O corniculata Population Movement
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INTRODUCTION
The semi-terrestrial amphipod Orchestoidea corniculata
Stout, 1913, inhabits the higher intertidal zone on sandy
beaches. Of the several species of Orchestoidea common in
California, O. corniculata is the one most abundant on
short, steep beaches with sand grains poorly sorted for size
(Bowers, 1964). The amphipods are basically nocturnal,
emerging from burrows at dusk and reburrowing at dawn. During
the hours of darkness, they move about the beach by hopping
or crawling, and feed on fresh wrack.
Identification of 0. corniculata in both juvenile and
adult instars has been facilitated by the keys in Bousfield
(1957, 1959, 1975), the studies by McClurkin (1953), and es-
pecially by the field identification key presented by Bowers
(1963). Bowers (1964) also investigated the natural history
and some aspects of the behavior of 0. corniculata in his ex¬
amination of niche separation between this species and O.
californiana. Further field studies on the distributions of
both burrowed and active O. corniculata were made by Craig
(1973). He sampled a population using pitfall traps and sub-
surface cores, but he did not extend his studies over the full
range of tidal conditions. Osbeck (1970) in laboratory studies
demonstrated two separate endogenous rhythms of activity, one
rhythm having a circadian periodicity, the other possibly
reflecting the tidal cycle. McGinnis (1972) found clear evi¬
dence of an endogenous circatidal rhythm in adult 0. corn¬
O. corniculata Population Movement
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Baker and Yip
iculata. The question of orientation in movement has also
been investigated by several researchers (Craig, 1971, 1973;
Enright, 1961; Hartwick, 1976).
Previous studies of O. corniculata have not provided a
detailed picture of population activity and movements in the
field. It was the objective of the present study to gather
the information necessary to provide such a picture. An ef¬
fort was made to sample the population over a series of dif¬
ferent tidal conditions sufficiently varied to make possible
the development of a simple predictive model for the move¬
ment of the population in space and time. Particular emphasis
was placed on obtaining data regarding directional movement
and on differences between adult and juvenile activity, since
little quantitative information is currently available on
these aspects of Orchestoidea biology.
FIELD STUDIES
Methods
Field studies were carried out between 24 April and 1
June, 1978, on a population of Orchestoidea corniculata on
the west beach of the Hopkins Marine Station of Stanford
University, located on Mussel Point in Pacific Grove, Cal¬
ifornia. A straight line transect was staked out from just
below the land vegetation down to about +2 ft. above mean
lower low water. Sampling stations were set up along this
transect at 4 m. intervals. Although the stations were
equal distances apart measuredalong the surface of the
O. corniculata Population Movement
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Baker and Yip
beach, the change in elevation between stations ranged from
1.7 to 1.9 ft. To sample the amphipod population, Pyrex
dishes 4.5 cm in depth and 9 cm in diameter were buried in
the beach at each station, with the lip of the dish level
with the sand surface. Amphipods were thus caught whether
walking or hopping. When the dishes were dry, the amphipods
were able to hop or crawl out of them easily; with an inch
of water in the bottom, the dishes retained both adults and
juveniles very effectively. Some dishes were made into
directional traps by placing a semicircular piece of wire
window screen, 10 inches high and 6 inches across the opening,
either above the dish and facing toward the ocean or below
the dish and facing away from the ocean
(Fig. 1). ob¬
Se
servations in the field showed that the amphipods neither
climbed nor jumped over the screens. Two directional traps
and one nondirectional trap were maintained at each station
on the beach. At certain times spatially intermediate
stations were established between permanent stations as
dictated by the height of the water.
Our own preliminary studies and earlier literature in¬
dicated that O. corniculata is mostly active during the hours
of darkness, therefore each sampling run was started at 1900
hrs Pacific Daylight Savings Time, and samples were taken at
hourly intervals through 0700 hrs the next morning. At each
sampling time, the traps were filled with water and allowed
to stand for 10 minutes. Then the water and any amphipods
in the traps were poured into marked fingerbowls to be counted.
The traps were immediately reburied on the beach. Once
O.
corniculata Population Movement
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Baker and Yip
counted, the amphipods were released back on the beach at
a distance judged sufficient to prevent any effects on the
results of subsequent samples.
Two tests were made to check the reliability of our
sampling techniques. In one preliminary study, traps were
established along 3 parallel transects 5 m. apart. Sampled
at hourly intervals, all profiles showed the same trends in
population fluctuations, and while absolute numbers caught
showed some variation there was no evidence of clumping.
In
a second test, the catches of duplicate directional traps
set 20 cm apart were analyzed statistically using the Rx C
test (Sokal and Rohlf, 1969). The duplicate traps showed no
significant differences in numbers caught during identical
periods (p.5), allowing the conclusion that variation
between duplicate traps was random and not due to clumping
of animals.
Data were plotted each time the population was sampled,
and the raw data immediately graphed to yield a stylized time-
lapse picture of the transect. See Fig. 2. For each hourly
observation the number of amphipods in each trap was plotted
at a point on the vertical axis representing height on the
beach. Information for large and small amphipods was treated
separately. Animals less than 6 mm in length in normal pos-
ture with the rear segments curled under the body were classed
as juveniles. All animals of greater length were classed as
adults though some probably were not sexually mature. Ori¬
In pre¬
ginal data are on file at the Hopkins Marine Station.
senting the results, these data are analyzed in three steps to
vield information on (1) temporal distribution of movement ac-
corniculata Population Movement
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Baker and Yip
tivity of the adult and juvenile populations, (2) direction¬
al movements of the amphipods up and down the beach, and (3)
distribution of adults and juveniles in space.
Results
Fig. 3 summarizes data showing the amount of movement
activity in relation to the time of day. The total amount of
activity for each hour is a number obtained by summing the
numbers of amphipods caught in traps at all levels of the
beach at that hour. Summing was done separately for adults
and juveniles. This number is an indication only of move¬
ment activity, and takes no account, for example, of amphi¬
pods which are stationary and feeding on the wrack. Adults
and juveniles show somewhat separated periods of activity.
Peaks of activity show a relationship to the times of dusk
and dawn, and to the period following a night time high tide,
accentuated in Fig. 3 by the line connecting the peaks of
the high tide.
In juvenile O. corniculata there are two peaks of ac¬
tivity, one around sunset and one around sunrise. The peak
of activity at dawn was also noted by Craig (1973) in a
short study. Another smaller and less regular increase of
activity occurs in the middle of the night and is as¬
sociated with a descending tide.
Movement activity of adult O. corniculata is more sharp¬
ly limited to the period of darkness, and generally shows a
bimodal pattern. When the high tide occurs before dark (eg.
May 1-2), the first peak of activity occurs shortly after
corniculata Population Movement
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Baker and Yip
dark. The animals move about relatively little as they feed
through the night, then a second peak of activity is seen
prior to dawn. As the high tide occurs progressively later
in the evening, the peak of activity is correspondingly
shifted in time. With the high tide after dark but before
midnight, essentially no adult activity is observed before
the high tide (eg. May 4-5). When the high tide moves past
midnight, only a single peak is seen after the high tide, but
as this occurs, another phase of activity begins to appear
after sunset (eg. May 10-11). These adults emerging near
dusk will burrow into the beach before the tide reaches its
highest level. A second peak of activity occurs that night
after the high tide has passed.
The peaks of activity shown in Fig. 3 exhibit a range of
magnitudes. A broad peak is usually correlated with a fall-
ing tide. Some apparent variability in both the height and
the timing of the peaks is probably not real, but results
from the fact that activity was only sampled hourly. Hourly
sampling adequately reveals the general pattern of activity
but a precise picture would require essentially continuous
sampling. The two periods which show least clearly the ex¬
pected pattern of activity (Fig. 3, May 4-5, May 15-16),
were those with the most extreme conditions of high wind and
surf. Amphipod movement activity is much reduced in harsh
weather conditions. In Fig. 3, the greater the number of days
elapsed between sequential observations, in general the great-
er was the degree of change in the pattern of activity. As
expected, data taken on two successive nights (May 24-25, May
p. 8
O. corniculata Population Movement
Baker and Yip
25-26), with similar weather conditions, show highly similar
activity patterns. Consistently, there is a separation of
adult and juvenile peaks of activity, even though this is
sometimes less clear on a descending tide.
The question arises as to whether the temporal separation
of adult and juvenile activity is accompanied by a spatial
separation, especially when significant numbers of both size
classes are active on the beach. Fig. 4 examines the distri¬
bution on the beach of adult and juvenile amphipods at speci¬
fic times. Although there is a tendency for juveniles to be
more widely distributed, there is no distinct spatial separa¬
tion of adults and juveniles.
The major directional movements of the O. corniculata
population on the beach surface are shown in Fig. 5. For
each station on the beach the numbers of amphipods caught in
a pair of oppositely oriented directional traps are compared;
the smaller number is subtracted from the larger. The dif-
ference is represented as a vector for net directional move¬
ment at that-station. If the difference between the two traps
was greater than 5 animals, a vector arrow is plotted on Fig.
5. This graph shows net directional movement in relation to
time, tide, and vertical position on the beach. At the times
of major activity (as shown in Fig. 3) the movement appears
to be directional. The net movement of amphipods is usually
in the direction that the tide is flowing. In the few ex¬
ceptions where movement is in the direction opposite to that
of the tide (eg. at 0130 hrs, May 8), the animals appear to
be going toward thelevel of a major concentration of wrack on
corniculata Population Movement
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Baker and Yip
the beach.
Vertical movements of amphipods along the slope of a
beach during changes in tidal height result in changes in
population distribution. In Fig. 6, distribution of the
population for a given night is described by histograms
showing the total numbers of amphipods trapped at each sta-
tion during that whole night. When a high tide occurs near
midnight, both the adult and juvenile amphipods are usually
trapped in greatest numbers at the relatively restricted level
where wrack is concentrated under these conditions. In con¬
trast, when the tide is descending through the middle of the
night, the adults are more dispersed; wrack is generally
abundant at several levels under such conditions. The ju¬
venile population may show a concentration higher up on the
beach, since the water level is higher during their peaks of
activity. During periods of strong surf, very little wrack
may be deposited on the beach, and the adults are again more
dispersed. These results substantiate the conclusion that
wrack is an important factor influencing the spatial distri-
bution of O. corniculata and that there is population move-
ment toward the wrack on the beach.
Discussion
The pattern of activity and directional movement of O.
corniculata discerned from the field studies appears to be
well-suited to a species living in the somewhat precarious
environment of the steep beach intertidal zone. Nocturnal
corniculata Population Movement
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Baker and Yip
activity provides maximal protection from visual predators
such as shore-birds (Bowers, 1964). Further, the amphipods
seem unable to tolerate much desiccation, and by burrowing
in the daytime when the surface sand is driest, they find
suitable moisture in addition to protection (Bowers, 1964).
A circatidal rhythm within the constraints of the circadian
rhythm provides even more precise behavioral adaptations.
For moisture and food, the amphipod must be active in the
area of the beach that is wetted by the high tides. However,
there is the danger of being swept away by the water. A
tidal rhythm of behavior which includes burrowing at high tides
greatly reduces this risk. The peak of movement activity
shortly after the high tide is advantageous to the animal
because it corresponds to the best feeding conditions. It
is important that net seaward movement is associated with
this peak in activity. By moving down the beach with the
descending tide, the amphipods find maximal opportunity for
feeding as they encounter the successive bands of fresh wrack
which are left by the receding water. Correspondingly, by
moving landward with a rising tide, the animals increase
their safety from wave action. Some animals actively move
up the beach, others are carried landward by the swash, rafted
aboard the wrack on which they are feeding (Bowers, 1964).
Landward movement is generally associated with burrowing. It
may also be advantageous for the animals to move landward
because if they burrow too low on the beach, they may be co¬
vered by water for a greater part of the tidal cycle, decreas¬
ing the time during which they can emerge and feed.
corniculata Population Movement
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Baker and Yip
Bowers (1964) described the distribution of the O.
corniculata population on a beach only about 300 m. over¬
land from our study site. He noted that distribution was
related to the semi-lunar spring-neap tidal cycle. Bowers
observed that the population was higher on the beach during
the spring tides and moved down the beach along with the
shift to neap tides and as the wrack each day was deposited
lower down. The beach he studied (Monterey Boatworks beach
faces northeast rather than west at Mussel Point, has a much
gentler slope, and is much more protected from high surf.
In the present study, as data were gathered over a period of
roughly two semi-lunar cycles, such a pattern was looked for
in the spatial distribution of the population on the beach.
However, it became apparent that on our steeper west-facing,
and much more exposed beach, weather conditions have a much
greater effect on the height of water and the presence of
wrack on the beach than does the spring-neap tidal regime.
In addition, the analysis in Fig. 6 shows that the popula-
tion distribution of active animals is modified by the timing
of the high tide. The situation with regard to the semi-lunar
pattern of distribution appears to be somewhat more complex
than that described by Bowers, even given constant weather
conditions.
Having considered the general advantages to the amphipod
population of the movement and activity pattern seen in the
field, we now address the question of what selective advantages,
if any, are accrued by a temporal separation of adult and ju¬
venile activity. Available information suggests several pos-
O corniculata Population Movement
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Baker and Yip
sibilities. One selective advantage might be reduction of
intraspecific competition, especially for food. Since no
actual experimentation was done in this area, speculation
on it is reserved for later discussion.
A second selection pressure that might encourage sepa¬
ration of adult and juvenile activity peaks was suggested by
an observation made in the field. A small individual nudged
into a burrow occupied by a larger amphipod was immediately
rabbed and eaten by the latter. Clearly, cannibalism by
adults on juveniles might provide a selective pressure to¬
ward separating peaks of activity of the two principal size
classes.
LABORATORY STUDIES
To investigate the possibility of predation by large
O. corniculata on smaller individuals, laboratory tests were
carried out using 500 ml flasks plugged by foam stoppers
Control flasks contained only juveniles, experimental flasks
contained both juveniles and adults. Factors varied in the
experiment included the amount of sand, the number of large
individuals, and the amount of fresh kelp put in the flasks
as food for both juveniles and adults. Unless otherwise
noted, all adults were captured24 hours before the experiment
began, and thus were without food for one night. Juveniles
were caught and placed in the flasks the day of the experiment.
The flasks were then placed in a dark cabinet and left over-
night. The number of juveniles recovered from each flask was
O. corniculata Population Movement
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Baker and Yip
compared with the number originally placed in the containers.
Both the procedures and the results are pictured in Fig. 7
for these experiments. In general, it can be seen that while
100% of the juveniles were recovered from all the control
flasks, juveniles were often missing from those flasks which
did contain adults, and those could only have been eaten.
Field and laboratory observations show that cannibalism
is one possible selction pressure for temporal separation of
adult and juvenile movement activity at the beach surface.
It would appear advantageous for the juveniles to be active
when adults are burrowed and to burrow and seek concealment
during periods of adult activity. Assuming that burrowing
does provide protection, we expected reduced cannibalism in
laboratory experiments as the depth of the sand layer in the
flasks increased. Instead, the least juvenile mortality was
seen in flasks floored with a monolayer of sand grains;
greater mortality was found in the flasks with more sand,
though those with 1/2 inch of sand and those with 1 inch of
sand showed no significant difference. Direct observations
of the animals in flasks indicate that a monolayer of sand
over glass does not provide a substrate allowing effective
locomotion and normal behavior, resulting in reduced juvenile
mortality. In the flasks with more sand, no observations were
made which might explain the lack of advantages of the greater
amount of sand. The experiments indicate that adults of both
sexes feed on juveniles under laboratory conditions. The
presence of a surplus of an alternate source of food such as
the kelp Macrocystis does not necessarily prevent cannibalism.
corniculata Population Movement
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Baker and Yip
Having established the temporal separation of adult and
juvenile activity peaks, and confirmed one possible selective
advantage thereof, the next area we explored was the mechanism
of separation. One possible mechanism for achieving activity
separation of the size classes might be an active avoidance
of adult amphipods by juveniles.
In studying direct behavioral interactions of adults and
juveniles, two different techniques were used: direct ob¬
servation, and photography. For direct observations, the
animals were kept in 500 ml flasks and watched under normal
laboratory lighting. For the photographic studies, the amphi¬
pods were placed in a petri dish with a diameter of 15 cm
lined with damp filter paper to provide a substrate suitable
for movement of the animals. The dish was divided in half by
a cardboard partition to allow simultaneous photography of two
physically isolated populations. The animals were kept in to-
tal darkness except when photographed with an electronic flash.
Direct observations show that juveniles are indifferent
to the presence of active adults, even when physical contact
occurs. Photographic studies support this conclusion. Distri¬
bution of juvenile amphipods in one half of a petri dish does
not significantly change after an adult is introduced into
the container and neither is there a significant difference
in the distribution of juvenile amphipods between two physi¬
cally separated halves of a dish when one half contains an a¬
dult and the other does not.
To investigate the possibility that differences in en¬
dogenous rhythms between adults and juveniles result in dif-
O. corniculata Population movement
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Baker and Yip
ferently timed peaks of activity, a laboratory setup was de-
signed to determine if juveniles deprived of environmental
cues exhibit peaks similar to those seen in the field, and if
these peaks might be affected by the presence of adults in a
more subtle way than could be revealed by our other studies.
Three-gallon circular cartons were obtained and a circu¬
lar hole cut in the bottom of each to allow insertion of a 6
cm in diameter
cm long plastic tûbevopen at both ends. When the carton was
filled to a depth of 4 cm with sand, the sand just reached
the level of the top of the tube on the inside and the tube
acted as a pitfall trap: any animal falling into the tube
would drop through to a dish of water placed below the bot¬
tom of the tube. The tube could be plugged at most times,
enabling amphipods to jump out again if they fell in, and un-
plugged only at sampling times. The animals for this experis
ment were collected the day the experiment began. Some of the
cartons contained only small individuals, others both large
and small. All experiments were run between 1930 and 0830 hrs.
The cartons were kept in constant dim light and were sampled
for five minutes at hourly intervals.
Fig. 8 shows the setup, the details of each experiment
and the results. All four cartons show juvenile activity pat¬
terns similar to those observed simultaneously in the field.
Peaks in the number of juveniles trapped were around the hours
of sunsét and sunrise. No significant differences were obser¬
ved between the cartons that contained juveniles and those that
did not. An earlier experiment in which the juveniles (200 per
carton) were kept in constant dim light for a period of 18
hours before sampling began yielded similar results.
O. or
iculata Population Movement
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Baker and Yip
GENERAL DISCUSSION
The field population of O. corniculata studied exhibits
a pattern of movement and activity that shows adaptation to
the constantly changing conditions of their intertidal beach
habitat, maximizing both protection from hazards and stress
and opportunity to feed on fresh wrack. Fig. 9 is an ideal-
ized schematic diagram showing the general pattern of surface
movement and activity in adult and juvenile O. corniculata
as indicated by our field data. The pattern of behavior in¬
cludes a temporal separartion of peaks in adult and juvenile
activity. For achieving this separation, one mechanism
would be an avoidance response by juveniles to adults, the
juveniles burrowing or moving away as adults approached. No
indication of such avoidance is found. Instéad, our labora¬
tory studies of juvenile movement activity, together with
the studies of adult activity by Osbeck (1970) and McGinnis
(1972), strongly suggest that it is differences in juvenile
and adult activity rhythms that account for the differences
in juvenile and adult peaks of activity. Further, the fact
that the juvenile pattern is maintained after 48 hours of
constant light and moisture conditions suggests that the
rhythm is endogenous. Although this rhythm seems unaffected
over a two day period by absence of adult activity, the long
term effects of such absence are not known.
Interesting questi ons arise as to when and how the acti¬
vity pattern of the juveniles gives way to or become the
adult pattern. Although these studies did not directly ap-
corniculata Population Movement
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Baker and Yip
proach this problem, qualitative observations suggest a gradual
transformation. Smaller, probably sexually immature sub-adult
amphipods were generally caught in the traps at least one hour
before the larger adults appeared. This suggests a gradual
shifting from juvenile to adult activity patterns.
With regard to the possible selective advantages for the
temporal separation of juvenile and adult activity peaks, can -
nibalism of young by adults has been demonstrated. This
phenomenon can be significant in the laboratory.
It occurs
in the field, but its significance there and its dependence
on environmental parameters are unknown.
A second possible advantage to temporal separation of a¬
dult and juvenile activity peaks is the reduction of intra¬
specific competition. Juvenile amphipods are basically mini¬
atures of the adults. Both feed on fresh wrack, especially
Macrocystis, the giant kelp. Temporal separation of activity
peaks is a way of increasing the food resources and making
more efficient use of the supply. Since wrack bands are con¬
stantly being deposited or washed away, increasing the amount
of time that the population as a whole is out and active in¬
creases the amount of wrack available to the population.
Speculating further, it seems likely that the juveniles can
relatively safely emerge before darkness and remain on the
surface for sometime after dawn, allowing them periods of
feeding before and after adult activity. We found the juveniles
to be much smaller and faster moving than the adults, thus they
may be poorer targets for shorebirds, the major amphipod pre¬
dator (Bowers, 1964), which have peaks of feeding activity
p. 18
O. corniculata Population Movement
Baker and Yip
at dawn and dusk.
The daily temporal partitioning of the food resource is
amplified over portions of the tidal cycle by the fact that as
the high tide occurs at different times, the amount of wrack
available to amphipods at a given time of the night also
varies. Thus, the juveniles sometimes have an opportunity to
feed on wrack that is not available to the adults, for example:
any wrack that is washed away between the time that the juve¬
niles emerge and when the adults become active. While adults
and juveniles do not show any clear separation in space, the
trapping data indicate that the juveniles are spread out more
widely on the beach than the adults. This would permit them
to make greater use of the smaller pieces of wrack spread
more widely on the beach, while the adult amphipods center
on the greatest concentrations of food.
O. corniculata Population Movement
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Baker and Yip
SUMMAR
1.
A population of Orchestoidea corniculata on a steep sandy
beach at Mussel Point, Pacific Grove, California was studied to
determine movement activity at the sand surface. Sampling was
done with directional and nondirectional pitfall traps on a
transect taken from the land vegetation to the sea.
The adult field population shows peaks of activity that
2.
exhibit both the circadian and circatidal rhythms reported by
earlier investigators. Adults are burrowed in the sand by day
and activity on the beach surface is confined to the hours of
darkness. When high tide occurs before midnight, the adults do
not emerge until the tide begins to ebb. They show a peak of
movement activity soon after emergence, followed by a period of
feeding during which little movement occurs, and then a second
peak of movement activity before sunrise. If high tide falls
after midnight, the adults emerge sometime after sunset, burrow
before the high, and then reemerge, showing another peak of
activity on the descending tide.
The peaks of movement activity of adults and juveniles
3.
do not occur simultaneously but show a temporal separarion.
The juveniles usually show a peak of activity around sunset,
before the adults emerge, and another peak around sunrise, after
the adults have burrowed for the day. Juveniles sometimes show an¬
other peak in movement at the same time as the adults on a
descending tide during the night.
4.
There is no correspondingly distinct spatial separation
between adult and juvenile activity on the surface of the beach.
O corniculata population Movement
p. 20
Baker and Yip
Generally, O. corniculata move up and down the slope of
the beach in the direction that the tide is going. This be¬
havior is advantageous in maximizing both protection from surf
and desiccation, and opportunity to feed. Seaward movement is as¬
sociated with feeding on the bands of wrack deposited by the
receding tåde. Landward movement is often associated with bur-
rowing, which provides further protection from wave action.
6.
Distribution of active adult and juvenile amphipodseon
the beach over the entire night is a function of the tidal
cycle and the location and amount of wrack present. The timing
of the night high and low tides is crucial, as most directional
movement occurs during the amphipod activity peaks. A low tide
in the middle of the night favors adult dispersal; low tides at
dusk and dawn allow juvenile dispersal. Adults tend to gather
near major wrack concentrations, while juveniles are more widely
distributed. However, when very little wrack is available the
adults also tend to disperse widely.
Cannibalism of juveniles by adults, demonstrated in field
and laboratory, provides one selctive pressure for temporal
separation in adults and juveniles of peak activity. Laboratory
studies show no significant correlation bétween extent of can¬
nibalism and 1) the amount of sand available for juveniles bur¬
rowing, and 2) the presence of alternate food such as Macrocystis.
Temporal separation of adult and juvenile activity peaks also
reduces intraspecific competition for food, and extends the time
available for wrack feeding.
No avoidance of adults by juveniles was demonstrated, even
8.
The mechanism for separating a¬
when physical contact occured.
dult and juvenile activity appears to be a difference in the
timing of rhythmic behavioral patterns.
0. corniculata Population Movement
p. 21
Baker and Yip
ACKNOWLEDGEMENT
We would like to thank the faculty, staff and students
of Hopkins Marine Station, in particular Larry Harding and
Robin Burnett. Most especially, we would like to thank
Dr. Donald P. Abbott for his exuberant guidance.
corniculata Population Movement
p. 22
Baker and YIp
LITERATURE CITED
Bousfield, E.L. 1957. Notes on the Amphipod Genus
Orchestoidea on the Pacific Coast of North America.
Bull. Southern Calif. Acad. Sci. 56: 119-129.
Bousfield, E.L. 1961. New Records of Beach Hoppers
(Crustacea: Amphipoda) from the Coast of Calif¬
ornia. Bull. Natl. Museum Canada, Contrib. Zool.
172: 1-12.
Bousfield, E.L. 1975. Morphological Key to Talitridae,
pp. 352-355. In Smith, R.I. and Carlton, J.T.
Intertidal invertebrates of the central California coast, 3rd ed,
(eds.) Light's Manual: University of Calif. Press,
716 pp.
Bowers, D.E. 1963. Field Identification of Five Species of
Amphipods. Pacific Sci. 17: 315-320.
Bowers, D.E. 1964. Natural History of Two Beach Hoppers of
the Genus Orchestoidea (Crustacea: Amphipoda) with
Reference to their Complemental Distribution, Ecology,
15: 627-696.
Craig, P.C. 1971. An Analysis of the Concept of Lunar
Orientation in Orchestoidea corniculata (Amphipoda),
Anim. Behav. 19: 368-374.
Craig, P.C. 1973. Behavior and Distribution of the Sand
Beach Amphipod Orchestoidea corniculata, Mar. Biol.
23: 101-109.
O. corniculata Population Movement
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Baker and Yip
Enright, J.T. 1961. Lunar Orientation of Orchestoidea,
Biol. Bull. 120: 148-156.
McClurkin, J.I. 1953. Studies of the Genus Orchestoidea
(Crustacea: Amphipoda) in California, unpublished PhD.
thesis, Stanford Univ. 207pp.
McGinnis, J.W. 1972. A Tidal Rhythm in the Sand Beach Amphi-
pod Orchestoidea corniculata, unpublished Bio. 175H
paper, Hopkins Marine Station of Stanford Univ. 23pp.
Osbeck, B.L. 1970. Circadian and Tidal Rhythms in the Sand
Beach Amphipod Genus Orchestoidea (Talitridae), un-
published MA. thesis, Univ. of Calif. Santa Barbara,
47pp.
Sokal, R.R. and Rohlf, J.F. 1969. Biometry, W.H. Freeman,
San Francisco, 776pp.
Hartwick, R.F. 1976. Aspects of Celestial Orientation Behavior
in Talitrid Amphipods, pp. 189-197. In De Coursey, D.J.
(ed.) Biological Rhythms in the Marine Environment, Univ.
of South Carolina Press, Columbia, 233 pp.
p. 24
O. corniculata Population Movement
LIST OF FIGURES
Fig. 1.
Diagram of the beach transect sampled, seen from
above. Circles represent pitfall traps and the
larger semicircles represent wire screens placed
either above or below the traps to insure direc¬
tional catches. The elevation of the beach sur-
face at each station is given as height above
mean lower low water.
Graphical representation of field data from one
Fig. 2.
night of observations. Bars without arrows show
numbers of animals caught in nondirectional traps
and bars with arrows those caught in directional traps.
The lengthx width of the bar is proportional to
the number of animals caught. The level at which
the amphipods were trapped is marked by the center of
the bars without arrows, or the bagse of the bars with
arrows. The tip of each arrow indicates the direction
of travel implied by the type of trap in which the
amphipods were caught. Location and relative size
of the wrack bands are indicated. Sunset was at 2000
hrs and sunrise was at 0600 hrs, Pp81.
Movement activity of juvenile and adult 0. corn¬
Fig. 3.
iculata ds shown by the total number of amphipods
of the two size groups trapped at all stations
each hour. Each square contains the data for one
night of sampling. Predicted tidal height and
corniculata Population Movement
p. 25
Baker and Yip
actual height of the swash are shown only to indicate
timing of the tides and severity of the surf.
Fig. 4.
Spatial distribution of adult and juvenile O.
corniculata at selected times (right column),
and total number of animals trapped versus time
for the same period (left column). Histograms in
kere
the right column show levels on the beach,amphipods
were trapped, and numbers caught. Adult/juvenile
composition of the actively moving population
at the particular times analyzed can be obtained
from the left column (see arrows).
Vectors representing net directional movement up
Fig.
and down the beach in relation to time, tidal
cycle, swash, and wrack deposits. Graph depicts
nine nights, showing different relations of diel
and tidal cycles.
Cumulative spatial distribution of active amphi¬
Fig. 6.
pods over the entire night for six differnt nights;
horizontal black lines in center columns show
total numbers of juvenile and adult amphipods
caught during the whole night at each level of the
beach sampled. Two columns at margins show tidal
regime, swash, and location of wrack on the beach
for each night studied, adjacent to the distribution
curves.
Fig. 7.
Graph showing procedures and results of laboratory
corniculata Population Movement
p. 26
Baker and Yip
experiments investigating cannibalism of juveniles
by adults. Each bar represents the results obtained
from one flask. Total length of bar shows number of
juveniles added to flask in the evening, black
portion shows number consumed by adults. Ad. -
adults, Juv. - juveniles.
Fig. 8.
Results of experiments comparing activity patterns
of juveniles maintained in 3 gallon cartons under
constant dim light, in the presence and in the
absence of adults. The experimental setup is
pictured at the top of the graph. The graph shows
the numbers of juveniles caught in a pitfall trap
in a 5 minute sampling period at hourly intervals,
and reflect the amount of movement activity in
the carton. All experiments were run in duplicate:
mean values are shown by the thick bars, ranges by the
the thin vertical lines thereon.
Fig. 9.
An idealized model, based on field data, showir
changing patterns of activity, and directional
movement in O. corniculata. Zigzag horizontal lines
portray tidal rise and fall. Five days are shown,
representative of a full tidal cycle. Squares re¬
present feeding without much accompanying movement
activity. The diagonal dashed line is merely a
visual aid connecting the high tides occuring
2-3 days apart. Triangles on baselines indicate
major periods of activity for adults (dotted lines)
e
p. 27
O. corniculata Population Movement
Baker and Yip
and juveniles (solid lines). The arrows indicate the
direction of net movement of adults (white) and
juveniles (black).
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Time of Day
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XXX Wrack
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100
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E 250
6 150.
100
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E 100
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Time of Day

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May 4-5


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200
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250
150
100
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100
Time of Day
.
Darke
May 18-19

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—
May 24-25


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May 31—
June


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Time of Day
Key
—.——
Adult Amphipods
Juvenile Amphipods
Predicted Height of Tide
-Height of Swash on Beach
F
Time of Day
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Time of Day

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5


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—
No. Adults No. Juveniles
Scale: E = 50 Anlmals
sunsel
sunrise
TIME

dirkness
Moy i-2
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111


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May 15.16
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TIME OF DAY
12
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sunrise
TIME
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May 18-19


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e
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May 24-25

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—

Moy 312June1
XXXOXIX
111


— — —1—
TIME OF DAY
KEY
— Predicied Height of Tide
---- --- Height of Swash on Beach
XX
Wrack
SCALE
Juvenile Amphipods
=25 Animals
Adult Amphipods
12
10
8
512
50
S
6
TIME
Number of Amphipods Trapped During Night
TIME
ai Each Station
100 0 100
100 200
200 100
May 10-11
May 7-8
XXXXXXX.
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221
May 31—June
May 18—19
XXXXXXXX
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p;
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May4-5/
May 15—16
XXXXXXX
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—
—
200—
100
100
200
Number of Amphipods Trapped During Night
TIME OF DAY
at Each Station
TIME OF DAY
—Predicted Height of Tide
— —-Height of Swash on Beach
X X Wrack
Juvenile
auit
/Amphipods
Amphipods
Sand-
No.
Juveniles
Added
No. Not
Recovered
X Kelp
15 Juv.


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15 JUV.




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15 Juv.
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15 Juv.
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adults not starved overnight
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May 31
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X 2
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40
320

Juveniles
40
X 2
Adults
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20+
10+
H
8
TIME
Fig


4
Time of Day

Darkness
4


1