the Point on a
flat, horizontal rock surface. There are numerous large rocks
in the lower intertidal region which act as a break and
greatly lessen the impact of waves at high tides. Waves tend
to roll over the pools, not crash over them. Area II is on
the west shore of the Point and is exposed to much stronger
waves then Area I. The pools are on the seaward edge of a
horizontal plateau. They receive much spray and splash at
high tides.
Pools 1 and 2, located in Area I,are connected at high
tides by a narrow channel which fills with water. Pools 3
and 4, in Area II, are adjacent and separated by a large crack.
Pool 4 is connected with another pool at one end. All pools
are granite, covered with a thin film of algae. Macroscopic
algae is found only in Pool 1 which has two small bunches
of Rhodoglossum affine. There are no rocks or sediment in
the pools and their water is not turbid.
The pools vary greatly in depth. Pool 2 is the shallow¬
est with a maximum depth of 3 cm. Pool 1 is the deepest with
a maximum depth of 8 cm. Pools 3 and 4 have intermediate
depths of 5 cm and 7 cm respectively.
Two studies were carried out in Areas I and II. The
first consisted of two 60 hour observations of gross
population movement. The second was a 53 hour investigation
Vertical Distribution of L. scutulata
Merchant, Noroian
of the movement of individual snails.
Materials and Methods
A photographic sampling technique was used. A one-half
meter square quadrat, divided into one-hundred 5 cm x 5 cm
squares, was placed over a pool in a stadardized position. A
photograph was then taken on Ecktachrome64 color positive
film using a Nikon F2AS camera and either a 105 mm or a 55 mm
Nikkor macro lens. To minimize parallax the photograph was
taken from the same position each time. At night a Vivitar
292 electronic strobe with a duration of approximately
0.OOl seconds was used as a light source. Slides were
projected on a screen with a Kodak slide projector using a
Kodak Ektanar zoom lens. Counts were made from the projected
image.
The photographic sampling technique was chosen for two
reasons. First, it minimized disturbance of the snails and
the area, especially in terms of exposure to light during the
night. Second, it recorded distribution at a point in time
rather than over the period of time necessary for observations
by eye. While replicate counts of the slides significantly
differed between the two investigators, their combined counts
give an error of less than 6% at the 95% confidence level.
A possible problem with the method is the inability to
resolve small snails, biasing the study towards the behavior
of larger individuals.
White fingernail polish was used to mark snails with
Vertical Distribution of L. scutulata
Merchant, Noroian
numbers or dots. A toothpick was used to apply the nail
polish. Only the largest snails, greater than or equal to
O.5 cm, were used.
Results
The first 60 hour study was done April 28-30, 1978;
the second was done May 16-18. The two studies were per-
formed 18 days apart in efforts to separate the tidal and
diel components of the changes in L. scutulata distribution.
The weather and surf conditions were also different. The
first study period was balmy and overcast until late afternoon.
The surf was mild. The second study period was clear and warm
but the surf was high, calming down slightly the second day.
The fluctuations in population distribution were
monitored between two categories, In Pool and Out of Pool.
In Pool indicates snails which are completely submerged.
The Out of Pool category includes snails completely out of
water and at the air-water interface. These categories
were selected because fluctuations between them appear to be
the sharpest change in behavior which the snails exhibit.
Large fluctuations in both distribution and total
population size of L. scutulata were observed in all of the
pools during both observation periods. The results for the
individual pools are presented in Figures 3 and 4. The
percent of the population Out of Pool is shown by the solid
line. The majority of the individuals in the Out of Pool
classification were found at the interface. The dashed line
Vertical Distribution of L. scutulata
Merchant, Noroian
represents the population size. Notes on the amount of
splash and wash throughout the observation period were only
collected during observation period two.
Three generalizations can be made about the fluctuation
in percent Out of Pool. These hold for all pools and both
observation periods. First, there is consistently a large
percentage of snails Out of Pool during descending tides at
night. Second, there is consistently a small percentage Out
of Pool during daytime low tides. Third, more snails tend
to be Out of Pool during the night than during the day. Large
percentages are also frequently found, but not consistently,
Out of Pool at ascending and high tides. They are seldom
found at daytime descending tides.
Each pool has its own pattern of fluctuations. The
snails in Pool 1 come out of the pool with the receding tide
at night and stay out. Most have returned to the pool by
the first daytime low or receding tide.
Pool 2 snails show similar behavior except that a small
percentage migrate into the pool at the nighttime low only to
come out again as the tide comes in. During the second
observation period a very interesting aberration occurs when
the percent Out of Pool peaks sharply at the afternoon
ascending tide. Pool 3 snails exhibit behavior very similar
to that of snails in Pool 2 during the second observation
period; however, it is less regular. Pool 4 shows the most
aberrant behavior. Distribution fluctuates with little
regularity during the first observation period and with
Vertical Distribution of L. scutulata
Merchant, Noroian
regular peaks at descending tides during the second observa¬
tion period. When the data for all the pools are combined,
what appears to be regular movement out of the pool at night
and into the pool during the day is apparent. See Figure 5.
In order to follow the movements of individuals, all
snails were removed from the vicinities of Pools 2 and 4.
The total populations of these pools were approximately 130
and 150 respectively. Ten snails selected from the Pool 2
population were numbered 1 through 10. Fifteen were numbered
from the population in Pool 4. In addition, twenty snails
from each pool were marked with a white spot on the tip of
the shell.
Labeled snails were returned to their respective pools.
Observations were made over a 53 hour period at major points
in the tidal cycle and the midpoints between them starting
four hours after the snails were returned to their pools.
Four categories were used to score the position of
individuals at Pool 2. In Pool signifies thoseindividuals
totally submerged, as before. Interface signifies those found
at or within 1-2 cm above the air-water interface. Snails
at the edge of the pool with their shells protruding out of
the water were also included in this category. Out signifies
those individuals not within the normal confines of the pool
and more than 2 cm from the edge of the water. Crack is a
separate category, not exclusive of the others and represent¬
ing snails found residing in cracks inside or outside of
the pool. The same categories were also used for Pool 4
vertical Distribution of L. scutulata
Merchant, Noroian
with the exception that In Pool was divided into two sub¬
categories: Bottom, for those found at the extreme depths
of the pool, 4.5-6.0 cm below the surface, and Side, for
those occurring between the interface and 4.5 cm under
the water.
The observation period lasted from June 1-3, 1978. The
weather was unusually calm. It was overcast both days
except for late afternoon of the first day and most of the
afternoon of the second day. There was little wind and the
surf was calm. Although it was a spring tide period, the
pools received little spray or wash except at high high tide.
All snails remained in their respective areas except for
two of the numbered ones in Pool 2 which were lost. Signif¬
icant migration of marked individuals occurred during the
observation period. 34% moved from their pool to nearby
pools or cracks. After 50 hours, 10 individuals from Pool 2
had migrated to Pool 1. Similarly, 6 individuals moved
from Pool 4 into the pool contiguous with it. Some back and
forth movement was also observed in both areas. An influx
of unlabeled animals was also observed. A maximum of 11 and
40 unmarked snails were found in Pools 2 and 4, respectively.
In Figures 6-9, the number of L. scutulata found at
different areas at each pool are compared with the tidal
and diurnal cycles.
Figure 6 shows the number of snails at the interface
plus those out of the pool. In general, peak numbers occur
during low tide periods. The highest peaks occur at low
Vertical Distribution of L. scutulata
Merchant, Noroian
tides in the afternoon at both pools. At Pool 2 this represents
from 71-82% of the population. At Pool 4 it is from 23-50%.
Figure 7 is a comparison of the number of individuals
at the interface of the pools. As in Figure 6, peaks occur
at low tides in Pool 2. Similarly, the peaks at low tides
occurring during the day are much higher than for those occur¬
ring during the night. This represent from 43-61% of the
total population. As might be expected, both numbered and
dotted subpopulations exhibit the same behavior. At Pool 4
there is no clear pattern of movement to the interface and
the number found there is very low in comparison with the
total population.
The number of snails observed in cracks is shown in
Figure 8. At Pool 2 movement to this category does not
show any regularity, though there are slight increases in
the crack population at high and low points in the tidal cycle.
At Pool 4 there appears to be an initial correlation between
the crack population and the tidal cycle with a large peak,
65% of the total, occurring at the first high high tide and
another, 43% of the total, occurring just before the second
high high tide.
Figure 9 compares numbered individuals found on the bot¬
tom and side of Pool 4. Most of the numbered snails were
submerged during the observation period. In general, the
bottom population increases at ascending and high tides.
Although the sample size is small and fluctuations represent
only a few individuals, a constant population was under
Vertical Distribution of L. scutulata
Merchant, Noroian
observation.
In general, individual movement appears to be the same
as that of the larger population. This can be seen most
clearly in Pool 2. See Figures 10 and 11, where the movement
of individuals is shown and compare with other figures.
The same picture for Pool 4 is not as clear.
There is much
more variability in individual movement.
Discussion
Changes in the vertical distribution of L. scutulata
appear to be related to the tidal cycle. These changes are
modified by the day-night cycle. This is as expected since
movement on their substratum is positively correlated with
dampness (Kops, 1964), a function of both tide and the diel
cycle.
Ascending, high and descending high tides are all
associated with wetness as a result of splash or spray from
the waves. These three periods differ in amount of wetness
and intensity of wave shocks. High tide is by far the most
severe and wettest of these periods, followed by ascending,
then descending tide. Wave shock can present a substantial
danger by dislodging L. scutulata from the rocks (Peterson,
1964). Therefore, it may be important when considering the
behavior of L. scutulata and its distribution. During the
day the rocks are dried more quickly when not exposed to
constant splash.
Decreases in the Out of Pool population during high tide
10
Vertical Distribution of L. scutulata
Merchant, Noroian
in the first two observation periods suggest either movement
into the pool for protection or removal by wave action.
During the first and calmer period of observations there was
no decrease at the peak of the high tide.
The amount of movement Out of Pool during ascending and
descending tides appears to be influenced by two factors.
The first is severity of wave shock and the second is phase
of the diel cycle. The high surf during the second observa¬
tion period is correlated with a suppression of the ascending
tide peaks but not the descending tide peaks at Pool 4 which
is the most exposed. Perhaps both the incoming and high
tides were too rough for movement out of the pool. The
descending tide became the only available wet period which
allowed "safe" movement.
Daytime is correlated with complete suppression of the
peak at descending tide and only partial suppression of the
peak at ascending tide. Coming out of the pool as the tide
recedes during the day could be particularly dangerous because
the sun can quickly dry the substratum and leave the snail
faced with problems of desiccation. There are two other
possible explanations, not associated with wetness, for
a peak in the Out of Pool number during an afternoon ascending
tide. This peak occurs regularly only at the shallowest pool,
Pool 2, and only during the warmer observation period. These
factors, combined with the lack of wash during the day may
have resulted in temperatures exceeding the tolerance range
of L. scutulata. This may have caused movement to the
11
Vertical Distribution of L. scutulata
Merchant, Noroian
interface where evaporation results in a decrease in temper¬
ature.
The other possible explanation is that a sharp peak
occurs closer to high tide and was not detected due to the
length of interval between observations. This possibility
seems unlikely because it is the presence of the peak rather
than its absence which is the aberration. However, a study
on short-term fluctuations where data were collected every
15 minutes for one and one-half hours indicated that signifi¬
cant changes in distribution do occur in times as short as
45 minutes.
The effects of tide on L. scutulata appear quite variable.
These effects depend on roughness of surf, degree of exposure
of a pool to the surf, and even the particular geometry of
the pool. Day and night, on the other hand, affect all the
pools in the same way. There is less danger of desiccation
at night and more during the day. When the gross population
distribution data for all four pools are combined as in Figure
5, the interpool variability results in a loss of obvious
tidal effects. At the same time, the effects of day and
night become even more obvious.
It was hypothesized above that L. scutulata may move to
the interface during the late afternoon when heat from the
sun would increase the temperature of the pool to an un¬
comfortable level. This could occur if the pool did not
receive any water during a high tide occurring during the
day and if the pool were shallow enough to allow a significant
12
Vertical Distribution of L. scutulata
Merchant, Noroian
rise in temperature. It has been observed that there is a
significant temperature differential between the pool, inter-
face, and surrounding rock. The temperature of Pool 2 at the
interface was found to be 6° C less than the pool or sur¬
rounding rock in the late afternoon on a warm day when the
sun was out for only a few hours. This idea, that the snails
use the interface as an air conditioner at times of heat
stress, finds support in observations of individual movement.
There is a significant increase in the number of snails at
the interface during the late afternoon in Pool 2 which has
a maximum depth of 3 cm. This pattern is not seen in Pool 4
which has a maximum depth of 7 cm and a lower pool temperature
than Pool 2. These observations are not conclusive. More
information on response to heat in L. scutulata and temp-
erature fluctuations of pools is needed.
It has been noted in Pool 4 that some individuals tend
to go to the bottom of the pool at times correlated with high
tide. At Pool 2 this is not so apparent. Pool 4 is exposed
to much greater wave shock at high tide than Pool 2 and it
is possible that snails either seek the depths of the pool
for protection or that they become dislodged during high
tide and are sent to the bottom of the pool. In order to
better understand this phenomenon more must be learned about
the ability of L. scutulata to hold on during wave shock.
Other questions in this regard are: How fast do dislodged
snails recover and attach themselves? Does attachment to
the bottom provide greater protection than attachment at the
13
Vertical Distribution of L. scutulata
Merchant, Noroian
interface or elsewhere?
In Pool 4, the majority of the marked population was
in cracks during the first high high tide. Movement into and
out of the cracks then showed a rhythmicity associated with
the tide; however, during subsequent high high tides a
decrease in the peak number and a shift towards ascending
tides occurred. It seems reasonable to hypothesize that the
snails seek cracks during high tides as cracks are protected
areas which afford shelter from wave shock. Hence, the
rhythmic movement described above may be a general response
to disturbance, anticipating rough conditions. Removing
the snails, marking them, and replacing them may have
provided this disturbance. Since it was not very rough
during the period of observation, the rhythmic response
eventually damped out and "normal" behavior resumed.
The general pattern of movement observed in the study
with marked individuals, i.e. an increase in the Out plus
Interface population at low tides and a corresponding
decrease at high tides, is different from that observed
in the first study. There are several possible explanations
for this difference.
The results obtained in the last study may be biased
for any of three reasons. First, an increase in the number
of individuals at the interface was noticed at low tide.
Since it was calm, it is possible that increased pool temp¬
erature during the day induced them to go to the interface
to cool off, i.e. air conditioner hypothesis. This would
14
Vertical Distribution of L. scutulata
Merchant, Noroian
heavily weight the number in the Out plus Interface category.
Second, the population was greatly disturbed by marking them.
Removal or perhaps nail polish itself could have induced
abnormal behavior or triggered various general responses
to disturbance, e.g. movement to cracks for protection.
Third, by significantly reducing the total population in
Pools 2 and 4, intraspecific interactions may have been
disrupted, resulting in a change of behavior.
It is also possible that the bias lies with the first
study. During the last study, migration was seen to occur
quickly. Short-term fluctuations in population size in a
given pool due to migration can influence vertical distribu¬
tion. For example, Pool 4 is connected by water to another
pool, which can act as a source of In Pool snails. It would
not be necessary for the snails to move into Pool 4 from the
outside; they could just enter without leaving the water.
By not observing a constant population in one pool, migration
effects may introduce large errors.
One of the most interesting aspects of monitoring indiv-
idual movement is the sequence of movements observed. There
are two important things to be noted in the plots of the
numbered individuals. In general, they exhibit the same
movement pattern as the total population. This is seen best
in Pool 2. However, not all individuals conform;
there is a rhythmic cycle which the population follows but
at the same time there are individual fluctuations depend-
ent upon the recent history of a particular individual.
Vertical Distribution of L. scutulata
Merchant, Noroian
Summary
1.) Fluctuations in the vertical distribution of Littorina
scutulata in tide pools were observed at Muscle Point.
Pacific Grove, California.
2.)
L. scutulata appear to move into and out of tide pools
as a function of the tidal cycle.
This behavior is modified by the day-night cycle.
3.)
4.)
The outward movement is positively correlated with
dampness and negatively correlated with wave shock.
The actual pattern of the in-out migration in each pool
5.)
is complex and dependent on a large number of environ¬
mental variables which are not yet fully understood.
The reasons for the in-out migration are still not known.
6.)
There are some indications: Movement into the pool and
also into cracks seems to offer protection against
wave shock; movement to the interface during the day
may be a mechanism to regulate temperature.
The night¬
time migration from in pool areas, where algae are
abundant, to out of pool areas, where algae appear
scarce, is not easily explicable.
Individual behavior shows a general agreement with
7.)
gross population behavior, but deviations were noted.
16
Vertical Distribution of L. scutulata
Merchant, Noroian
References
Glynn, P.W. 1965.
Community, Composition, Structure...of
the Endocladia-Balanus Association in Monterey Bay,
California. Beaufortia 12: 1-198.
Hewatt, W.G. 1937. Ecological Studies on Selected Intertidal
Communities of Monterey Bay, California. Amer. Midl.
Nat. 18: 161-206.
Kops, E. 1964. The Effect of Certain Environmental Factors
on the Activity Pattern of Littorina planaxis and L.
scutulata. Hopkins Marine Station Spring Course Report.
North, W.J. 1954. Size, Distribution, Erosive Activities
and Gross Metabolic Efficiencies of the Marine Intertidal
Snails, Litto
prina
lanaxis and Littorina scutulata.
Biol. Bull. 106: 185-197.
Peterson, R.E. 1964. Lower Limits of the Habitats of Littorina
scutulata and Littorina planaxis. Hopkins Marine Station
Spring Course Report.
17
Vertical Distribution of L. scutulata
Merchant, Noroian
Acknowledgment
We wish to thank the faculty and staff of Hopkins
Marine Station for making this endeavor worthwhile. We
extend special thanks to Dr. John Phillips for his
incredible patience, imagination and advice which helped
us in playing the sorts of games we wanted to play this
quarter.
18
e
Vertical Dist
tribution of
scutulata
Merchant, Noroian
Figure 1. Photographs of Area I and its pools. Pool 1
Is the lower of the two. The maximum dimensions of Pool 1
are 85 x 20 cm with a depth of 8 cm. The maximum dimen¬
sions of Pool 2 are 35 x 20 cm with a depth of 3 cm.
Vertical Distribution of L. scutulata
Merchant, Noroian

20
Vertical Dis
ribution of L. scutula
Merchant, Noroian
Figure
Photographs of Area II and its pools. Pool 3
is on the left in the bottom picture. The maximum dimen¬
sions of Pool 3 are 45 x 22 cm with a depth of 5 cm. The
maximum dimensions of Pool 4 are 50 x 30 cm with a depth
of 7 cm.
Vertical Distribution of L. scutulata
Merchant, Noroian
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 3. Change in the total population and percent
of the population Out of Pool during the observation
period April 28-30, 1978. Tidal and diel cycles are
also indicated. Changes in the total population are
indicated by a dashed line. Changes in the percent Out
of Pool population are indicated by a solid line.
Dotted line indicates areas of uncertainty in percent
Out of Pool.
Vertical Distribution of L. scutulata
Merchant, Noroian
tide
Pool1
60-




40-


20-
0
80-

W
Po 2
60-
40-

20-

1 80-
Pool 3
60
§ 40

20
80-
Pool 4
60
40-
V

20-

tide,
1700
000
0100 o900
o900 1700 0100
1700
-160
-120
-80
-40
-160
-120
80
-40
-0
160
-120
-80
-40
-0
-160
120
-80
—40
Vertical Distribution of L. scutula
Merchant, Noroian
Figure 4. Change in the total population and percent of
the population Out of Pool during the observation period
May 16-18, 1978. Tidal and diel cycles are also indicated.
The amount of splash and wash is indicated by 's. One *
for light and two for strong. Changes in the total
population are indicated by a dashed line. Changes in the
percent Out of Pool population are indicated by a solid
line.
tide
80-
60-
40-
20-
P 80-
60-
40-
+ 20
7 0.
8 80.
60-
40-
20
80
60-
40
20
tide
(ft.)
1500
Vertical Distribution of L. scutulata
Merchant, Noroian
Po01
P


X Pool 2

1
* X PooI 3 * * *






* +4
*
Pool 4



500
2300
-160
-120
-80
-40
-160
-120
-80
-40
-160
-120
-80
-40
-160
-120
-80
-40
26
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 5. Total numbers of L. scutulata In Pool and Out
of Pool for Pools 1-4 for the two periods of observation.
Solid lines represent Out of Pool. Dashed lines represent
In Pool. Tidal and diel cycles are also indicated.
Vertical Distribution of L. scutulata
Merchant, Noroian
tide
April 28-30
240
200+

160
Number
120-
of

Snails
80-

A0-
0100
0000
1/00 00
1ino ot
5+
tide
May 16-18
280-
240-
200-

Number
160-

Snails
120-

80-
40—

1500 2300 0700 1500 2300
500 2300 0700
Time of Day
28
Vertical Dist
tribution of L. scutulata
Merchant, Noroian
Figure 6. Change in the Out plus Interface population
during the observation period June 1-3, 1978. Tidal and
diel cycles are also indicated. The amount of splash and
wash is indicated by *'s. One * for light and two for
strong.
Vertical Distribution of L. scutulata
Merchant, Noroian
tide
25-
Pool 2
20
Number
15-
of
Snails
10-

Pool 4
20-
Number
15-
Snails
10—
5-




1700 ojoo o9oo 1700 0100 o900 1700 0100
Time of Day
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 7. Change in the Interface population during the
observation period June 1-3, 1978.
Tidal and diel cycles
are also indicated. The amount of splash and wash is
indicated by *'s. One * for light and two for strong.
tide
number
snails
number
snails
Vertical Distribution of L. scutulata
Merchant, Noroian
4
l6-
14-
Pool 2
12-
j0-
8 -
6-
A-
2-
L
0 +
Pool 4
tta

1700 0100 0900 1700 0100
Time of Day
* *
0900 1700 0100
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 8. Change in the population in cracks during
the
observation period June 1-3, 1978. Tidal and diel cycles
are also indicated. Changes in the crack population in
Pool 2 are indicated by a solid line. Changes in the
crack population in Pool 4 are indicated by a dashed line.
Vertical Distribution of L. scutulata
Merchant, Noroian
4
tide,
24-
22-
20
18-
16-
Number
of
Snails 12-
10-

8 -
4—


2-

10o ol00 0000
Vo ol00 000 100
Time of Day
o100
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 9. Change in the Bottom and Side populations in
Pool 4 during the observation period June 1-3, 1978.
Tidal and diel cycles are also indicated. The amount
of
splash and wash is indicated by *'s. One* for light and
two for strong. Changes in the Bottom population are
indicated by a solid line. Changes in the Side population
are indicated by a dashed line.
tide
Number
Snails
Vertical Distribution of L. scutulata
Merchant, Noroian
*
14—
12-

24
10-
6-
2—
L
1700 o1oo 0900 1700 o10o o900 1700 0100
Time of Day
36
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 10. Change in location of 8 numbered individuals
at Pool 2 during the observation period June 1-3, 1978.
Tidal and diel cycles are also indicated. Categories
are identified as follows: O indicates Out; I indicates
Interface; P indicates Pool.
Vertical Distribution of L. scutulata
Merchant, Noroian
tide
p
—


—
kkll


P
nO-
9


P

p



P


P
Voo oloo o/00 100
1700 0100 o900
Time of Day
000
Vertical Distribution of L. scutulata
Merchant, Noroian
Figure 11. Change in location of 14 numbered individuals
at Pool 4 during the observation period June 1-3, 1978.
Tidal and diel cycles are also indicated. Categories
are identified as follows: O indicates Out; I indicates
Interface; S indicates Side; B indicates Bottom.
Vertical Distribution of L. scutulata
Merchant, Noroian
- -p  0-o

r
slr  lr

p
1121
2
1P2


40