0
OESTRACT
Measurements were made of the force required to break
the barnacle Halanus glandula while the animal was attached
to tour different rocky substrata. Force was exerted on the
barnacle normal to the substrate surface and the
dislodgement force was recorded. In general, force was not
proportional to barnacle size and barnacles were stronger om
stronger rocks. The results are discussed in the context of
the environmental hydrodynamic forces encountered and the
population genetics of B. glandula.
INTFODUCTION
The larvae of Beglandula must undergo two
metamorphoses before becoming a sessile adult. The barnacle
during these two stages is pelagic and free-swimming, but
the second stage larvae, referred to as the cyprid, must
settle permanently before the second metamorphosis occurs.
Ihis settlement takes place on a wide variety of substances.
whose physical properties, such as surface strengt, can vary
greatly. Strength is the amount of force per area that must
be applied to cause the pieces uf the material to break.
he barnacle has three types of body plates: basal,
shell, and opercular. The basal plate, which forms the
base of the barnacle's truncated cone-like shape, cannt be
cemented to the shell plates, which comprise the side walls,
if the barnacle is to expand its diameter during growth.
Consequently, 'fixation fibres' hold these two plate types
tagether. Cement is exuded beneath the calcareous basal
plate to anchor the barnacle to the substratum. Os
barnacles cam enjoy a fairly lang lifetime, ane would expect
that they would attempt to maximise the strength of this
anchoring system.
The cement af the adult balanoid barnacles has beem
studied with regard to its strength of adhesion to slate,
steel, and rubber surfaces (Carderelli,18; Yule ?
Walker,1784) and the work of Yule and Walker (1984) provides
a stress value of 1.7 X 10 Fa for barnacle cement before
cohesive tailure occurs. Yet even knowing the breaking
stress value (another term for strength) for the cement, one
still does not know how biplogically relevant this value is.
Although the cement may be able to withstand stresses of um
to 170,000 Fa, other components, such as the substratum to
which the barnacle is attached, the shell plates, or the
fixation fibres may not. The present study was designed to
see what part of the entire barnacle-rocky substratum
complex would fail first, and at what stress level. The
experiments were carried out on four different rock types to
test for any effect of the substratum.
MATERIALS AND METHODS
B. plandula were studied at three locations near
Monterey, California comprising four different rocky
substrata: (1) The north side of Whale Feak at Garrapata
State Fark, located about S miles south of Carmel, with two
types of granite, (2) Stillwater Cove, in Carmel Bay next to
the Febble Beach golf course, with a type of sandstone, (I)
Natural Bridges State Fark in north Santa Cruz, CO, with
Monterey shale.
For each test, a straightened paper clip was used to
remove the barnacle's body from the shell and a "O-Tip" was
then used to absorb as much of the remaining fluid as
possible. Ofter the inside of the barnacle was allowed to
dry for a few minutes, "Five Minute" epoky was applied to
the inside of the barnacle shell. A pre-bent paper clip,
with a loop at one end and a right angle bend at the other,
was then placed into the epoxied shell and held normal to
the substratum, until the epoxy hardemed enough to maintain
the paper clip in this position (see Figure 1). The length
and width of the barnacle's basal area were then measured to
the nearest millimeter with a ruler. Length here is defined
as the longest line in the area of contact between substrate
and barnacle; width is the largest dimension perpendicular
to length (see Figure 2). For the tests where the substrate
broke, the length and width were measured again after the
specimen was dislodged. These measurements were compared to
the original measurements obtained and if any discrepancies
existed, the latter set of values was used. The bazal area
was assumed to e best approximated by an ellipse and
therefore was calculated using the equation: Orea - T X
(length/2) X (width/2).
Ofter the epoxy had hardened, a recording spring scale
(zimilar to that used by Jones and Demetropoulos, 1968) was
used to determine the force required to remove either the
barnacle, the substratum to which it was attached, the paper
clip, or some combination of rock and barnacle. A carabimer
attached both to the spring scale and through the loop of
the modified paper clip made certain that the sample would
not be lost in the process of removal (see Figure 3). Every
attempt was made to administer a steady pull, although this
was accomplished by hand. Once dislodged, the sample was
examined to determine what part of it had brokem, and the
force required for removal was noted.
Simple linear regressions were run on the tests in
order to determine if a relationship existed between the
basal area of the barnacle and the force needed to dislodge
the specimen. These were performed for each group in the
study, a group being defined by the rock type the barnacle
was attached t and the location of the break.
RESULTS
123 tests were performed. For 35 of these, the paper
clips were pulled out, while the rack and the barnacle
remained attached; these tests will not be considered
further. In only pne case was breaking force correlated
wtih area: that for the first type of granite when the
barnacle broke. The relationship was as follows: Force
required to break barnacle (Newtons)- 272424.à Fa X Basal
area (m) + 2.87 M (r -.4401, 29 D.F.).
For the forty-two tests where the barnacle broke, the
mean breaking stress values differed significantly among
rock types, with the exception of those barnacles on the
sandstone and shale (see Figure 4 and Table 1).
Twenty-one of the 12 tests broke the rock before the
barnacle or paper clip dislodged. The granite broke just
once (this was the first type of granite), while the
sandstone and shale broke a total of twenty times. The mean
surtace breaking stress for the Monterey shale found in
Santa Cruz was approximately three times greater (161,338.89
Pa) than that for the Stillwater Cove sandstone (49,350.00
Pa). Ihe one case of the granite breaking was for a
feldspar crystal in the quartz and mica matrix and required
295,000 Pa to dislodge it. Otherwise, all that can be said
for the two types of granite is that at least JO2,077.42 Pa
for the first type and 523,500 Fa for the second must be
applied before they break. These values come from the mean
breaking stresses of the barnacles on the granite substrata,
since granite did not break before the barnacles did.
Additionally, twenty-five tests broke some combination of
barnacle and sandstone or shale (see Tables T and 3). For
both groups, analysis of variance demonstrated mo
significant differences between mean breaking stress values.
DISCUSSION
As larger barnacles receive larger wave forces tham
smaller ones due to a greater exposed area, one would expect
that they would have to be attached with a greater temacity
in order to survive. Unexpectedly though, the results
obtained here do nat support this line of reasoming. Only
barnacles attached toone of the four types ofrock
demonstrated a significant relationship between force and
area.
When a B. glandula suffered a break, the basal plate
usually remained fixed to the substratum, while the shell
plates broke free from it. In order for this to occur, the
fixation fibres holding the two types of plates together
must be broken. The fracture between the base and the shell
was not always "clean"; often part of one or several of the
shell plates remained attached to the base and consequently,
the fracture split the shell plate itself. In this latter
case, the fixation fibres probably were not broken. This
variability of the severing fracture's location within B.
glandula could contribute to not finding a force versus area
relationship, as different components of the barnacle's
structural system will most likely posses varying breaking
stresses.
Several experimental factors should be considered whem
analyzing this study. First, forces normal to the rocky
substratum were the only ones considered here. Because
waves strike the barnacle from many directions, forces in
other directions, such as parallel to the rock, need to be
also studied.
Second, it is possible that the entire basal area of B.
glandula is not the correct biological parameter to attempt
to relate to force. For instance, a significant
relationship has been demonstrated between the force
required for removal of Balanus balangides and the cement
area present between it's cuticular basal plate and the
slate surface to which it is attached (Yule and Walker,
1784), although the actual cement area is different from the
basal area. Thus, relating basal area to force might be
contributing to the lack of a relationship between force and
area noted here. Twenty-five tests resulted in breaking
both the barnacle and the rock on which it sat. Here also
the variability in location of break probably contributed to
the negative finding.
Ihe lack of a force versus area relationship for the
twenty-ane tests where the rocky substratum broke beneath
the barnacle is not too surprising, considering the highly
variable treatment rocks are subjected to in the intertidal
rone. A variety of organisms affect the surface
characteristics of the sutstratum either by physical
actions, such as burrowing, or chemical means, such as
secreting mucus on the rock which contains a wide variety of
chemical compounds. These compounds may alter the integrity
of the bonds within the rock which comprise its strength.
Fhysical forces from the sun and the waves also affect
different portions of the rock to varying degrees. Since
these factors weaken the rock's breaking strees in different
amounts and their location of attack is more-or-less random,
one must expect to find a variety of values for the rock's
breaking stress when testing at different sites on the rock.
Only a small number of tests were performed for many of
the groups, which defined by break location and the rock
type on which the barnacle sat. As mentioned above, some of
these group sizes would most likely not increase, no matter
how many tests were conducted. For other groups though,
additional tests would prove relatively easy to obtain. If
tests belonging to these latter groups were to be obtained
in larger numbers, a more confident and knowledgable
analysis of possible trends could be completed.
The cyprid larvae of Balanus balanoides have been shown
to be able to detect and respond to different
characteristics of the surface on which they are exploring
as a possible permanent home site (Yule and Crisp, 1983).
In the conteat of these studies, the results shown in Figure
4 suggest that at some time in the life cycle, B. glandula
is able to determine the strength of the rock on which it
sits. If it realizes that it resides on a rocky substratum
whose surtace strength is low, it apparently diverts energy
away from producing a stronger cement, attachment fibers or
shell. This raises the possibility that the barnacles on
weak substrata invest energy into the more rapid growth o¬
the reproductive system. This speculation assumes that the
populations present at the three locations studied are
genetically similar. If they are not, then this scenario
may turn put to be simply an example of differences between
populations. Unfortunately, not enough is known about the
genetic diversity of B. glandula on the coast of California
to determine whether this study deals with one or more
populations, although this field of research is an active
one (personal communication, Lani West). Therefore, any
conclusion concerning the results contained in Figure 4 will
have to wait until more conclusive studies have been
conducted on the species' genetic diversity.
Ihe mean sur face strength of the sandstone found at
Stillwater Cove is roughly one-third of that of the north
Santa Cruz shale and this shale value is approximately half
that of the minimum strength for the weaker of the two types
of granite (see Tables I and 2). Although the values for
the sandstone and shale are not significantly different, the
wide mange encompassing all the rock surface strengths
demonstrates that the argument in the previous paragraph is
plausible, as they are direct evidence for the existence of
substrata with differing surface strengths.
ACKNOWLEDGEMENT:
First and for most, I would like to thank Mark Denny for the
time he spent helping me perform and think about my study.
I would also like to thank Doug Stomer for assistance with
statistics and Lani West for the information concerning the
population genetics of B.glandula. Judy Thompson was
helpful in a wide variety of ways; additionally, the rest of
the staff here at Hopkins have heen both friendly and
helpful. Thanks to Allen and the library staff for
minimizing my wandering time before actually finding what I
wanted. Finally, thanks to my fellow 175H students for
providing thought-provoking comments and simply for their
friendship.
HEFERENCES
Carderelli, N. F., 1968. Barnacle Cement as a Dental
Restorative. 49 pp. Maryland: National Institute of
Dental Hesearch. IFublication no. 151.J
Jones, W. E. and A. Demetropoulos, 1968.
Exposure to wave action: Measurements of an important
ecological parameter on rocky shores on Onglesey.
J. Exp. Mar. Biol. Ecol. 2: 46-63.
Yule, A. B. and D. J. Crisp, 1783. Adhesion of cypris
larvae of the barnacle, Balanus Balanoides, to clean
and arthropodin treated surfaces. J. Mar. Bicl, Oss.
U. K. 63: 251-271.
Yule, A. B. and G. Walker, 1784. The adhesion of the
barnacle, Balanus Balanoides, to slate surfaces. J.
Mar. Biol. Ass. U. K. 64: 147-156.
Table 1
BREAKING STRESS VALUES OF BALANUS
GLANDULA
ROCK TIPE
MINIMUN MAXIMON
MEAN
50
STRESS
(N/mXm)
302077.42
51500
133488.76
721000
5500.00
162000
890000
302511.16
89630.7
166433
64300
32000
57200
191475.00
142904.20
315000
ROCK TYPE
Table 2
BREAKING STRESS VALUES OF SUBSTRATA
MEAN
MININUM MAXIMUM
S.D.
STRESS
(N/mXm)
295000.00
49350.00
47200
51500
3040.56
161338.89
149396.76
46800
630000
Table 3
BREAKING STRESS VALUES OF A COMBINATION
OF BALANUS GLANDULA AND SUBSTRATUN
ROCK TYPE
MEAN
MINIMUM MAXINUN
S.0.
STRESS
(N/ mXm)
42900
146325.00 138840.13
343000
136938.10 146206.05
43600
734000
Figure 1: Diagram of a prepared specimen. Note that the
barnacle cement layer is exaggerated in size for purposes of
clarity.
Figure 2: Diagram of the top view of the barnacle's basal
area, illustrating the measurement conventions used in this
study.
Figure 3: Diagram of the apparatus used to measure the
breaking stress value.
Figure 4: Graph of rock type versus the mean strength of E.
glandula. The middle symbol of the three points for each
rock type is the mean value, while the upper and lower
points represent the range of the 957 confidence interyals.
Analysis of variance and the GTT method were used to obtain
the values shown.
Figure 1
EPOXV
BASAL
PLATE
ROCK
—PAPER CLIE
BARNACLE
SHELL
BARNACLE
CEMENT
ZLAVER
Figure 2
TOP VIEW
BASAL AREA
LENG


WIDTH
Figure 3
STEEL
WIRE
PAPER
CLIF
1
„RECORDING
SPRING
SCALE
ZCARABINER
BARNACLE
—

—
Figure 4
MEAN BREAKING STRESS VALUES OF BALANUS
GLANDULA ON FOUR DIFFERENT ROCK TYPES
600000
500000
0000
300000
0000
100000-