INTRODUCTION
Tricolia pulloides is an abundant intertidal prosobranch
gastropod inhabiting many algae of the mid-intertidal zone. In
the area studied around Hopkins Marine Station of Stanford Univer-
sity, Pacific Grove, California, most T. pulloides inhabit the
abundant red alga, Gigartina papillata. Combined abundances of
organism and habitat provide ample opportunity for the study of
T. pulloides and its exploitation of various microhabitats within
G. papillata.
Goals of this study were: 1) to determine changes in distri¬
butions of populations of Tricolia pulloides on Gigartina papillata
during changing conditions of light, surge, and exposure in the
field; 2) to examine isolated responses to changes in each physical
factor in the laboratory; 3) to explain observed field and labora¬
tory behavior in terms of avoidance of possible stresses brought
on by these changing physical factors.
MATERIALS AND METHODS
Field Study
Investigation of Tricolia pulloides' behavior began with an
intensive field study of populations inhabiting pairs of algae at
1.6 m, 1. 8 m, and 2.0 m above mean lower low tide. Distributions
of these populations were observed every 2-3 hours through one
complete tidal cycle beginning at 1800 on 2 May.
Algae were upright, 3-5 cm high, located on a sloping granite
face with no adjacent algal cover or sediment. Waves rarely broke
on the area, but there was strong surge at times. Each alga was
inhabited by 20-50 snails. These populations were supplemented
by approximately 20% to amplify distributional shifts. Population
densities remained within reasonable limits, however; many other
algae the same size were inhabited by nearly a hundred snails.
Distributions of Tricolia pulloides within Gigartina papillata
in the field and laboratory were quantified by subdividing the alga
into three regions: 1) outer region, comprised of all broad, thin,
leafy frond ends beyond at least one major branch point; 2) inner
region, comprised of granite substratum immediately beneath the
alga and thick, narrow frond stems up to the first major branch
point; 3) middle region, comprised of all remaining frond area.
Radial distributions of populations in all experiments were charac¬
terized by recording the number of snails in each of these areas.
In the field study, exact counts were obtained of the number of
snails in outer and inner regions. Numbers for middle regions,
however, were estimated. The difficulty of gently searching through
an alga with fine forceps while on SCUBA during strong surge made
exact counts of larger middle populations virtually impossible.
All other counts were exact.
Several distinct changes in distributions of Tricolia pulloides
populations were observed during the field study. However, environ¬
mental factors, particularly light, surge, and exposure, were
fluctuating concurrently, so that distributional shifts could not
be attributed conclusively to changes in any single environmental
factor. Controlled laboratory experiments were necessary to deter¬
mine important environmental parameters and their effects on radial
distributions of T. pulloides populations.
Laboratory Experiments
Response to Light: Initial laboratory experiments tested for
a response by Tricolia pulloides on Gigartina papillata to light
from above and from the side. In this and other laboratory experi¬
ments, snails were collected from the field, marked with silver
nail polish, with no apparent detrimental effects, and submerged
in 13°C sea water with G. papillata for 24 hours prior to the
experiment. Four algae, similar to those chosen for the field
study, were collected from the field complete with granite chip
and holdfast. The algae were cleared of biota, anchored in model¬
ling clay, submerged, and stocked with 25 T. pulloides. Each
system of four algae and 100 gastropods was equilibrated in darkness.
After sixty minutes, numbers of T. pulloides on sides of fronds to
be illuminated were counted under dim red light. Thirty watt
microscope lamps, positioned 10 cm above one system and 10 cm to
the side of the other, were turned on. Numbers of T. pulloides on
lit sides of fronds were recorded at thirty minute intervals. The
vertical experiment was terminated after 60 minutes. The lateral
experiment was terminated after 150 minutes.
Because no significant phototactic response was detected during
the laterally-lit experiment, despite a significant downward popu¬
lation shift in the vertically-lit experiment (see RESULTS), a
second laterally-lit phototaxis experiment was conducted, in an
artificial environment. At approximately 1100, 1300, 1900, and
2300 on 1 June, roughly 100 snails were submerged in clear plastic
jars filled with sea water. Jars were placed on a lab bench two
meters from a window. The water was swirled until Tricolia pulloides
were scattered about the jar bottoms. After 30 minutes approximately
80% of the gastropods had climbed or were climbing the jar walls.
Paper divided into eight sectors of 45° each was taped around the
jars. Numbers of gastropods on the jar walls in each sector was
recorded.
Response to Surge: Tricolia pulloides'response to surge also
was tested in the laboratory. Surge was simulated by shuttling a
plywood board attached to PVC pipe back and forth in an aquarium
filled with 13°C sea water.
On 21,22, and 23 May, five algae were collected from the field,
cleared of biota, anchored in modelling clay, and tacked upright
to the aquarium floor. One hundred Tricolia pulloides collected
and marked the previous day were placed on the submerged algae.-
After 120 minutes equilibration in calm water, radial distribution
of T. pulloides on each alga was recorded. The plywood board,
which extended about halfway down the water column, was moved back
and forth across the entire length of the aquarium. Manual surge
was accelerated gradually to ten cycles per minute. After 30
minutes the radial distribution of each population was recorded.
A minute of manual surge separated each reading, to prevent unre¬
corded populations from shifting. After 30 minutes calm, distri¬
butions were recorded again.
Response to Exposure: Finally, exposure was tested in the
laboratory for its effect on radial distributions of Tricolia
pulloides populations. In order to simulate the natural tidal
cycle, a clock was positioned above an aquarium, with the aquarium
drain hose attached by string to the hour hand of the clock. As
the hour hand made a revolution every 12 hours, the drain hose, and
hence aquarium water level, rose and fell. Thus algae gradually
were exposed and submerged twice daily.
On 1 June three pairs of algae were collected from the field,
cleared of biota, and anchored in clay. One pair was placed on the
aquarium floor, one pair on a brick, and one pair on two bricks in
the aquarium. At 1700 each pair was stocked with 50 snails collected
the previous day. Algae were submerged completely, in synchrony
with the high tide at 1711. Populations were allowed to equilibrate
until 1800, at which time radial distributions were recorded.
Observations were made under constant light every two hours, through
two high and low tides.
Statistical Analyses
Row-by-column contingency tests (Zar,1974) were performed on
most data. Outer, middle, and inner regions or lit and shaded sides
were rows. Levels of physical factors were columns. Thus
RxC tests indicated levels at which distributions, rows, were con¬
tingent upon levels of physical factors,columns. A chi-square test
(Zar,1974) was used to compare distribution around jar wall for the
dark experiment to an even distribution.
RESULTS
Field Study
Radial distributions recorded during field studies are shown
in Figure 1. An RxC test was performed on data for submerged
populations with subjective estimates of surge levels, calm, mod¬
erate, and rough, as columns. Radial distributions, rows, were
contingent upon surge levels (p £ 0.001). An RxC analysis of inner
and outer data, eliminating estimates of numbers of snails in
middle regions, also indicated radial distributions were contingent
upon surge levels (p + 0.001).
An RxC test also was performed on all field data, with exposed
and submerged as columns (see Figure 1). Radial distributions,
rows, were contingent upon presence of water (p £ 0.001). Elimination
of estimates of populations in middle regions produced similar
results (p « 0.001).
Laboratory Experiments
Response to Light: Figure 2 shows distributions on lit and
columns indicated radial distributions were independent of prior
surge on 23 May (p » 0.25). On 22 May, though, distributions were
contingent upon prior surge (p £ 0.025). However, it appears that
atypical initial distribtuions on 22 May might have been the source
of unexpected difference between initial and final distributions.
Response to Exposure: Radial distributions for the exposure :
experiments are presented in Figure 5. RxC tests with exposed and
submerged as columns indicated radial distributions were contingent
upon submersion (p £ 0.001). Interestingly, when data for the one
alga which did not remain upright upon exposure is discarded, the
G value nearly doubles (21.3 vs. 38). Qualitative observation
showed gastropods on this alga moved inward to a lesser extent when
exposed than did gastropods on those algae which remained upright.
DISCUSSION
In all experiments, Tricolia pulloides populations shifted to
outer regions of Gigartina papillata during optimum conditions of
calm submersion in dim light. Mooers (1979) has shown diatoms to
be T. pulloides' principal food item, and Chun (1979) has shown
that diatoms and T. pulloides' grazing tracks are found almost en¬
tirely on outer regions of a similar red alga, Rhodoglossum affine
Thus, under optimum conditions, T. pulloides populations may move
to outer areas of G. papillata to feed.
Desiccation is a considerable threat to Tricolia pulloides:
an 1D50 of 18 hours was reported for laboratory exposure
(Firestone, 1979), and may be considerably less in bright sunlight.
Thus, negative phototaxis seen in natural and artificial surround¬
ings would seem an adaptive response. The sensitivity of this
response is indicated by jar experiments: areas of highest snail
density on jar walls shifted east as areas of incident sunlight
shifted west during the day (see Figure 3). Similarly, T. pulloides
response to exposure seen both in the field and the laboratory would
minimize the period in which the snail is entirely without moisture
and subject to eyaporative water loss.
Inward movement of Tricolia pulloides during strong surge
minimizes the threat of being swept off the alga, since inner regions
swing through a smaller arc and with less tangential velocity than
frond ends. During trial surge experiments snails on outer areas
of algae often were swept off if strong manual surge was begun
suddenly. Thus, inward movement of T. pulloides populations during
surge also appears to be an adaptive response.
FIGURE LEGENDS
Figure 1: Radial distributions of approximately 150 Tricolia
pulloides on 3 pairs of Gigartina papillata at different
tidal heights. Upper, middle, and lower sections of
histograms represent, respectively, outer, middle, and
inner algal régions. Widths of histogram sections are
proportional to numbers of snails in appropriate regions
of algae. Approximate total number of snails observed
per reading shown below. Populations above dashed line
were exposed when observed. Populations below dashed
line were submerged when observed. Time line shown,
black bar indicates darkness.
Figure 2: Summed distributions of 100 Tricolia pulloides on lit
side, clear bar, and shaded side, hatched bar, of fronds
of four Gigartina papillata before and after exposure to
light from above and from the side. Sizes of clear and
hatched areas are proportional, respectively, to numbers
of snails on lit and shaded sides of fronds. Exact
numbers enclosed. Significance levels (p values) are
from RxC contingency tests (Zar,1974) performed to
determine if snail distributions are independent of
lighting.
Figure 3: Distribution of Tricolia pulloides populations around
walls of clear jars after 30 minutes in sunlight. Total
numbers of snails per experiment indicated in parentheses.
Thicknesses of sectors are proportional to numbers of
snails on jar walls in each of eight 45° sectors. Sig¬
nificance level (p value) for night experiment is from
a chi-square test comparing this distribution to an even
distribution. Significance levels (p values) for morning,
afternoon, and evening experiments are from an RxC con¬
tingency test (Zar, 1974) performed to determine if
snail distributions around jar walls were contingent
upon directional sunlight. North indicated by arrow
labeled N. Direction of incident sunlight shown by
unmarked arrows.
Figure 4: Summed radial distributions of 100 Tricolia pulloides
on Gigartina papillata after 60 minutes calm, 30 minutes
manual surge, and 30 minutes calm on three consecutive
days. Upper, middle, and lower sections of histograms
represent, respectively, outer, middle, and inner algal
regions. Widths of histogram sections are proportional
to numbers of snails in appropriate regions of the 5
algae. Actual numbers enclosed in appropriate sections.
Significance levels (p values) are for RxC contingency
tests (Zar, 1974) performed to determine if radial
distributions were contingent upon manual surge.
Figure 5: Radial distributions of 150 Tricolia pulloides on three
pairs of Gigartinä papillata during a 24 hour simulated
tide cycle. Upper, middle, and lower sections of histo¬
grams correspond, respectively, to outer, middle, and
inner regions of pairs of algae placed at high, medium,
and low levels in an aquarium. Nidths of histogram
sections are proportional to summed numbers of snails
in appropriate regions of the 3 pairs of algae. Actual
numbers enclosed in appropriate sections. Populations
above dashed line were exposed when observed. Populations
below dashed line were submerged when observed. Time
line and date shown below.
LITERATURE CITED
Chun, Stephen A., 1979.
Qualitative and quantitative effects of
herbivore grazing on the epiphytes on Rhodoglossum affine.
Unpublished manuscript on file at Hopkins Marine Station,
Stanford University, Pacific Grove, California.
Firestone, Alan L., 1979. Distribution, abundance, and size-class
distribution of the prosobranch gastropods Tricolia pulloides
and Barleeia haliotiphila in the rocky intertidal zone at
Mussel Point and Point Pinos. Unpublished manuscript on file
at Hopkins Marine Station Library, Stanford University, Pacific
Grove, California.
Mooers, Mary G., 1979. Diet and reproductive biology of the rocky
intertidal prosobranch gastropod Tricolia pulloides. Unpub¬
lished manuscript on file at Hopkins Marine Station Library,
Stanford University, Pacific Grove, California.
Zar, Jerrold H., 1974. Biostatistical analysis. Prentice-Hall,
Inc. Inglewood Cliffs, New Jersey. 41-54 pp.
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PHOTOTAXIS ON THE PLANT
LIGHT FROM ABOVE
Distribution after 60 minutes
Distribution prior to light
(equilibrated 60 minutes)
exposure to light from above
46
32
lit side
— P.05
shaded side
54.
68

etel
LICHT FROM THE SIDE
Distribution prior to light
(equilibrated 60 minutes)
134
66

P9.25
lit side
shaded side
Distribution after 150 minutes
exposure to light from the side
139
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morning
(n=105)
afternoon
(n=75)
evening
(n=91)
night
(n=85)
PHOTOTARIS INAJAR
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