ABSTRACT
A study of the activity patterns of Collisella scabra showed
that the most prevalent movement occurred during periods of
submergence with no difference between daylight and darkness.
Furthermore, studies showed that cephalic tentacles are not
essential to the homing ability of this species, but they do play
a role in the ability to detect the directionality of mucous
trails.
INTRODUCTION
Collisella scabra is a homing species of limpet. This
particular species grows its shell margin to fit the substrate at
its home site (Davis, 1895; Underwood, 1979; et al). This
provides protection against removal by predators, as well as
providing resistance to desiccation while the limpet is emerged
at low tide (Branch, 1981, 1986). Because these factors depend
on the ability of C. scabra to return to its home site and orient
properly after a foraging run, a rigid homing instinct is of
paramount importance to its survival.
The mechanism which C. scabra uses to return home is a topic
of some dispute. Several mechanisms have been proposed to
explain this homing ability; these mechanisms fall into three
categories (Cook 1969,1971; Thomas 1973; et al):
1) Use of external cues: these mechanisms rely on
factors not associated with the limpet or the limpet's
home rock. These include the position of the sun and/or
moon, identifying coastal landmarks, polarization of
light, magnetism, and gravity.
2) Dead-Reckoning: this relies on the ability of the
limpet to recall either distances travelled and angles
rotated or linear and angular accelerations and
"compute" an acceptable route home.
3) Use of endogenous clues: these mechanisms rely on
factors associated with or incorporated onto the
surface of the rock. These include following a mucous
trail home, and possessing a topographic memory of the
rock surface.
The ability of limpets to home after the home rock was displaced
or rotated have demonstrated that C. scabra do not rely
exclusively on external clues for their homing mechanism
(Underwood, 1979). Likewise, the ability of limpets to home in
displacement experiments has eliminated dead-reckoning as the
sole homing mechanism in C. scabra; limpets cannot rely on
remembering their outward trail and be able to return home after
being displaced from their original path (Branch, 1981). The
third category of mechanisms has proven to include the most
likely strategies employed by a limpet to return home following a
foraging excursion. Ability to follow mucous trails has been
demonstrated for many species of gastropods, including C. scabra
(Cook and Cook, 1975; Peters, 1964; Connor, 1984a,b; et al.)
Before a detailed study of the homing mechanism used by C.
scabra could be undertaken, it was necessary to perform a set of
field experiments which would place the homing ability of this
species in its proper context. Populations of C. scabra were
observed over a diel cycle and its general activity pattern
The size of limpets was measured at different
recorded.
intertidal heights, as well as distance travelled on a foraging
run, and time spent foraging. Furthermore, the paths taken by C.
scabra in its foraging excursions were recorded. Completion of
these observations set the stage for a study of the mechanism(s)
used in the homing ability.
Several researchers have examined the role that cephalic
tentacles play in the homing of gastropods, both in field and
laboratory situations. The cephalic tentacles of gastropods are
a tactile and an olfactory organ (Peters, 1964), and may be of
great importance in the trail-following ability of C. scabra.
Field experiments involving the removal of cephalic
tentacles have yielded conflicting results. Davis (1895) removed
the tentacles from 2 individuals and displaced them 5-6 cm from
their home site -- these animals returned to their home site and
Davis concluded that tentacles are not indispensable to the
homing ability of C. scabra. However, Jessee (1968) removed
limpets from their home sites, anesthetized them with magnesium
chloride and left them in an aquarium for 3 days to recover
before similar displacement. Most of the limpets in this study
did not return home and those that did may have relied on an
alternate mechanism since directionality in mucous trails is only
present for 48 hours (Connor, 1984b). Using a mucous trail to
return home requires that the limpet not only be able to detect
the mucous trail, but also to distinguish its trail from others
and to determine the directionality of the trail.
Experimental manipulations of gastropods have been the key
factor in demonstrating the ability of these organisms to follow
mucous trails, and are the only studies which have demonstrated a
Unfortunately, only a
directional component to the trail.
handful of studies have examined the role of the cephalic
tentacles in trail-following experiments. Peters (1964) reports
that Littorina planaxis requires its cephalic tentacles to detect
mucous trails at all. Behavioral observations by Connor (1984b)
suggest that Collisella scabra uses its cephalic tentacles to
detect the directional component of the trail. She describes C.
scabra touching alternate sides of a trail with its tentacles as
it progresses along it.
Two studies were undertaken using C. scabra in order to
examine the role of the cephalic tentacles in the homing ability
of this species. First, a field study was performed to resolve
some of the conflict as to whether C. scabra is able to home
after excision of its cephalic tentacles, and second, a trail¬
following experiment was conducted to determine the role of
cephalic tentacles in this type of homing ability.
MATERIALS AND METHODS
ACTIVITY PATTERNS
Observation of the movements of Collisella scabra were made
at 15 minute intervals over a 24-hour period at Cabrillo Point,
Pacific Grove on May 30, 1990. Three relatively flat study sites
were chosen along a transect at .83 m, .98 m, and 1.29 m above
MLLW. Snorkel equipment was used to observe movement during
periods of submergence. The shell length of individual limpets
was first measured along the line of greatest distance and then
their home sites were marked using Wet N' Wild nail color with
one mark placed on the anterior portion of the shell and a
corresponding mark placed adjacent to the home site. Clear
acetate sheets were then placed over the 1 m study sites and a
number was written on the transparency corresponding to
The home site marks and the
individual home sites.
transparencies denoting the position of the home site facilitated
observations which required distinguishing those limpets which
were on their home site from those that were away from it. The
positions of 100 individuals in each of the three study sites
On a subsequent period of
were recorded in this manner.
activity, 25 individuals in each study site were chosen at random
from the group of motile animals; the time spent foraging and the
maximum distance travelled by each of these limpets were
recorded.
These transparencies were also used to record the paths
taken on foraging runs. At 15 minute intervals, the position of
a given limpet was marked on the transparency using a lead
pencil.
TAGGING
Individuals which were used in the tentacle removal needed
an identifying mark so that they could be replaced adjacent to
the correct home site. Limpets were identified using a system of
different colored dots to represent Roman numerals: Green - 1;
blue - V; Orange - X. These markings remained up to 6 weeks.
Home sites were identified using transparencies in the manner
described above.
TENTACLE REMOVAL IN THE FIELD
In removing the cephalic tentacles, I strayed from the
methods of previous researchers and found that an ounce of
patience was worth a liter of solution of Mgcl, isotonic to
seawater. Previous studies involving the removal of cephalic
tentacles employed the use of just such a solution to anesthetize
animals and facilitate the tentacle excision (Peters, 1964;
Jessee 1968). However, C. scabra will readily extend their
cephalic tentacles if submerged and placed on their shell. To
facilitate tentacle removal and reduce the size range of limpets
used, C. scabra ranging from 12-15 mm were used in this study.
Although Hewatt (1940) reports limpets smaller than 14 mm do not
home, Jessee (1968, and personal observations) indicates that
only limpets smaller than 6 mm fail to home. A subsequent report
by Sommer (1982) indicates that limpets as small as 2 mm display
homing ability although a higher degree of home site changing is
observed.
In this study, 25 limpets were placed on their shells in a
large petri dish and the tentacles, once extended, were removed
using a pair of iridectomy scissors. 25 other limpets were
removed from the rock and placed in a similar petri dish on their
shells to serve as controls. In these field experiments, limpets
were then immediately replaced to within 6 cm of their home site
on a rising tide, and their ability to return home was noted on
the subsequent low tide.
MOVEMENT DURING SIMULATED TIDE
A tide tank was set up to simulate the rising and falling of
the tide. A series of 7/16" holes were drilled in 1" PVC pipe
and used as spraybars to simulate wave-action. This was attached
to a filtered seawater system through a Rainmatic 2000. This
device has the capacity to execute 8 on/off cycles and hence
could be programmed for up to 4 days (two high and two low tides
per day). A series of 7/16" holes was also drilled vertically in
a PVC standpipe to allow for a gradual rising and receding of the
tide.
TRAIL FOLLOWING EXPERIMENTS
Using techniques described by Connor (1984b, modified from
Cook & Cook, 1975), the ability of limpets to follow mucous
trails was examined. Glass plates were scrubbed with Alconox,
dipped in a suspension of carbon particles, and rinsed with
Carbon particles adhere to mucous, and
distilled water.
therefore, exposing the plates to this suspension ensured that
mucous was removed from previous experimental trials. The glass
plates were then placed in the tide tank described above and over
a period of 24 hours collected a layer of fine particles. Trails
laid down on these plates appeared as clear trails through the
layer of sediment.
Once a trail was laid down, the limpet was removed and the
plate rotated 90° to account for any external cues. Depending on
the experiment, the same limpet or a conspecific was placed .5 cm
from the middle of the trail. Equal numbers of limpets were
placed on each side of the trail to account for any innate
turning preference. Responses to the trail were: 1) no movement
(not used in analysis); 2) crossing the trail; 3) following the
trail toward the origin; or 4) following the trail away from the
origin. Individual limpets were used only once. A trail lasted
30 minutes, and a limpet had to move at least 5 cm along a trail
in order for that trial to be recorded as a positive following
experience.
This trail-following experiment described by Connor (1984b)
was repeated using C. scabra collected at Cabrillo Point, PG. In
addition, limpets with excised tentacles were tested for their
After
trail following ability using a similar technique.
excision of the tentacles, but before placement on the glass
plates, limpets were placed in an aquarium for a period of
approximately one hour or until notable movement was observed.
From this point, the procedure described above was repeated.
RESULTS
ACTIVITY PATTERNS
Figures la,b,c display the activity of C. scabra over a 24
hour period at three distinct intertidal heights. No activity
was observed while the limpets were emerged at low tide (15
daylight observations and 2 during low tide at night). The
period of greatest movement was found to correspond to daytime
submerged periods at each site, although substantial movement was
also observed during awash periods, both day and night, and
during nighttime submerged periods. In 8 weeks of observations,
C. scabra was found to strictly adhere to this pattern.
Figure 2 represents the relationship between the size of the
limpet and the study site in which it was found. This figure
shows a significant relationship between the size of a limpet and
its position in the intertidal region (r - .528, p « .0005). On
the average, smaller animals are located lower in the intertidal
zone, with the size of the animal increasing with intertidal
height. Figure 3 displays the relationship between the size of a
limpet and the maximum distance travelled in a given foraging
excursion. A signifacant relationship was also found to exist in
this case (r - .448, p « .0005). In general, larger animals
travelled greater distances while foraging.
These relationships do not represent a random sample of the
population since study sites were chosen for the presence of C.
scabra; however, there is a significant relationship between the
size of an animal, its position in the intertidal, and the
maximum distance travelled in a given foraging trip.
Furthermore, subsequent observations have shown these patterns to
be representative of this population of C. scabra.
Figures 4a,b,c display some of the common foraging patterns
of C. scabra. Platzfrass (fig. 4a) behavior is evidenced by a
limpet raising its shell and turning on its home site.
Platzfrass or "eating in place" behavior was so termed by Funke
(1968 in Connor 1984b) based on behavioral observations which
suggested that limpets were feeding on a ring of algal material
which grows around the home site in between the perimeter of the
shell and the area occupied by the foot of the limpet. The other
foraging patterns are similar to patterns observed in other
species of homing gastropods. Figure 4b represents foraging runs
in which limpets return to their home site by retracing their
outward paths; figure 4c represents another common foraging
pattern employed by limpets where the outward trail is not
retraced in the journey home.
TENTACLE REMOVAL IN THE FIELD
After allowing sufficient time for the limpets to return to
their home sites (one period of submergence), limpets were
classified as: 1) home -- on its home site and oriented properly,
2) not home -- away from the home site or not oriented properly,
or 3) missing -- not found within reasonable distance of the
study area. Results from this field experiment are presented in
table I. It is evident from this table that no significant
differences in homing ability were found to exist between control
limpets and those with excised tentacles.
Observations of the movement patterns of these two groups
were made on four subsequent daytime high tides and no
substantial differences were seen in numbers of limpets moving,
average distance travelled, and average foraging time of the two
groups.
TRAIL FOLLOWING IN THE TIDE-TANK
Activity patterns of limpets placed in the tide-tank
described above were consistent with those displayed in figure 1
(a-d).
The results of the trail following experiments were analyzed
using a Chi square test comparing observed results to 503
expectation for trail following vs. trail crossing (i.e., if
trails had no effect, half of the limpets would follow the
trails, and half would cross it). Of those limpets which
followed the trail, frequency of trail following toward the
origin vs. trail following away from the origin were again
compared to 508 expectation values (Table II).
Results of the experiment on intact individuals were
consistent with those found by Connor (1984b). Intact
individuals followed trails more often than they crossed them.
Individuals placed next to their own trails followed them
preferentially toward the origin, while individuals placed next
to trails of conspecifics followed trails preferentially away
from the origin. Limpets with excised tentacles followed
trails more often than they crossed them.
However, the
directional preference found in intact individuals was lost in
the limpets lacking cephalic tentacles.
DISCUSSION
A study of the activity patterns of Collisella scabra at Big
Fisherman's Cove on Santa Catalina Island, CA, reports movement
only during daytime awash periods (Wells, 1980). Wells defines
an awash period as "the time interval during which the zone of
turbulence accompanying the tide is present at a particular
intertidal height." Apparently, the lack of severe wave action,
and accompanying turbulence, at Big Fisherman's Cove caused the
awash period,as Wells defines it, to be restricted to hour long
periods when the limpets were just covered by the ebbing tide.
This pattern of movement is similar to that found by Hewatt
(1940) and Wells (1917). However, Brant (1950) reports that
feeding excursions occur at high tide with no apparent
differences in activity between daylight and darkness.
Furthermore, Villee and Groody (1940) report only small
individuals of C. scabra were seen to move at all. However,
these observations, as well as those of Hewatt (1940) and Wells
(1917) were largely restricted to low tide, with high tide
observations being made without the use of snorkel or SCUBA
equipment.
With these extreme differences in activity patterns between
different populations of C. scabra, I found it necessary to
define the times of greatest movement for the population of
limpets used in this study. The sites employed in this study
were characterized by moderate wave action. The zone of
turbulence was present throughout the entire submerged period,
even at the study site lowest in the intertidal region. The
difference in the amount of wave action in different studies, and
hence the amount of time spent in the zone of turbulence, could
account for the differences in the amount of limpet movement at
varying degrees of submergence found in these studies. This
suggests a detailed study of the correlation of limpet movement
with wave action in both the same and geographically isolated
sites to determine the relationship between turbulence and
activity as well as its potential adaptiveness.
This study shows that limpets higher in the intertidal move
more during an awash period than animals lower in the intertidal,
suggesting that time spent submerged is a factor in the amount of
movement seen at different degrees of submergence. In addition,
with larger limpets being located in the higher intertidal region
(see fig. 2), the size of a limpets could also be playing a role
in the different amounts of movement observed during submerged
periods. Tidal location, wave exposure of the site, and limpet
size may all play a role in a limpet's activity patterns.
The differences in nighttime movement seen in these studies
It would seem here that the
is more difficult to explain.
limpets are already more active at Cabrillo Point than at
Catalina and thus experience less pressure to be actively feeding
at night. However, the amount of movement a limpet is required
to undertake for its survival at a given location depends on the
type of food, its abundance and the population density, as well
as the growth rates of individuals and their reproductive
potential. Another explanation is the differences in the types
of predators found at different sites. Wells (1980) examined the
effects of octopus, a nocturnal predator of limpets, on activity
patterns of C. scabra. The lack of nighttime movement of C.
scabra recorded by Wells could be due to effects of predation by
At Cabrillo Point,
octopus or other nocturnal predators.
predation by octopus is a rarity at best, and is certainly not
common enough to affect activity patterns of C. scabra found at
this site. A site or geographic difference in the basic foraging
behavior of a limpet with a predator's presence brings up the
interesting question of genetic versus learned determination.
Studies on this could give ecologists new perspective on species
interactions important in population dynamics.
The paths a limpet follow in a given foraging run can be
grouped into the three categories shown in figure 4 (a,b,c),
These are: 1) Platzfrass, 2) taking different outbound and
return routes, and 3) retracing the outward bound path. Each of
these foraging patterns represents a unique aspect of the limpets
homing ability. Limpets partaking in Platzfrass behavior need
only reorient themselves properly on their home site when
finished. As simple as this process sounds, it is actually quite
complex. C. scabra do not orient themselves on their home site
by a trial-and-error process of attempting to obtain a proper
shell fit with the substrate, but instead will leave their shell
raised and turn on their home site until oriented properly and
then lower their shell (Sommer, personal communication and
personal observation). Mucous laid down previously by the limpet
in the Platzfrass behavior, in addition to trapping microscopic
algae and diatomaceous material for later feeding (Connor,
1984c), could serve as a directional clue for proper orientation.
Limpets employ the use of their cephalic tentacles in this
orienting behavior, feeling the area around the home site. Three
of the four limpets which failed to return home in the field
tentacle removal experiment were classified as "not home" due to
improper orientation on the home site. However, the high degree
of proper orientation in limpets lacking tentacles, and the
inability of these limpets to detect trail directionality suggest
that an alternate method may be employed for proper orientation.
Lindberg and Dwyer (1983) reported on the mechanism employed
by C. scabra in excavating their home site depression. They
found that C. scabra secrete acidic mucopolysaccharides from the
foot and acidic mucopolysaccharides and carbonic anhydrase from
the mantle edge. These substances were shown to soften the rocks
to allow for excavation by the radula. To date, the only reports
of Platzfrass have been behavioral observations and it could be
that the limpets are excavating their home site depression in
addition to, feeding on the algal material around the home site
which is believed to occur during Platzfrass behavior. This is
further evidenced by the fact that limpets placed on glass plates
in the tide-tank showed a profundity of Platzfrass behavior,
possibly attempting to form the depression of its new home site,
as limpets placed away from their home site are apt to do. The
limpet could use textural clues of the excavated home site in
orienting properly. It may be that C. scabra possess either or
both of these mechanisms for proper orientation. Davis (1895)
suggested that the mantle tentacles could be used in detecting
textural cues associated with the rock; this suggests that
cephalic tentacles aid in proper orientation, but are not
indispensable to it.
The mechanism used in the second type of foraging pattern
where different outbound and return routes are taken by C. scabra
is not well understood. It is most likely that the animal makes
use of clues associated with the rock surface, although those
mechanisms which have been ruled out as primary mechanisms (e.g.,
dead-reckoning or visual clues) could play a role in the ability
of a limpet to home along a different return route. Some
researchers report that mucous trail-following is the only homing
mechanism employed by C. scabra (Hewatt, 1940; Underwood 1979; et
However, using the trail-following theory of homing
al).
exclusively makes it difficult to explain why C. scabra do not
always return home along the same path that was laid down at the
beginning of the foraging run. It is possible that limpets not
returning along the same path are encountering mucous trails
previously laid down, but this would be difficult to detect
without prolonged monitoring of behavior at a site.
Directionality is only present in trails for a period of 48
hours, and it is also unlikely that in displacement experiments
limpets encounter mucous trails with such regularity as to be
able to find their way home more than 808 of the time,
Furthermore, Jessee (1968) showed that scrubbing the surface of a
rock with a 328 solution of NaoH to remove mucous trails did not
inhibit limpets from homing. Similar mucous trail removing
experiments have yielded similar results (Sommer, 1982; Branch
1981). Taken together, these results suggest that one or more
mechanisms exist for limpets to home other than trail-following.
The last type of foraging pattern taken by C. scabra is
typified by retracing an outbound path in the return home and can
be easily explained by the mucous trail-following ability in this
species (Table II). An individual following its own trail to the
place of its origin has obvious consequences on the ability of
the limpet to return home. Upon encountering a current trail,
the limpet need merely retrace it to its home site. It is not
known whether limpets are able to detect directionality of
retraced trails, and limpets encountering a previously retraced
trail may not have the ability to detect its directionality and
follow it home. However, if the limpet encounters one of its
trail which hasn't been retraced, such as that in figure 4b, it
would be able to follow this trail to its home site.
A limpet encountering the current trail of a conspecific
would turn and follow the limpet laying down the trail, or meet
that same limpet head-on if the latter was on its return journey
home. A limpet encountering a conspecific's previously laid,
unretraced trail would follow this trail to the other limpet's
home site. The tendency of a limpet to follow a conspecific's
trail in the opposite direction is less well understood. Several
explanations exist, all of which are based only on observations
and theory. The first explanation of this phenomenon is a
territorial behavior; C. scabra could use this mechanism to avoid
territories of other limpets as well as guard their own territory
Alternatively, this mechanism could be
against intruders.
employed by limpets who abandon their home sites to search for
the home site of a conspecific. Since mucous trails ultimately
lead to a home site, it would be beneficial to any limpet away
from its home site (e.g., in the tide tank) to follow the trail
of another limpet in order to find a ready-made home site.
Finally, it could be a form of clustering behavior similar to
that demonstrated in C. digitalis (Millard, 1968).
LITERATURE CITED
Brant, D.H., 1950, A quantitative study of the homing behavior of
the limpet Acmaea scabra. Unpubl. Spec. Prob. Reprt., Dept.
Zoology, Univ. Calif. Berkeley.
Branch, G.M., 1981, The biology of limpets: physical factors,
energy flow, and ecological interactions. Oceanogr. Mar.
Biol. Ann. Rev. 19: 235-380.
Limpets: their role in littoral and
Branch,
G.M.,
1986,
sublittoral community dynamics. The Ecology of Rocky Coasts
ch. 7: 97-116.
Connor, V.M., 1984a, Foraging energetics in two intertidal limpet
species. Unpub. Reprt., Dept. Zoology, Univ. Calif. Davis
94-115.
the mechanisms underlying
Connor, V.M.,
Analysis of
1984b,
directional trail following in the limpets Collisella scabra
and C. digitalis. Unpub. Reprt., Dept. Zoology, Univ. Calif.
Davis 117-150.
Connor, V.M., 1984c, Stimulation of food species growth by limpet
mucus. Science 225: 843-844.
S.B., 1969, Experiments on homing in the limpet Siphonaria
Cook,
normalis. Anim. Behav. 17: 679-682.
S.B., 1971, A study of homing behavior in the limpet
Cook,
Siphonaria alternata. Biol. Bull. 141: 449-457.
S.B., O.S. Bannford, J.D.B. Freeman, & D.J. Teideman, 1969,
Cook,
A study of the homing habit of the limpet. Anim. Behav. 17:
330-339.
S.B. and C.B. Cook, 1975, Directionality in the trail¬
Cook,
following response of the pulmonate limpet Siphonaria
alternata. Mar. Behav. Physiol. 3: 147-155.
Davis, J.R.A., 1895, The habits of limpets. Nature 51:511-512
Hewatt, W.G., 1940, Observations on the homing limpet Acmaea
scabra (Gould). Amer. Midland Natural. 24 (1): 205-208.
Jessee, W.F., 1968, Studies of homing behavior in the limpet
Acmaea scabra. The Veliger 11: 52-55 (Supp.)
Lindberg, D.R. and K.R. Dwyer, 1983, The topography, formation
and role of the home depression of C. scabra. Veliger 25
(3): 229-233.
Millard, C.S., 1968, The clustering behavior of Acmaea digitalis.
Veliger 11, (Supp.) 45-51.
Peters, R.S., 1964, Function of the cephalic tentacles in
Littorina planaxis (Philippi). The Veliger 7 (2): 143-148.
Sommer, F., 1982, Homing behavior of the limpet Collisella
scabra. Unpub. MS on file at HMS Lib.
Thomas, R.F., 1973, Homing behavior and movement rhythms in the
Proc.
pulmonate limpet Siphonaria pectinata (Linnaeus).
Malac. Soc. Lond. 40: 303-311.
Underwood, A.J., 1979, The Ecology of intertidal gastropods.
Adv. Mar. Biol. 16: 111-210.
Villee, C.A. and T.C. Groody, 1940, The behavior of limpets with
reference to their homing instinct. Amer. Midland Natural.
24 (1): 190-204.
The behavior of limpets with particular
M., 1917,
Wells,
reference to the homing instinct. Journ. Anim. Behav. 7
(6): 387-395.
Wells, R.A., 1980, Activity pattern as a mechanism of predator
avoidance in two species of Acmaeid limpet. J. Exp. Mar.
Biol. Ecol. 48: 151-168.
FIGURE1
ACTIVITY PATTERN


4.83 m

l
E
S
I
T
D 4.98 m
AE
C 11.29 m
l-
—l
—
2000
1200
0400
HOURS
Fig. I -- The activity pattern of Collisella scabra over a 24-hour
period (May 30, 1990); bars represent the percentage of
the population active; light conditions indicated by light
and dark bars; S, submergence; E, emergence; A, awash period.
2-
20 -
18 -
18 -
14 -
12 -
10
4-
2-
2.8
19 —
18 -
17 -
18 -
15 -
14 -
13-
12 -
11
10 -
9 -
5 -

FIGURE 2: SHELL LENGTH VS. INTERTIDAL HEIGHT

R = .528
n - 75
P (.0005
2.8 3
32 34 38 38 4 42
Intertidal Height (tt)
FIGURE 3: SHELL LENGTH VS. DISTANCE TRAVELLED
R =.448
n = 75
P6.0005
+


1
+
tt
12
16
20
SHLLET (mm)
3
.
. 1
0 E.
5
3
0
0
0
—
—
0
0

0
a
5
8
0
—O1O
o
+

310
0
oo
22

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