ABSTRACT
The effects of wave action on the dynamics of intertidal communities
are often modeled viewing seawater as a pure fluid. This assumption
overlooks the potential consequences of solid ocean debris, including rocks,
animal shells, and other hard objects, which can be "thrown" into the
intertidal zone at high velocities.
This study investigates the survivorship of the owl limpet Lottia
gigantea in relation to its bombardment by wave-borne projectiles. Over a
five-week period, the presence of new shell damage in 160 limpets was
recorded at seven different rock faces that received varying levels of wave
exposure and that contained varying levels of nearby rocks and debris. During
the same period, maximum wave force measurements were taken daily, and
Styrofoam targets placed at each rock face were used to record the projectile
impacts.
Wave projectile action at directly wave-exposed areas can play a
substantial role in the survivorship of L. gigantea. Limpets living on rock
faces at directly-exposed sites experienced substantial levels of wave projectile
damage on days with high wave action. Limpets living on rock faces that
experienced deflected waves or waves that "ramped" off lower rock beds were
significantly less affected by wave-projectiles. This suggests that the effects of
wave action on limpet survivorship can depend on the local topography as
well as the disturbance caused directly by hydrodynamic forces.
INTRODUCTION
The ability of marine invertebrates to survive in the wave-swept
intertidal zone has led to many studies on how wave action affects the
abundance, life cycle, and mortality of these species (Ricketts et al., 1985;
Branch 1981; Denny et al., 1985). Though the consequences of hydrodynamic
forces on intertidal organisms has generally been well characterized, these
studies usually model seawater as a pure fluid. In reality, seawater may
contain solid ocean debris such as rocks, logs, and animal shells, and can
"throw" these objects into the intertidal zone through wave action (Dayton
1971; Shanks and Wright 1986). Given that water velocities upwards of 10
m/s have been measured on wave-swept shores (Denny 1985), it seems
plausible that a modest-sized object propelled at such speeds could injure or
kill intertidal organisms where wave action alone might not.
In the one major study on the effects of wave-projectiles on intertidal
organisms (Shanks and Wright 1986), the impact of projectiles on the barnacle
Chthamalus fissus and the limpet Lottia gigantea were studied. The study
concluded that wave action could "throw" rocks as projectiles, and that the
survivorship of organisms living in intertidal areas with loose pebbles and
rocks partially depended on the damage caused by wave-borne missiles. The
study also concluded that the effects of wave projectiles can vary within
microhabitats (rock edges vs. rock faces vs. rock tops).
The hypotheses and conclusions of the Shanks and Wright study,
while thorough, have not been re-tested or expanded upon in the past decade.
In their report, the authors noted several shortcomings of their experiments.
Importantly, they note that extensive tag loss during their study prevented
repeatable observations of the same limpets, and thus made qualitative
measurements of survivorship in L. gigantea more difficult. In addition,
their limpet studies were conducted at only one location, San Nicholas Island
in southern California. The phenomenon of wave-borne projectiles has not
been experimentally confirmed and documented at other locations.
This paper describes an attempt to repeat the study of Shanks and
Wright at a separate location, as well as to examine several additional factors
not considered in the original experiments. My intent was also to provide
more quantitative data on effects of wave projectiles on survivorship of the
owl limpet Lottia gigantea using careful and extensive tagging, as well as to
record data more frequently than in the previous study. This paper also
considers aspects that control the severity and frequency of wave projectile
action, such as local topography, height above MLLW, and wave velocity.
MATERIALS AND METHODS
STUDY SITES. The measurement of wave projectile action was conducted at
Hopkins Marine Station of Stanford University, Pacific Grove, California.
The intertidal zone of Hopkins Marine Station consists of areas of increasing
wave exposure, from highly protected rocks to areas where waves break
directly on the rock face. Seven vertical rock faces containing substantial
numbers of Lottia gigantea were selected as study sites. Each site was
observed to experience substantial wave exposure. Three of the seven sites
experienced direct wave exposure during the majority of the tidal cycle. Four
of the seven sites were located in areas with partial deflection of incoming
waves or in areas where striking waves "ramped" off outlying rock beds. The
locations of the sites are shown in Figure 1, and characteristics of each site are
listed in Table 1.
TRACING LIMPET SURVIVORSHIP. The measurement of limpet shell damage
and survivorship was conducted by tagging 20 to 25 limpets at each study site.
At each site, the vertical extent of L. gigantea was visually estimated and a
horizontal transect across the rock at the approximate mean level of local L.
gigantea was marked. Twenty to twenty-five limpets were randomly chosen
that lived within one foot of the horizontal transect. Limpets were
deliberately excluded from the random sampling if they were already severely
damaged, or if they resided near a horizontal ledge, which would make them
susceptible to oystercatcher predation (Hahn and Denny 1989).
Each chosen limpet was tagged either using numbered Dymo-label6
tags affixed with Splash-Zoneë epoxy, or with weatherproof paint. The shell
damage of each tagged limpet at each site was recorded every three days. To
compare this running data with the "historical record" of limpets at each site,
the shells of every limpet falling within the transect band was also observed
at the beginning of the study, and the percentage of damaged limpets per site
was counted. In addition, the length and width of each limpet within each
transect band was measured, and the surface area of each limpet shell was
calculated by approximating the shell's shape as an ellipse.
MEASURING MAXIMUM WAVE FORCE. To provide some measure of the wave
force of each day of the study, six maximum wave force meters (as designed
and described by Bell and Denny, 1994) were placed at Site 1. Their heights
varied from 5.35 ft to 9.35 ft above MLLW. These meters consist of practice
golf balls attached to springs, which are attached to the rock face. A rubber
indicator allowed for daily measurements of maximum spring extension,
which could consequently be converted into wave force and wave velocity
estimates, as described by Bell and Denny (1994).
MEASURING WAVE PROJECTILE ACTION. Measurement of wave projectile
action was conducted by placing styrofoam targets on the rock faces of the
study sites. The styrofoam targets were all cut from the same styrofoam board
to ensure consistency in stiffness and hardiness. Targets were square blocks
measuring 5 cm X 5 cm X 1 cm thick, creating a 25 cm2 target area. At least
one major face of each target was carefully smoothed to allow detection of
small impacts. Five targets were affixed to each rock face at each study site at
the mean height of the limpets using Splash-Zoneë epoxy. Epoxy was
applied all around the edges of the target as well as under it, which helped to
prevent the edges of the target from chipping away due to wave action. Daily
observations were made of each styrofoam target, and the number of new
impacts was recorded. Targets that were severely damaged were replaced as
needed.
SHELL IMPACT MEASUREMENTS. To measure the ruggedness of the limpet
shell, fifteen limpets were collected and tested. The size of the tested limpets
was approximately the same, and was equal to the average size of the tagged
limpets averaged over all the study sites (51 mm length). Limpets were
placed horizontally on a rigid metal surface. Impacts were generated by
dropping a 133 gram steel rod, guided through a PVC pipe, onto the center of
the limpet shell. The rod's end was spherical, with a surface area of 50 mm2.
The rod was dropped from increasing heights in 5 cm increments, and
measurements were made to note when the shell first cracked or chipped,
and when the impact was lethal. A lethal impact was one where the shell
was cracked severely enough to expose the soft body underneath, or where
the rod completely penetrated the shell. Successive rod drops began at
heights where previous limpets had begun to chip in order to minimize the
number of impacts given to each shell and to reduce the amount of
cumulative shell weakening during the experiment.
ROCK TERMINAL VELOCITY MEASUREMENTS. The terminal velocity of rocks
sinking through water provides some information on the minimal velocity
necessary for a wave to lift and propel the rock as a missile. To measure the
terminal velocity of rocks sinking in seawater, rocks of various masses were
collected from West Beach at Hopkins Marine Station, near study site 7. Each
rock was approximately spherical, and weighed from less than 1 gram to over
300 grams. Each rock was dropped in the Kelp Forest tank at Monterey Bay
Aquarium, Monterey, California, next to a taut vertical line marked with 1
meter increments. Each rock was given five to six meters of sinking depth to
reach terminal sinking velocity, and the time for each rock to pass two lem
markers was measured. The data were then used to calculate terminal
sinking velocity.
RESULTS
LIMPET SHELL DAMAGE AND SURVIVORSHIP. Of the 160 limpets tagged for the
study, four disappeared over the five-week study period. The shells of three
of these four limpets were severely cracked in the last observations made
before their disappearance, and they were assumed to have died.
Of the 75 limpets marked on sites with direct wave exposure, incidence
of new limpet shell damage was positively correlated with water velocity (r =
0.849, p « 0.05, see Figure 2 and Table 2). At sites with deflected wave
exposure or "ramped" waves, no significant correlation was detected (r =
0.679, p » 0.05). However, the overall incidence of newly damaged limpets did
not significantly differ between the two types of sites (mean dir. sites -
0.30%/day, se = 0.15%, mean indir. sites = 0.27%/day, se = 0.18%; paired t-test,t
= 0.686, p» 0.5).
STYROFOAM TARGET MEASUREMENTS OF WAVE PROJECTILE ACTION. Incidence
of new styrofoam impacts was positively correlated with maximal water
velocity at sites with direct wave exposure (r = 0.885, p « 0.05, see Table 2 and
Figure 3). At sites with deflected or "ramped" waves, no correlation was
detected (r = -0.784, p » 0.05). The actual frequency of styrofoam impacts per
target also differed significantly between the two types of sites, with directly
wave-exposed sites experiencing greater impact frequency than wave-
deflected sites (mean dir. sites = 4.76%/day, se = 0.82%, mean indir. sites =
1.86%/day, se = 0.54%; paired one-tailed t-test, t = 2.015, p « 0.05).
The relative size of styrofoam impacts clearly varied, but over the
course of the five-week period styrofoam dents that occurred early during the
study had noticeably shrunken in size, probably due to seawater-induced
expansion of the styrofoam. This precluded any accurate examination of
relative impact severity. From personal observation, however, the deepest
and widest impacts were recorded from sites 3, 5, and 7, which were the sites
with direct wave exposure.
LIMPET SHELL IMPACT MEASUREMENTS. The average-sized limpet (50.3 mm
length) could withstand a direct blow from the steel rod dropped from a
height of 32 cm + 10.99 cm (mean + sd, see Table 3). The first visible sign of
shell damage was most frequently chipping to the outer portion of the shell
(12 of the 15 cases). The other three limpets suffered hairline cracks
originating from the apex. The strength of the blow necessary to fatally
wound the limpet varied from 30 cm to 75 cm; however, even these
measurements are probably underestimates because of the shell weakening
already caused by previous shell damage.
TERMINAL VELOCITY OF ROCKS SINKING IN SEAWATER. See Table 4 and
Figure 4.
DISCUSSION
There are many factors that affect the frequency of rocky debris being
"thrown" into the intertidal zone, and this inherent complexity leads to high
levels of variation when trying to measure the effect experimentally. The
results of this study confirm many of the sources of variability encountered in
the original 1986 study (e.g. wave velocity, local substratum relief). For
instance, Shanks and Wright noted that the frequency of projectile hits across
a single rock face can vary by several orders of magnitude, depending on
whether one measures the rock edge, rock top, or rock face itself. In more
than one instance in this study, two styrofoam targets separated by less than
two feet (both the same height above MLLW) experienced more than a five-
fold difference in projectile impact frequency.
This study further suggests that the frequency of wave projectile
impacts relies on macroscopic topographical factors in addition to very small
distance separations. If the limpets' rock face was protected by a rock bed that
partially impeded the incoming waves, there appeared to be a decrease in the
amount of projectiles "thrown" against this rock face as measured by the
styrofoam targets. The extent of protection did not need to be absolute nor
was it always immediately obvious; even rock beds that caused waves to
'ramp" off and crash into the limpets' rock face appeared sufficient enough to
reduce wave projectile impacts. Often, these "ramped" waves created a
greater visual perception of wave impact by generating loud breaking noises
and throwing up whitewater several meters into the air. In contrast to
appearance, though, the rock faces hit by these ramped or deflected waves
seemed less susceptible to projectile hits than more modest-sized waves that
struck rock faces directly.
The deflection of incoming waves probably serves to deter wave-borne
projectiles through several mechanisms. The sudden bump in the rocky
floor may cause the wave to break sooner, affecting the ability or the timing of
the projectile "throw." Early breaking or deflection of the wave also causes it
to slow down, possibly decreasing the intensity and range of the projectiles.
Finally, offshore rocks or deflecting rock beds may be hit by the projectiles
first, serving to sift and shield more inshore rock faces from projectile hits.
While the frequency of styrofoam impacts at directly wave-exposed
sites was significantly greater than at deflected-wave sites, the fraction of
limpets experiencing new shell damage did not significantly differ between
the two types of sites. This indicates that other shell-damaging factors, such as
bird or crab predation, may be affecting one or both types of sites, and that
these factors can be as important as wave projectiles in causing limpet shell
damage. While wave projectiles may play a role in limpet survivorship, they
are clearly not always the dominant factor even at highly wave-exposed sites.
As a final confounding note, there are several unexplored factors that
determine limpet shell strength and susceptibility to shell damage. Lottia
gigantea frequently play host to rider limpets (probably McClinktockia scabra),
that piggyback on the shells of L. gigantea and etch an oval scar in the shell.
These scars remain even if the rider limpets fall off. L. gigantea 's cracks,
when observed in the field, tended to originate from these oval scars. This
implies that these rider limpets may cause some substantial shell weakening.
In the assessment of limpet shell strength, three of the limpet shells tested
contained rider limpets, and these three shells were also among those to be
damaged at the lowest impact energies. This further implicates a role for
these rider limpets in the survivorship of their hosts.
If the data from the direct-exposure sites in this study are pooled over
the entire experiment period, then 0.3% of the limpets at these sites were
newly-damaged per day for a five-week period in April and May. In the study
by Shanks and Wright it was measured that 0.5% of limpets were newly
damaged per day in March, while 0.2% of limpets were newly damaged per
day in June. Shanks and Wright took measurements only in March, June,
September, and November. Interestingly, both studies not only agree on the
approximate incidence of new limpet damage, but this study also finds a
value intermediate to those found in Shanks and Wright's March and June
measurements, as one might ideally expect. This does not rule out chance or
coincidence, but this does provide some confidence in the results, and more
importantly, indicates that one can still measure repeatable data despite the
complexity of the factors that influence wave projectiles.
Assuming that the incidence of new limpet shell damage is well
approximated by the data from both studies, it is possible to extrapolate the
results to investigate the effects of wave projectiles in a limpet's lifetime. By
taking the least-squares regression line that correlates wave velocity with
limpet damage and normalizing the results per surface area, one finds that a
50 mm long (about 1500 mm *) L. gigantea, living in a model world with
constant wave exposure and average wave-induced water velocity of 5 m/s,
would experience an average of 22.2 "hits" on its shell each year, and that 2.6
of these "hits" would cause visible shell damage (see Figures 5, 6, 7). If 0.3% of
limpets are hit per day in five weeks (35 days) as this study's data suggest, one
can also extrapolate the probability of a limpet receiving a hit over a series ofx
days (see Figure 8). These data indicate that a single limpet will have a 50%
chance of receiving a damaging hit in a period of 227 days. While at first
these might seem like high estimates and that this data suggest that all
limpets should be cracked, in practice the estimates are not unreasonable. It
has been observed that limpets are capable of repairing cracks and chips to
their shells in less than 3 months (Shanks and Wright 1986; Bulkley 1968),
and with this ability, limpets could plausibly endure over 2 damaging hits per
year and still manage to look fully unscathed for several months in the vear
because of their rapidly healing shells. Transect data (Table 1) also indicate
that a substantial portion of limpets are damaged at any one time, ranging at
study sites to fractions up to 24.2%. Both these observations may account for
the high estimated incidence of projectile-mediated damage.
If one extrapolates this data further into the limpet's 15-year life span
(Abbott 1980), then the average limpet would expect 333 "hits" on its shell and
39 shell-damaging cracks over the course of its life. Even as an
approximation, this still represents a substantial influence on the life and
potential survivability of the organism. By extremely conservative estimates,
if the frequency of shell damage made in this and the 1986 study were both
erroneously a full order of magnitude too high and the average yearly wave
velocity were approximated at 3 m/s instead of 5 m/s (during the entire study
in May, even during relatively calm periods, there was never a period that
averaged below 3 m/s), then the average limpet would still experience at least
17 "hits" on its shell and more than a 50% chance for a single shell-damaging
hit sometime in those fifteen years.
These approximations have been made without any attention towards
the possible severity of limpet damage. By dropping the rod onto a control
piece of styrofoam from a height normally sufficient to deliver a completely
fatal blow to a limpet (as determined from the shell impact experiment), and
comparing the resulting dent to styrofoam impacts in the field, it became
apparent that some of the field impacts were qualitatively comparable to the
impact severity needed to fatally wound a limpet. Wave projectiles can
therefore act in several different ways; they can merely "knock" the limpet's
shell, they can chip or crack the shell, increasing the limpet's susceptibility to
predation or desiccation, or they can deliver a fatal impact to the limpet.
While qualitative and quantitative data both suggest that increasingly serious
damage to limpets occurs increasingly rarely, these effects are still large
enough to be seen in the field over a short period of time.
CONCLUSIONS
The data from this study largely agrees with the information found in a
similar study conducted by Shanks and Wright (1986), and finds that 1) waves
"throw" rocks into the intertidal zone, 2) the intensity and frequency of these
wave projectiles positively correlates with wave velocity, and also depends on
macroscopic topography in addition to availability and size of projectiles, and
3) these projectiles have a real and detectable effect on health and
survivorship of Lottia gigantea.
ACKNOWLEDGEMENTS
This project could not have been possible without the help and
patience of many people. Mark Denny waded through several drafts of this
paper and poked and guided the project into shape. Jim Watanabe, Brian
Gaylord, and Emily Bell suggested brilliant ideas and located obscure tools and
equipment that I needed. Steven Randle and Gi-Soo Lee braved ten-foot
waves and early morning chills to help me at 5 a.m. low tides.
Special thanks to Chris Harrold and the staff and volunteer divers at
the Monterey Bay Aquarium who offered their Kelp Forest Exhibit and their
Thursday afternoon to help an undergraduate student drop rocks in their
tank, and to explain to dozens of inquiring tourists that this was "real science
going on.
REFERENCES
Abbott, D.D., E.C. Haderlie, R.H. Morris, eds., 1980. Intertidal Invertebrates
of California. Stanford University Press, Stanford, CA, USA.
Bell, E.C. & M.W. Denny, 1994. Quantifying "wave exposure": a simple
device for recording maximum velocity and results of its use at several
field sites. J. Exp. Mar. Biol. Ecol., Vol. 181, pp. 9-29.
Branch, G.M., 1981. The biology of limpets: physical factors, energy flow and
ecological interactions. Oceanogr. Mar. Biol. Annu. Rev., Vol. 19, pp. 235
380.
Bulkley, P.T., 1968. Shell damage and repair in five members of the genus
Acamea. Veliger, Vol. 11 (Suppl.), pp. 64-67.
Dayton, P.K., 1971. Competition, disturbance and community organization:
the provision and subsequent utilization of space in a rocky intertidal
community. Ecol. Monogr., Vol. 41, pp. 351-389.
Denny, M.W., 1985. Wave forces on intertidal organisms: a case study.
Limnol. Oceanogr., Vol. 27, pp. 178-183.
Denny, M.W., T.L. Daniel & M.A.R. Koehl, 1985. Mechanical limits to size in
wave-swept organisms. Ecol. Monogr., Vol. 55, pp. 69-102.
Hahn, T. & M. Denny, 1989. Tenacity-mediated selective predation by
oystercatchers on intertidal limpets and its role in maintaining habitat
partitioning by 'Collisella' scabra and Lottia digitalis. Mar. Ecol. Prog. Ser.,
Vol. 53, pp. 1-10.
Hobday, Alistair, 1995. Body-size variation exhibited by an intertidal limpet:
Influence of wave exposure, tidal height, and migratory behavior. J. Exp.
Mar. Biol. Ecol., Vol. 189, pp. 29-45.
Ricketts, E.F., J. Calbin, J.W. Hedgpeth & D.W. Phillips, 1985. Between Pacific
Tides. Stanford University Press, Stanford, CA, fifth edition, 652
Shanks, A. & W. Wright, 1986. Adding teeth to wave action: the destructive
effects of wave-borne rocks on intertidal organisms. Oecologia, Vol. 69, pp.
420-428.
TABLES
INITIAL
SITE
PERCENT L.
SITE CHARACTERISTICS
HEIGHT ABOVE MLLW (FT)
GIGANTEA
DAMAGED
West facing rock; At most times, waves
7.35
splash strongly on rock face after ramping off
24.2
mussel-bed rocky outcropping
West and northwest facing rock above a large
6.9
6.20
mussel bed that breaks incoming waves and
puts limpet in direct splash area.
Northwest facing rock; at high and medium
5.71
18.8
tide, waves break directly onto rock. At low
tide, some wave deflection from lower rocks
Northwest facing rock; receive only deflected
5.94
and ramped waves which splash off large
10.0
mussel beds and rocky outcroppings below
West-northwest facing surge channel; direct
5.22
13.3
wave exposure from waves rushing into and
striking back of channel
North and northwest facing crevice; splash
3.57
20.5
exposure from waves that strike outside of
rocky outcropping
Northwest facing rock at highly wave¬
5.05
0.0
exposed site; waves break directly onto rock
face
TABLE 1. CHARACTERISTICS OF STUDY SITES USED TO STUDY WAVE
PROJECTILE ACTION. Sites 3, 5 and 7 were subjected to breaking waves and
experienced primarily direct wave exposure. Sites 1, 2, 4 and 5 received primarily wave
splash, "ramped" wave exposure, or had waves deflected by other nearby rock faces. The
percentage of limpets damaged was calculated by observing all limpets across a horizontal
transect at each site. From personal observation, the amount of loose rocks and boulders
increased when moving from site 1 towards site 7.
Site
4.05 m/s
3.70 m/s
3.85 m/s
4.81 m/s
7.02 m/s
7.13 m/s
010
1/1
010
0/2
111
070
170
110
370
070
070
070
170
2/1
0 70
21
3/1
070
070
070
070
071
1/ 0
170
110
011
170
171
211
070
070
170
170
170
1/2
210
—210
210
2 11
170
2 11
TABLE 2. SYTROFOAM IMPACTS AND NEW LIMPET SHELL DAMAGE WITH
INCREASING WAVE VELOCITY. The number of styrofoam impacts (left number) and
the number of new limpets damaged (right number) are shown for the average maximum
wave velocity of time periods at each study site. Shaded sites are rock faces that receive
direct wave exposure for the majority of the tidal cycle.
ILIMPET HEIGHT TO DAMAGE ENERGY TO DAMAGE
(centimeters
(Jouleslmeter2)
39.29
35
91.67
78.58
170.25
25
65.48
25
65.48
78.58
104.77
78.5
78.58
65.48
78.58
78.58
104.77
15
78.58
83.82
Mean
8.78
S
1385.
TABLE 3. LIMPET SHELL IMPACT ASSESSMENT. A steel rod with impact area of
50 mm2 and mass 133.5 grams was dropped from increasing heights through a PVC pipe
onto the center of an L. gigantea shell. The first height that created visible limpet shell
damage (chip, crack, or lethal blow) was recorded and shown above. The corresponding
energy per square meter was also calculated and shown. The mean length of limpets tested
was 50.3 mm, and the standard deviation of sizes tested was 3.9 mm.
TERMINAL
ROCK MASS
SINKING TIME
VELOCITY (m/s)
(grams
(seconds/2m
4.8
0.93
0.41
2.00
0.43
2.68

0.51
3.70
0.60
6.26
3.15
0.63
12.70
3.18
0.63
15.19
2.87
0.70
2.7
0.72
36.32
36.78
2.78
0.72
92.39
2.50
0.80
2.00
1.00
312.11
TABLE 4. TERMINAL SINKING VELOCITY OF ROCKS IN SEAWATER. Spherical
rocks of varying masses were dropped into the Kelp Forest Exhibit tank in Monterey Bay
Aquarium, Monterey, CA. Each rock was allowed to sink for 5 to 6 meters to achieve
terminal sinking velocity. The time for each rock to fall the last two meters in water was
recorded and the time used to measure the terminal sinking velocity
FIGURE LEGENDS
FIGURE 1. LOCATION OF STUDY SITES ALONG ROCKY INTERTIDAL ZONE
OF HOPKINS MARINE STATION, PACIFIC GROVE, CA. The numbered study
sites were all vertical rock faces located at the designated points along the intertidal zone
FIGURE 2. NEWLY-DAMAGED LIMPETS VS. WATER VELOCITY AT SITES
WITH DIRECT WAVE EXPOSURE. Correlation between measured maximum water
velocities of sample periods and the number of limpets with newly cracked or chipped
shells (r = 0.849, p « 0.05). The data shown are from sites with direct wave exposure
during the majority of the tidal cycle.
FIGURE 3. STYROFOAM IMPACTS VS. WATER VELOCITY AT SITES WITH
DIRECT WAVE EXPOSURE. Correlation between measured maximum water velocities
of sample periods and the number of styrofoam impacts recorded (r = 0.885, p £ 0.05).
FIGURE 4. TERMINAL VELOCITY OF ROCKS SINKING IN SEAWATER.
Measurements were made by dropping spherical rocks into the Kelp Forest tank in
Monterey Bay Aquarium, Monterey, CA. Rocks were dropped next to a taut vertical line
with marked 1-meter increments and allowed to sink 5 to 6 meters to achieve terminal
sinking velocity before speed measurements were made
FIGURE 5. STYROFOAM IMPACTS AND NEW LIMPET DAMAGE NORMALIZED
TO SURFACE AREA. The frequency of impacts on each is normalized to the number of
impacts per square meter. The surface area of each limpet was estimated by approximating
shell shape as an ellipse and measuring the semimajor (a) and semiminor (b) axis lengths,
with surface area equal to Hab.
FIGURE 6. THEORETICAL SHELL-DAMAGING PROJECTILE HITS PER YEAR
VS. WATER VELOCITY. This line is constructed from the surface area of a 50 mm long
limpet (1500 sq. mm surface area) and the least-squares regression line from Figure 2 to
give the average number of shell-damaging projectile hits a limpet would experience per
vear in relation to average yearly water velocity. By extrapolating the number of hits per
limpet surface area to the number of impacts per square meter, the number of hits per
square meter per day can be measured. Scaling this factor by the average limpet surface
area allowed the theoretical yearly impact frequency to be estimated.
FIGURE 7. THEORETICAL TOTAL PROJECTILE IMPACTS PER YEAR VS.
WATER VELOCITY. This line is constructed from the least-squares regression line from
Figure 3 and the surface area of the styrofoam targets to give the average number of total
shell impacts a limpet (50 mm length, 1500 sq. mm) would experience per year in relation
to average yearly water velocity.
FIGURE 8. THEORETICAL PROBABILITY OF A LIMPET RECEIVING A HIT
AFTER CONSECUTIVE DAYS. 10.7% of the limpets at directly-hit sites were damaged
over the period of 35 days. This allows an estimation of the number of new limpets
damaged per day. By subtracting this "hit" probability from 1, the probability of a limpet
escaping a hit after x consecutive days can be calculated. The projected estimates for the
first 365 consecutive days are shown above. The dashed line demonstrates that an average
limpet can survive 227 days before reaching the 50% probability of receiving a damaging
hit.
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kolp
S
kolp

e.


S


8
s
N

14





10 5
I.
J
o
Hopkine
ee.


der,g v. Slre.

Dewey
Sst.
Avo.
Eardleyt
Ast.
Ave.
Figure 1
4 ka-
-
-
o /6


Water Velocity (m/s)
Figure 2
—
4
O

Water Velocity (m/s)
Figure 3
1.1-
0.9-
0.8.
0.7-
0.6
0.5
0.49
V - 0.440x°.144

200
100
Rock Mass (grams)
Figure 4

300
200-
150-
50
Damaging Limpet Impacts
• Styrofoam Impacts

D

6
2
Water Velocity (m/s)
Figure 5
0+
y = 1.105x - 2.965
3 4 5 6
Yearly Average Water Velocity (m/s
Figure
40—
30
10-
y = 5.238x - 4.029
+
Average Yearly Water Velocity (m/s)
0.5-
04
y = (1 - 0.00304)

nneccesszxxznnecccnnnxx:
100
200
300
Consecutive Days
Figure 8