LABORATORY OBSERVATIONS
ON THE
NOCTURNAL
VERTICAL MIGRATION
BEHAVIOR
OF
TEGULA PUL.
G0
(GASTROPODA, TROCHIDAE)
Charles J. Wright
Hopkins Marine Station
Mussel Point
Pacific Grove, CA
INTRODUCTION
Tegula pulligo is the commonest trochid gastropod found in the kelp
forests of Monterey County. Previous distributional studies on T. pulligo,
T. montereyi and T. brunnea by Lowry and Pearse (1974), Hunt (1977) and
Sellers (1977) indicated that the three populations stratified on
Macrocystis plants, with T. pulligo usually occupying the lower four meters
of the plant.
Preliminary subtidal observations in the kelp forest adjacent to the
Hopkins Marine Station at Mussel Point in mid-April yielded data suggesting
that T. pulligo underwent a diel vertical migration on the kelp, with up-
ward motion occurring at night. Subsequent field studies by Thornton
(reported in this volume) have shown there to be a statistically significant
migration of the population in our study area.
Laboratory studies were undertaken in attempt to 1) demonstrate the
observed field behavior in a laboratory environment and 2) to determine
what effect light, or its intensities, may have on any observed behavior.
If light was shown to be an important regulating factor, it was also hoped
to obtain a general light-preferred range and maximally tolerated intensity.
MATERIALS AND METHODS
All experiments were conducted using a clear acrylic cylinder measur¬
ing 2 m high and having an internal diameter of 14 cm. A single kelp plant,
usually having between eight and twelve stipes, and its holdfast, was
attached to a weighted plastic box and sunk in the cylinder. The box was
perforated and prevented animals from blocking water flow, and decreased
the directional nature of the flow by allowing multiple outlets or inlets,
depending on flow direction. Blades on each plant were trimmed as necessary
(2)
to allow viewing inside the cylinder. Animals collected at random from
the field were dropped in the cylinder and given between one and three
days to adjust to their new environment before observational periods began.
During several studies, natural light passing through a skylight above
the apparatus was the sole light source. In those studies which used
artificial lights, 8 75 watt GE reflector bulbs and 1 650 watt Sylvania
DWV movie light were used. To allow additional flexibility in establishing
desired gradients, 4 of the 75W bulbs and the 650W bulb were controlled by
a Variac regulator, and varying thicknesses of window screening were
attached to the outside of the cylinder. All light intensities reported
were measured with a Weston Model 703-60 Type 3A photometer on the outside
of the cylinder. A red-cellophane-covered flashlight with a peak intensity
of 50 Foot-Candles was used to determine the position of the snails at
night. Fresh sea water (about 14°C) flowed continually throughout the
system. Unless otherwise noted, all statistical analyses were done by
Student's T-test.
RESULT!
Figure 1 is a schematic presentation of the light intensities generally
encountered in the field. As the study progressed, the canopy thickened,
and ambient intensities decreased slightly.
Figure 2 presents the results from a study using natural daylight
and one day of acclimation. It shows a slight tendency for the animals
to move up in darkness, but statistical tests between the mean heights
during the dark interval and those of the lighted periods show no signifi¬
cance (p».9). Additional analysis between bright (200 ft.-candles) and
dimmer conditions heights did not show any significance to exist, either
(.5 «p «.9).
(3)
The animals represented by Figure 3 were allowed one day of acclima¬
tion and artificial light was used. There appears to be vertical movement
with peak height occurring in the brightest light interval. Testing be¬
tween the mean heights of the light versus dark periods showed a close to,
but not quite (p«.065) significant displacement of the animals with differ¬
ing light conditions. Further, no significance between dim and bright
conditions was demonstrated (.2 «p «.4).
In the experiment represented by Figure 4, the lighting was the same
same as Figure 3 but 4 days of acclimation was used. The population ex¬
hibited a clear migration upward corresponding with the dark interval.
The significance of the movement was dramatic (po.0001). Again, tests
were conducted between bright and dim mean heights, and they showed no
significant differences to exist (.2 «p«.4).
The data depicted on Figure 5 came from a study where dim artificial
light was used (a peak of 70 ft. candles) and one day was allowed for
acclimation. The mean height during the lighted intervals is rather
significantly lower (0.02 « p « 0.05) than the mean heights during the
dark condition.
Figure 6 presents the data from a displacement experiment. Animals
were removed, then dropped back into the cylinder in light. They were
given 6 hours of light followed by 9 hours of dark. One run showed a
highly significant (using 952 confidence intervals established by T-test)
increase in mean heights during the dark interval. The second run showed
a dramatic increase also, but it occurred during the lighted period. In
combining the two runs, however, no significant differences between dark
and light heights can be shown (.05 « p «.1).
(4)
Figure 7 depicts displacement studies during the night. In analyz¬
ing the first run, there was no significance evident between the light
and dark conditions. During the second run, however, highly significant
movement took place between the dark period of 1830-1000 and the following
lighted interval (p « .005), and motion significant to between .025 and
.Ol occurred during the dim period between 1700 and 1830.
DISCUSSION
As mentioned in the introduction, T. pulligo has been shown by field
observations to undergo a significant vertical migration up kelp plants
at night, with downward movement as daylight increases. Predation by
visually-cued animals known to prey on T. pulligo such as Enhydra lutris,
Hexagrammacus decagrammus, Damalychthys vacca and Octopus (Hunt, 1977)
could make such behavior selectively advantageous. During the day, though
some individuals may be found exposed on the blades and stipes, the bulk
of the population remains low on the plant, hidden from view, thereby
escaping detection by many predators. At night, a larger percentage of
the population moves up onto the stipes and blades feeding.
Such behavior, however, was not clearly and consistently seen in the
laboratory. With the notable exception of the population of animals used
for the Figure 5 experiment, those animals given one day or less to
acclimatize to the system showed no significant movement of mean height
with varying light intensities. Animals which had been in the system for
two days or more began to show movement bordering on significance (p «.05).
The animals allowed four days or more in the system before being observed
demonstrated a clear migration of population mean height as a function
of light intensities. Their peak height was attained during the dark
intervals, as was the peak of the field population. This would indicate
that the animals required a longer period of adjustment to the experimental
conditions than had been anticipated.
The data presented on Figure 5 also calls to attention the importance
of accurately determining the light intensities actually seen by the
animals, both in the field and within the apparatus. As seen in Figure 1,
the light experienced by the animals on the kelp appears to be in the 0-125
ft.-candles range. Only during the study presented by Figure 5 was the
light that dim; in the other studies, the light shown on the animals
tended to be higher for at least a part of the study. The longer period
of adjustment could be in response to those higher levels of light.
Determining the actual levels seen by the animals within the cylinder was
also difficult, since intensity varied as readings were taken around the
circumference. In future work, it might prove informative if distinctions
were made as to the location of an individual, i.e., behind a blade, in
the center of the cylinder, on an extreme wall. The variability of
responses within the laboratory system could be a reflection of not having
taken such considerations fully into account.
Individual variability was touched upon only superficially by tagging
and monitoring individual's positions, and could use more work. Samples
of the field population were haphazardly taken, and their treatment was
not standardized. Perhaps by controlling for such specific factors as
sex, size, time since last feeding, or collection site variability of
response might be reduced.
SUMMARY AND CONCLUSTONS
1) T. pulligo undergoes a nocturnal migration in the laboratory similar
to that seen in field studies.
(6)
2) T. pulligo requires at least three days to acclimatize to a
laboratory environment in which light intensities are higher than those
experienced in the field.
3) Animals knocked off plants undergo their greatest repopulation move-
ment at night.
LITERATURE CITED
HUNT, D. E., 1977. Population Dynamics of Tegula and
Calliostoma in Carmel Bay, with Special Reference
to Kelp Harvesting. M.A. Thesis, San Francisco
State University, San Francisco, CA. 81 pages.
LOWRY, L., A. MCElroy and J. Pearse, 1974. Sublittoral
Ecology of Kelp Beds of the Open Coast Area Near
Carmel, California. Biol. Bull. 122: 95-114.
SELLERS, R. G., Jr., 1977. The Diets of Four Species of
Calliostoma (GASTROPODA, TROCHIDAE) and Some As¬
pects of Their Distribution Within a Kelp Bed.
M.S. Thesis, Stanford University, Stanford, CA.
ACKNOWLEDGMENTS
I should like to recognize the people whose assistance
and help in the writing of this paper was indispensable.
The critiques and suggestions offered by my advisor, Dr.
Chuck Baxter proved timely and very valuable. The as¬
sistance and comments of Dr. John Phillips also are
gratefully acknowledged. Dr.Robin Burnett's help in the
statistical analyses of the data are greatly appreciated.
I would like to thank Sharon for having done the final
typing of the paper. And Kath's support and patience
have been absolutely astounding, and I can't thank her
enough.
FIGURE 1
Represented on Figure 1 is are the general light
levels found in the kelp forest off Hopkins Marine
Station. Intensities were measured between May 9 -
26, 1978.
(10)
10 M
OPEN WATER
0600
1500
750+
75-100
500-700
20-35
300-500
10-20
100-250
5-15
Sunny Day
10-15 Ft. Visibility
Slight Surge
Some Particulate in Suspension
50-100
0-10
Figure 1
(11)
so
10050

Sporophy
O-10


KELP BED
ANO
100-175
FIGURE 2
Animals were allowed one day to acclimatize under
natural light. Light intensities were measured at the
1.5 M and 0.5 M levels on the outside of the system. In
the lower figure, the dashed line indicates the mean
height of the entire population; the solid line depicts
the mean height of the animals found above the top of
the plastic box to which the plant was attached. The
top figure plots the distribution of snails as per¬
centages per quarter cylinder interval. The numbers
within the bottom interval refer to the percentage
of the total population located below the top of the
supporting box.
(12)
0. 5
50
25
50.0
200-
90
TIME
56.9
200-
28

—18
49.
38.6
50.456

345
8-0
28.9
130-
18
23
9o-
45.5
750+-
200
12

55.2
(13)
FIGURE 3
The group used in this experiment were given one day
within the system prior to observations under artificial
light. The dashed line is the mean height plot for the
total population; the solid line is that of those animals
above the top of the supporting plastic box. The top
figure plots the distribution of animals as percentages
per quarter cylinder intervals. The numbers within the
bottom intervals refer to the percentage of the total
population found below the top of the supporting box.
120
90
37.7
352
— — — ——
750+ - 70
TIME
20.7
18
450-
30
25.9
27.5
41.3
44.3
526

450-30
De
14
31.7
(15)
FIGURE 4
Animals were run simultaneously with those of Figure 3,
but had different history. Given one day initially to
adjust, the animals were used in a two day displacement
study, which involved being displaced four times during
the study. Thereafter, the animals were left undisturbed
in the same conditions as Figure 3 group. However, at
1600 on the first day of this study, the animals were
displaced in a move necessary to standard lighting seen
by both groups.
Light intensities were measured between the two
systems, and are in foot-candles. The dashed line is
the mean height curve for the entire population; the
solid line is the plot of mean height for those animals
above the top of the plastic supporting box. The top
figure shows distribution of the total population as
percentages per quarter cylinder intervals. The numbers
within the lowest intervals are the percentages of
the total population found below the top of the box.
(16)
120
60
30
55.1
750+-
70
4.7
450-
30
18 TIME
12.4
14.6
— ——
ure
31.2
52.7
450-30
513
7504-70

1
5.1
41.9
(17)
FIGURE 5
Animals were given one day to adjust under artifi¬
cial light. Peak intensity during the study was 70
foot candles; the cross-hatched interval indicates a
slight light leak.
The lower figure follows the mean height of the
animals above the top of the plastic box supporting
the plant. The upper figure shows the distribution
of the animals as percentages within quarter cylinder
intervals. The numbers adjacent to the lowest interval
indicates the percentage of the total population
located below the top of the plastic box.
75
50
64.5
57.6
31-0
51.4
50.0
47.2
55.
46.9
54.1
58.3
66-Oft. candles
18
TIME
00
Figure 5
06
12
(19)
FIGURE 6
Graphed is the data from a day displacement experi¬
ment. Animals were given one day under artifical lights
to adjust. They were then removed from the cylinder,
and dropped back in along the walls, to simulate being
knocked from plants by surge during the day. A period
of six hours light was followed by nine hours of dark.
The solid line traces the mean heights for the first
day; the dashed line follows the mean heights for the
same group on the second day, after a simulated night
displacement experiment.
105
35
70
+-
TIME
18
ure
06
(21)
750+ - 40


2
FIGURE 7
Figure 7 presents the data from a simulated night
displacement experiment. The same animals as were
used for the Figure 6 experiment were removed and
returned to the cylinder in darkness. After six
hours of darkness, lights were turned on. The lower
light regime plot indicates the times when a light
burned out, dimming ambient levels until a replacement
bulb could be obtained. The solid line indicates the
mean heights during the first day; the dashed line
outlines the mean height curve for the second day.
As no distinction between above or below the box top
were made, mean heights are for the entire population.
105
7504 40
H
I
750+ - 40
12 TIME
15
—11—
Figure 7
12
(23)