Laurie S. Hedden

ENTR
ODUCTION
The gastropod Olivella biplicata (Sowerby, 1825) is common
in many localities on the Pacific Coast of North America from
British Columbia to Baja California, and has been the subject
of several investigations. A large population occurs in and
around the harbor at Monterey, California (Smith & Gordon, 1948).
Wr. David W. Phillips of Hopkins Marine Station, Pacific Grove,
California (personal communication) had noted that individuals,
at least those in and near the intertidal zone, undergo more
or less regular shifts in orientation and position on the beach
with changes in tide level, and suggested a quantitative inves-
tigation of this behavior. Shifts in position and activity
with tide have been noted in other sand-dwelling gastropods,
eg. Oliveancillaria brasiliensis (Chemnitz, 1788) (see Gofferje,
ricula) auricaularia (Lamarck, 1810)
1950) and Olivan
cillaria (Lint:
(see Marcus & Ma
reus, 1959). The orientation and movement of
Olivella biplicata have been examined in one study (Edwards,
1965), but primarily from the point of view of differences
between size classes in natural populations. The purpose of
the present study was to confirm, and provide a measure of,
the tendency of Olivella biplicata to move downward on receding
tides and upwards on incoming tides.
Laurie S. Hedden


MATERIALS AND METHODS
All studies were carried out on the shore just east of
Fisherman's Wharf, Monterey harbor, California, an area
characterized by a very gentle slope and little wave action,
ransect lines parallel to the water's edge
I set up three
within the intertidal region occupied by Olivella biplicata.
The transects, separated by 15-foot horizontal intervals
on the beach, were labelled Station A (at the tidal height
of +1.1 ft), Station B (+0.25 ft) and Station C (-O.25 ft).
On separate occasions, both day and night, observations were
made at three selected times before, during and after the
period of lower low water (LLW).
At each sample time, Olivella at each transect area were
examined to see whether they were oriented facing upslope or
downslope. Animals headed in any direction upward of a line
roughly parallel to the wave front were counted as facing
ipslope; conversely for downslope. When any station was
submerged by more than 6-8 inches of water, animals found on
the sand surface provided the most reliable data on true
orientation. When the transect area was exposed at each LLW.
leaving a firmer substrate, nearly all the animals burrowed
below the surface, but the orientation of the burrowed Olivella
could be reliably determined.
Along with studies of orientation upslope and downslope,
at each observation time quantitative measures were made of
Olivella population present at each
ransect level, to provide
laurie S. Hedden
a measure of the actual movement of the population up and
down the beach. During each ebb tide, low tide, and flow.
four 0.25 m' quadrats were selected on each transect line, in
areas not previously disturbed by sampling. At each, a sheet
metal quadrat frame was inserted into the beach and all animals.
both those on the surface and those bur
rrowed to a depth of 3
inches, were counted. Animals were returned to the quadrat of
capture immediately after counting. Locations of quadrats
along the transect lines were changed with each sample time.
Night observations were made with the use of a flashlight
covered with red cellophane, since preliminary observations
had shown the animals respond much less to this than to white
light. Animals which were mating, and animals close enough
to an occasional Polinices to be exhibiting escape response
(Edwards, 1969) were not counted in the study,
pe
RESOLI
The results are su
mmarized in Figures 1 and 2.
During the ebb tides, there was a significantly greater
number of Olivella pointing downslope than upslope at Stations
A and B, while at Station C, still covered by more than one
foot of water, a larger number of animals were still oriented
upslope. At lower low water (LLW) almost all Olivella at each
station pointed downslope. The final counts, taken during
tidal flow about three hours later, showed a reversal in
orientation at the low station (C), while at the upper stations
the majority of the animals still pointed downslope.
laurie S. Hedden
Actual numbers of animals per unit area at the transects
(Figure 1), as based on the sample population counts, showed
a decrease in the total number of Olivella at Stations A and
B, complemented by an increase in the total number at C, as
the tide receded. During the rising tide, there was a reversal
of this distribution, and the numbers at A and B increased
again while those at C declined. The changes observed in
distribution are those which one would expect if orientation
upslope and downslope is a reflection of movement in these
directions. The variability between the four 0.25 m samples
taken at any one time on a particular transect was fairly great.
for Olivella are rather gre
jarious (Edwards, 1965) and show a
clustered, non-random dis
ribution. While the differences be-
tween populations at given transects at different times are
not statistically significant, the results obtained are strongly
suggestive of population movement up and down the beach. Data
in Figure 1 were obtained at night and early in the day. The
results shown in Figure 2 indicate that statistically significant
changes in orientation with phase of low tide also occurred
during daylight hours. Downslope orientation corresponded to
the receding tide, and, as before, there was a reversal of
direction with the incoming tide, and most observed animals
headed upslope.
Laurie S. Hedden
DISCUSSION
There is a clear indication of a change in orientation
of Olivella biplicata in the hours before and after lower low
water. Although only the differences in numbers oriented upslope
vs. downslope are significant, the variations in population at
each level on the beach are entirely consistent with these
data, suggesting that considerable numbers of animals move up
and down the beach with the tide. This is further corroborated
by qualitative field observations. Movement is not highly
directional; most moving Olivella follow rather tortuous paths.
However, they are capable of travelling up to several meters
per hour (Stohler, 1969). The stimuli involved in eliciting
this behavior have not been determined, though the possibilities
are numerous. There has been some speculation as to what the
advantages are to Olivella in maintaining intertidal populations.
Edwards (1965) has suggested that intra-specific competition
and protection from subtidal predators, eg. Polinices and
Pisaster, might force larger animals to move higher up on the
beach. In addition, a third possible factor could be the
high rate of turnover of detritus in this region of the beach,
providing a continuously rich food supply for Olivella. By
moving with the tide, the intertidal Olivella population can
minimize periods of exposure and spend a correspondingly greater
time in such activities as feeding and mating,
Laurie S. Hedden
SUMMARY
The intertidal sandy beach gastropod Olivella biplicata
exhibits downslope (seaward) directional orientation as the
tide ebbs toward lower low water and upslope orientation as
the tide rises. Some seaward and landward movement of beach
populations also occurs. These orientation changes occur
whether low tides come during the day or the night.
VMOUTP
ACKNOWLEDGEMENT
I would like to thank all the faculty, staff and students
of Hopkins Marine Station, Pacific Grove, California. Special
thanks go to David W. Phillips for his help which led to this
study, and to Dr. Donald P. Abbott for his patience and guidance.
particularly in the preparation of this manuscript.
Laurie S. Hedden
TTTEPA
LITERATURE CITED
Edwards, Dallas Craig
1965. Distribution patterns within natural populations
of Olivella biplicata, their underlying behavioral
mechanisms and their ecological significance.
Ph.D. thesis, University of Chicago.
1969. Predators on Olivella biplicata, including a
species-specific predator avoidance response.
Veliger 11: 326-333.
Gofferje, C. N.
1950. Contribuicao a zoogeografia da malacofauna do
litoral do Estado do Parana. Arq. Mus. Paranaense 8:
221-282.
Marcus, E. & E. Marcus
1959. Studies on Olividae. Bol. Zool. 22: 99-188.
Smith, Allyn G. & Mackenzie Gordon, Jr.
1948. The marine mollusks and brachiopods of Monterey
Bay, California, and vicinity. Proc. Calif. Acad.
Sci. 26 (8): 147-245.
Stohler, Rudolf
1969. Growth study in Olivella biplicata (Sowerby, 1825).
Veliger 11: 259-267.
Laurie S. Hedden
FIGURE CAPTIONS
1. Orientation and movement of Olivella biplicata before,
during and after lower low water (LLW) on two successive
days at Monterey harbor, California. Numbers of animals
heading upslope and downslope, and number/me, at levels A,
B and C on the beach are shown. Transverse lines on the bars
of the lower graph indicate numbers of animals in four
separate 0.25 m2 samples.
2. Orientation of Olivella biplicata before and after lower
4
low water at Monterey harbor, callfornia. All animals seen
at each of the three observation periods occurred between
the -1.0 ft and the +1.0 ft tidal levels.
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