Download Shell Hardness and Compressive Strength of the Eastern Oyster

Reference: Biol. Bull. 225: 175–183. (December 2013)
© 2013 Marine Biological Laboratory
Shell Hardness and Compressive Strength of the
Eastern Oyster, Crassostrea virginica, and the Asian
Oyster, Crassostrea ariakensis
SARA A. LOMBARDI1,*,†, GRACE D. CHON1,¶, JAMES JIN-WU LEE2,§,
HILLARY A. LANE1, AND KENNEDY T. PAYNTER1,3
1
Department of Biology, University of Maryland, College Park, College Park, Maryland 20742;
Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899;
and 3Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, P.O.
Box 38, 1 Williams Street, Solomons, Maryland 20688
2
sive force are likely explained by differences in thickness
and density between age classes and species. Further, we
compared the compressive strength of differing ages of
these two species to the crushing force of common oyster
predators in the Chesapeake Bay. By studying the physical
properties of shells, this work may contribute to a better
understanding of the mechanical defenses of oysters as well
as of their predation vulnerabilities.
Abstract. The valves of oysters act as a physical barrier
between tissues and the external environment, thereby protecting the oyster from environmental stress and predation.
To better understand differences in shell properties and
predation susceptibilities of two physiologically and morphologically similar oysters, Crassostrea virginica and
Crassostrea ariakensis, we quantified and compared two
mechanical properties of shells: hardness (resistance to irreversible deformation; GPa) and compressive strength
(force necessary to produce a crack; N). We found no
differences in the hardness values between foliated layers
(innermost and outermost foliated layers), age class (C.
virginica: 1, 4, 6, 9 years; C. ariakensis: 4, 6 years), or
species. This suggests that the foliated layers have similar
properties and are likely composed of the same material.
The compressive force required to break wet and dry shells
was also not different. However, the shells of both six- and
nine-year-old C. virginica withstood higher compressive
force than C. virginica shells aged either one or four, and
the shells of C. ariakensis at both ages studied (4- and
6-years-old). Differences in ability to withstand compres-
Introduction
Adult oysters possess two calcareous valves connected by
a hinge ligament that acts as a spring and allows an oyster
to separate the valves (gape), thereby permitting an exchange between the internal and external environments
(Carriker, 1996). Each valve is composed of three primary
layers. The outermost layer is the periostracum, which primarily consists of an organic (conchiolin) matrix and protects the shell from corrosion (Galtsoff, 1954, 1964; Bottjer
and Carter, 1980; Carriker, 1996; Fig. 1). The middle layer
is the prismatic layer, which contains curved, wedge-shaped
prisms of calcite crystals within a conchiolin matrix (Galtsoff, 1954, 1964; Mount et al., 2004). The innermost layer,
nearest the pallial space, is the calcite-ostracom layer, also
known as the foliated layer. The foliated layer constitutes
the majority of the shell and comprises sheets of calcite that
are associated with providing shell strength (Galtsoff, 1954,
1964; Chateigner et al., 2002; Mount et al., 2004). Since the
foliated layer is composed of parallel sheets of calcite
crystals, its microscopic structure is similar to that of the
nacre layer of aragonite structures (Checa et al., 2007).
Received 29 March 2013; accepted 26 November 2013.
* To whom correspondence should be addressed. E-mail: SaraAnn
[email protected]
† Current address: Department of Environmental Science, American
University, 4400 Massachusetts Ave. NW, Washington, DC 20016.
¶ Current address: Department of Natural Sciences, Hawaii Pacific University, 45-045 Kamehameha Highway, Kaneohe, HI 96744.
§ Current address: Department of Biological Sciences, Marshall University, 1 John Marshall Drive, Science Building Room 350, Huntington, WV
25755.
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S. A. LOMBARDI ET AL.
Figure 1. Primary layers of Crassostrea virginica oyster shells. This
cross-section of a valve illustrates the primary layers of the oyster shell
from superficial to deep: periostracum (A), prismatic layer (B), and the
calcite-ostracom, or foliated, layer (C). Each of these layers is present in
both the left and right valves. Modified from Galtsoff (1964).
Within oyster shells, the foliated layer acts as the main
defense that predators must penetrate to access oyster tissue.
Overall, the shells assist in survival, as they provide space
for internal organs and shield tissues from outside forces
that include predators as well as environmental stressors
such as salinity changes, desiccation, and anoxic or contaminated water (Carriker, 1996; White and Wilson, 1996).
Therefore, understanding the properties of the shells, in
particular the foliated layer, can contribute to understanding
oyster predation defenses and survival. Ultimately, the mechanical defense provided by the shells enables oysters to
perform essential ecosystem services such as habitat formation, water filtration, and benthic-pelagic coupling in marine
and estuarine systems (see Newell, 1988; Meyer and
Townsend, 2000; Steimle and Zetlin, 2000; Porter et al.,
2004; Grabowski et al., 2005; see Geraldi et al., 2009 for a
review of oyster ecosystem services).
Throughout the past century the Chesapeake Bay population of the Eastern oyster, Crassostrea virginica (Gmelin,
1791), has been in decline due to over-harvesting, habitat
destruction, and the introduction and spread of parasitic
diseases such as Dermo, caused by Perkinsus marinus
(Mackin, Owen, and Collier) Levine, 1978 (Newell, 1988;
Rothschild et al., 1994; Ford and Tripp, 1996). However,
the morphologically and taxonomically similar Asian oys-
ter, Crassostrea ariakensis (Fujita, 1913), is more resistant
to P. marinus infection and more resilient to its effects when
infected (Calvo et al., 2001; NRC, 2004). This led to the
consideration of C. ariakensis for introduction to the Chesapeake Bay to augment the native population, thereby aiding in ecosystem services and providing an alternative species for the commercial oyster industry. The potential
introduction of C. ariakensis to part of the native range of C.
virginica resulted in numerous studies comparing the anatomy and physiology of these two species (e.g., Calvo et al.,
2001; Matsche and Barker, 2006; Harding, 2007; Harlan,
2007; Lombardi, 2012; Lombardi et al., 2013). During one
such study, it was observed that when notching shells with
a grinding wheel, considerable more effort was required for
C. virginica than for C. ariakensis (Lombardi and Paynter,
unpubl.). This observation illustrated the need for a more
in-depth investigation of the mechanical properties of the
valves of these oysters, in order to understand the mechanisms for these apparent differences in strength and durability that may translate to differences in survival.
In this study, we investigate factors potentially affecting
oyster shell strength, including age, species, and shell wetness, and we discuss the implications of these factors for
predation pressures. The mechanical properties of hardness
(resistance to irreversible deformation) and compressive
strength (force necessary to produce a crack in a material) of
oyster shells were measured in various age classes of C.
ariakensis and C. virginica. Since oyster shell layers accumulate as the animal grows, different layers may have
different properties. Therefore, shells were also tested for
differences in the hardness between superficial and deep
foliated layers. Further, the compressive load at fracture, a
metric of the ability to withstand pressure without cracking,
was measured on the whole shell.
On the basis of perceived shell durability, we hypothesized that C. virginica shells would be more resistant to
fracture and indentation, and thus harder and able to withstand a higher compressive force than C. ariakensis shells.
We further hypothesized that age would be correlated with
thickness and that older shells would be able to withstand a
higher compressive force without cracking than younger
oyster shells. The results of this study provide insight into
the shell properties of two similar oyster species and may
contribute to an understanding of mechanisms for defense
against predators and for survival.
Materials and Methods
Oyster preparation
Crassostrea virginica individuals (ages 1, 4, 6, and 9
years) were obtained from hatchery-planted bars in the
Choptank River in the northern Chesapeake Bay, and Crassostrea ariakensis individuals (ages 4 and 6 years) were
obtained from University of Maryland Center for Environ-
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CRASSOSTREA SHELL MECHANICS
Figure 2. Protocol necessary to prepare oyster samples for microindentations. After samples of oyster shells
were cut, they were placed in resin and polished to create a reflective surface (Panel A). A microindenter (Panel
B) placed a diamond tip into a sample to create an indentation in the shell (Panel C). The square in Panel C is
a microindentation into a Crassostrea ariakensis shell sample.
mental Science’s Horn Point Laboratory Oyster Hatchery
program. Each oyster was scrubbed with a wire brush to
remove any debris and epifauna and then shucked to isolate
shells from oyster tissue. Shell height (mm) and total mass
(g) of each individual were also measured.
Hardness
The hardness, measured in gigapascals (GPa), of four C.
virginica shells from each age class (1, 4, 6, and 9 years)
and four C. ariakensis shells from each age class (4 and 6
years) was tested. A diamond wheel was used to cut two
pieces (about 10 mm by 20 mm) from the flattest part of
each valve. One piece was used to test hardness at the
deepest foliated layer, and the second piece was used to test
hardness at the most superficial foliated layer. In all samples, the area where the adductor muscle attached to the
valve was not used so as to maintain consistent valve
sampling.
Each valve sample was embedded in resin and polished
with a Buehler EcoMet 4000 Variable Speed Grinder-Polisher (Lake Bluff, IL) with water or suspensions of abrasive
particles between the samples and grinding pads, in order to
facilitate the collection of clear microindentations (Chinn,
2002). Each sample was polished for 10-40 min at each grit
size (400, 600, and 1200 grit with water, microcloth with
Buehler TexMet 1.0-␮m suspension, and microcloth with
0.5-␮m diamond suspension). Between each grit size, all
samples were placed in a sonicator for 5 min and then
observed under a microscope to inspect for uniform surface
scratches to determine if the sample was ready to progress
to the next grit size. After the finest grit polishing, the
samples were mirror-smooth (Fig. 2A).
Each valve sample was taken to the National Institute of
Standards and Technology where five indents on each sample were made using a Vickers pyramidal diamond microindenter (Zwick, GA; Fig. 2B). Each indent image (Fig. 2C)
was captured using a Leco Olympus PMG3 microscope
(Olympus, PA). Hardness values were calculated using the
standard Vickers hardness (HV) equation:
HV ⫽ 1.854 ⫻ F/d2,
where F is the applied force (1 kg and 9.8 m/s2) and d is the
mean length (mm) of the indentation diagonals. Mean hardness values for each sample were calculated.
Analysis of hardness between superficial and deep foliated layers, species, and age (4- and 6-year-olds) was performed using a fixed-factor split-plot analysis of variance
(ANOVA), with oyster age as the main plot and each
foliated layer as the subplot. A separate one-way fixedfactor ANOVA was conducted to test for intraspecific hardness differences among dry specimens of C. virginica at
different ages (1, 4, 6, and 9 years) using the foliated layer
with the lowest hardness value.
Compressive strength
Intraspecific and interspecific comparisons. Interspecific
analyses based on age classes (4 and 6 years) were con-
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S. A. LOMBARDI ET AL.
ducted using 10 dry shells of each species. Additionally, 10
dry C. virginica shells aged one and nine were used for
intraspecific analyses. Before the compressive strength was
measured, shell thickness within 5 mm of the ventral shell
margin of the curved valve was measured using digital
calipers (mm). The compressive force, measured in newtons
(N), needed to fracture each shell was determined by using
an Instron load compression machine with a flat load cell
(Instron, PA). The left and right valves of each shell were
realigned to mimic a closed live oyster. The flat right valve
was placed on the base of the machine, while the cupped left
valve pointed upward with the tallest point of the shell
aligned with the center of the machine. Compressive
strength was assessed by increasing the force until the shell
cracked and there was a major drop in load compression
force. After the drop in force was observed, the oyster was
examined to confirm the presence of a crack. In all samples,
the crack was visually identified and went through all layers
of the shell. Typically both valves were cracked, though on
occasion only the upper, cupped valve cracked. We observed complete radial cracks that originated in the center of
the shell and spread outward to the edge.
A two-way fixed-factor ANOVA was used to test for
differences in compressive strength based on age and species and to test for any interactions between the two. Additionally, a one-way fixed-factor ANOVA was used to test
for differences in compressive strength between all ages (1,
4, 6, 9 years) of C. virginica. To determine if thickness
could explain the patterns observed, we tested whether
thickness differed between age classes or species by
using a two-way fixed-factor ANOVA and two separate
analysis of covariance (ANCOVA) models (one with species as a factor and a second with age class) with thickness
as a covariate.
Wet versus dry shells. The relationship between shell wetness and compressive strength was also investigated using
six-year-old oyster shells; five shells of each species were
kept wet, and five shells of each species were placed in a
60 °C oven for 72 h to dry. Dry shells were stored in a
desiccator at room temperature (about 24 °C) until compressive strength data were collected using the procedure
described in the previous section on Intraspecific and interspecific comparisons. A separate two-way fixed-factor
ANOVA was used to test for significant differences in
compressive strength between wet and dry shells, species,
and any interaction between the two factors.
Shell density
Density was measured in 10 C. virginica shells for each
age (1, 4, 6, and 9 years) and 10 C. ariakensis shells for each
age (4 and 6 years). Shells measured for density were
different shells than those used for either compressive
strength or hardness tests but were from the same source
and were conditioned identically. The volume of each pair
of valves was calculated by displacement using graduated
cylinders, and the dry mass of the shells weighed on a
microbalance. Using these values, density (g/ml) was calculated for each shell. A separate two-way fixed-factor
ANOVA with age and species as factors was used to compare shell densities.
For all comparisons, statistical analyses were performed
in JMP (JMP, NC) and R 2.11.1 (R Core Team, 2013). In
some cases, log-transformed data were used to meet normality or homogeneity of variance assumptions. Since the P
values between transformed and non-transformed data were
comparable and the accept/reject decisions were the same,
results are presented on non-transformed data to ease interpretation.
Results
Hardness
Intraspecific Crassostrea virginica comparisons (ages 1, 4,
6, 9 years). The mean (⫾SEM) shell hardness values for
one-year-old individuals of C. virginica were 1.17 ⫾ 0.17
GPa, 1.55 ⫾ 0.22 GPa for four-year-olds, 1.47 ⫾ 0.16 GPa
for six-year-olds, and 1.20 ⫾ 0.14 GPa for nine-year-olds.
Hardness values were not significantly different between
age classes of C. virginica (P ⫽ 0.368). As there was no
significant difference in hardness between outer and inner
foliated layers (P ⫽ 0.274, see the following Results section
on interspecific comparisons), the intraspecific analysis was
performed using the lower hardness value from each shell.
This was done because most predators would need to penetrate only one valve, and therefore we chose to use the
weaker valve.
Interspecific comparison (ages 4 and 6 years). When we
performed the split-plot analysis, no significant two-way or
three-way interactions were found for shell hardness between age, foliated layer, or species (in all comparisons P ⬎
0.05). Crassostrea ariakensis exhibited a shell hardness of
1.45 ⫾ 0.13 GPa, which was not significantly different from
that of C. virginica shells (1.77 ⫾ 0.13 GPa; P ⫽ 0.090).
Shell hardness values did not significantly vary between
four-year-old (1.64 ⫾ 0.15 GPa) and six-year-old oysters
(1.57 ⫾ 0.12 GPa; P ⫽ 0.635) or between foliated layers
(␮deep ⫽ 1.50 ⫾ 0.12 GPa; ␮superficial ⫽ 1.72 ⫾ 0.14 GPa;
P ⫽ 0.274). It should be noted that C. virginica shell values
differ from those in the intraspecific analysis (previous
Results section), because the intraspecific analysis used
only one foliated layer per oyster, whereas this analysis used
two samples per shell.
CRASSOSTREA SHELL MECHANICS
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Figure 3. Compressive force, thickness, and density of Crassostrea virginica and Crassostrea ariakensis
shells. Panel A: Compressive force of different ages of C. ariakensis and C. virginica. Significantly more force
(N) was necessary to produce a crack in six- and nine-year-old C. virginica shells than in shells of four-year-old
C. virginica or four- and six-year-old C. ariakensis shells. Panel B: Thickness by species for each age class.
Shells of age six C. virginica were significantly thicker than shells of age four C. virginica and four and six C.
ariakensis. Panel C: Shell density by species. Crassostrea virginica shells were more dense than C. ariakensis
shells. In all panels, C. virginica values are shown in a darker color, error bars depict standard error, and different
letters indicate significantly different values at an alpha of 0.05.
Compressive strength
Intraspecific Crassostrea virginica comparisons. No significant difference in compressive strength was found between
one- (853.9 ⫾ 169.1 N) and four-year-old C. virginica
shells (1089.8 ⫾ 90.8 N; P ⫽ 0.235). However, a significant
increase in compressive force was needed to crack both sixand nine-year-old shells (mean forces were 3896.1 ⫾
560.7 N and 3513.1 ⫾ 476.3 N, respectively) compared to
one- and four-year-old shells (mean forces were 853.9 ⫾
169.1 N and 1089.8 ⫾ 90.8 N; P ⬍ 0.001, P ⬍ 0.001,
respectively).
Interspecific analysis (ages 4 and 6 years)
A significant interaction was found between age and
species for compressive strength (P ⫽ 0.001; Fig. 3A). The
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S. A. LOMBARDI ET AL.
Figure 4. Thickness and compressive force. Prior to testing the compressive force required to fracture the
shell, the thickness within 5 mm of the ventral shell margin of the curved valve was measured using digital
calipers. This figure illustrates the relationship between thickness (x) and the compressive force required to
produce a crack (y) in four- and six-year-old specimens of Crassostrea virginica and Crassostrea ariakensis.
ability to withstand compressive force did not statistically
vary between four- (971.4 ⫾ 84.1 N) and six- (1522.0 ⫾
315.1 N) year-old C. ariakensis (P ⫽ 0.109). However,
six-year-old C. virginica shells were able to withstand a
significantly higher compressive force (3896.1 ⫾ 560.7 N)
than both four- (P ⬍ 0.001) and six- (P ⫽ 0.002) year-old
C. ariakensis shells. Compressive strength did not differ
between shells of four-year-old C. virginica or age four- or
six-year-old C. ariakensis (P ⫽ 0.538).
Thickness, density, and wetness. A significant exponential
relationship between the compressive force required to produce a crack (y) and shell thickness (x) was found (y ⫽
523.6e0.381x, R2 ⫽ 0.4657; P ⬍ 0.001; Fig. 4)—thicker
oysters withstood higher compressive force. ANOVA results indicated that thickness varied on the basis of species
and age class (significant interaction between species and
age, P ⫽ 0.003). At age four, the thickness of C. virginica
shells (2.47 ⫾ 0.26 mm) was not significantly different from
that of C. ariakensis (2.07 ⫾ 0.24 mm; P ⫽ 0.275); however, at age six, C. virginica shells were more than twice as
thick as C. ariakensis shells (␮C. ariakensis ⫽ 2.01 ⫾ 0.38
mm; ␮C. virginica ⫽ 4.53 ⫾ 0.42 mm; P ⬍ 0.001). ANCOVA
indicated no significant interaction between thickness and
age (P ⫽ 0.172) or thickness and species (P ⫽ 0.871) and
that in each case thickness had a significant effect (P ⫽
0.001; P ⫽ 0.002, respectively). After accounting for thickness using ANCOVA, species were no longer different (P ⫽
0.358), but a significant trend persisted with age—valves of
older oysters had a higher compressive strength than
younger valves (P ⫽ 0.004).
When analyzing density, no significant interaction between species and age was found (P ⫽ 0.923). Age also had
no effect on density (P ⫽ 0.876), but C. virginica shells
exhibited a mean density of 2.18 ⫾ 0.06 g/ml, which was
more dense than C. ariakensis shells (1.44 ⫾ 0.12 g/mL;
Fig. 3C; P ⬍ 0.001).
No interaction was found between wetness and species
(P ⫽ 0.930), and the force needed to produce a crack did not
differ between wet (2512.8 ⫾ 441.8 N) and dry (2191.5 ⫾
532.7 N) shells (P ⫽ 0.547).
Discussion
This study investigated the mechanical properties of compressive strength and hardness across different ages of adult
Crassostrea virginica and C. ariakensis shells. We found no
differences in shell hardness by age, layer (innermost or
outermost foliated layer), or species. In view of the hardness
data, we believe the foliated layers of adult C. virginica and
C. ariakensis likely have similar compositions, giving rise
to similar strength and defenses. As similar calcite configuration likely yields similar strength, our findings are in
agreement with those of Checa et al. (2007), who found that
the crystallographic structure of the foliated layer had a
consistent arrangement across bivalve species. Further,
compressive force data revealed that the compression
strength of C. virginica shells increased with age, but no
difference was found in compression strength between C.
ariakensis age classes. Interspecifically, ages six and nine of
C. virginica could withstand significantly more compression force than either age class (4 or 6 years) of C. ariakensis.
In particular, the compression strength of six-year-old C.
ariakensis shells was 61% lower than that of six-year-old C.
virginica shells; this difference is similar to the one (64%)
observed between spat (oysters less than a year old) of these
species (Newell et al., 2007). Interestingly, however, at age
four no significant difference (⬃10%) existed between species. Therefore, the ratio of compressive strength between
C. virginica and C. ariakensis individuals likely varies by
age. While the reason for this is unknown, it may reflect
differences in growth pattern, growth rate, energy allocation, or life-history strategies between these species.
CRASSOSTREA SHELL MECHANICS
Compressive strength was not different between species
at age four years, but six-year-old C. virginica shells withstood a higher compressive force than six-year-old C. ariakensis, a pattern that may be explained by differences in
density and thickness. The shells of C. ariakensis were 34%
less dense than those of C. virginica. Regression analysis
indicated a significant relationship between valve thickness
and compressive strength, and six-year-old C. virginica had
the thickest valves. Additionally, when an ANCOVA was
performed with thickness as a covariate, species no longer
had an effect on compression strength. Local curvatures
notwithstanding, the dependance of loads required for fractures originating in the interior section of the shells and
other material are dependent on the square of thickness, as
seen in work presented in Qasim et al. (2006, 2007). Therefore, the protective nature of shell thickness versus the load
required to fracture the shell under a point load reveals that
small attenuations in shell thickness impact fracture loads
by a squared relation. However, our data do not fully adhere
to this pattern. We believe the discrepancy is due to the
shape of the shells, as curvature can distribute the load over
the entire structure, thereby influencing the compressive
strength (Sayegh and Dong, 1970). Therefore, the mechanical defenses of shells likely include the foliated layer
structure and the curve and thickness of the shell. In summary, we believe that the compressive strength differences
between species can be primarily explained by differences
in shell thicknesses and density; however, those factors do
not explain ontogenetic differences based on age.
Since many predators penetrate through the shell to access internal oyster tissue (White and Wilson, 1996), identifying differences in shell strength by age and species could
have implications for restoration and management of the
native population and the stability of the fishing industry.
Therefore, the differences in strength, illustrated by the
ability to withstand compressive force, between C. ariakensis and C. virginica could translate into differences in
predation. Although predators vary by location, common
predators of oysters include fish (namely drums, sheepsheads, rays, and skates); crabs; snails (especially drills); sea
stars; and flatworms (Mackin, 1959). However, species
have different mechanical abilities to penetrate the shell. For
instance, the cownose ray, Rhinoptera bonasus (Mitchill,
1815), is capable of bite forces of about 220 N (Maschner,
2000); and the bite of the large black drum, Pogonias
cromis (Linnaeus, 1766), can reach up to 1250 N (Grubich,
2005). Further, the rock crab, Cancer irroratus Say, 1817,
has demonstrated chelae crushing forces of up to 75 N
(Block and Rebach, 1998); and the blue crab, Callinectes
sapidus Rathbun, 1896, has exhibited a mean chelae force
of 27 N (Singh et al., 2000). In contrast, the mean compressive strength of C. virginica spat ranged from 51 to 71 N
and that of C. ariakensis from 18 to 25 N, depending on the
presence of predators (Newell et al., 2007); in this study we
181
found the compressive force needed to crack the shells of
one- and four-year-old C. virginica and four-year-old C.
ariakensis was about 1000 N, whereas six- and nine-yearold C. virginica withstood more than 3500 N. Thus, if a
predator attacks the center of the shell, then both spat and
adult C. virginica and C. ariakensis should be able to
withstand cownose ray, rock crab, and blue crab predation,
but C. ariakensis and young (ages four and younger) C.
virginica are still vulnerable to drum predation.
Another factor that may influence shell properties is the
ratio of chalky deposits to foliated sheets within the valves.
Naturally interspersed between foliated sheets are chalky
deposits (Galtsoff, 1964; Carriker, 1996). Although both
chalky and non-chalky layers are made of similar chemical
elements, they have different chemical compositions, and
chalky material is more porous than the folia (Yoon et al.,
2009; Lee et al., 2011). Galtsoff (1964) found that 50% of
C. virginica samples had no chalky patches, and only 3%
had more than 75% of the shell composed of such deposits.
However, no studies have quantified the chalky proportions
in C. ariakensis. In this study we were interested in assessing the hardness of the foliated layer; however, while collecting data we encountered regions of high variability
within C. ariakensis. We believe these represented samples
taken in the chalky layer, and we excluded them from
analysis (Appendix Fig. 1). Although some discrepancy
exists in the literature (Lee et al., 2011), Yoon et al. (2009)
and Ullmann et al. (2010) found the chalky layer of Crassostrea oysters species to be less dense than the foliated
layers. Therefore, since regions of high variability were
found only in C. ariakensis valves that were also less dense
than those of C. virginica, we hypothesize that C. ariakensis
may have a higher proportion of chalky material than C.
virginica. A difference in this proportion could also explain
the differences in strength and durability observed at the
macroscopic level—that is, the greater difficulty that Lombardi and Paynter (unpubl. data) experienced in grinding C.
virginica shells compared to C. ariakensis shells. Despite
providing less strength, a higher proportion of chalky material may be advantageous in an area dominated by gapeor claw-limited predators because chalk deposits may increase shell thickness at minimal cost to the oyster. Thus,
different proportions of chalky material to folia likely influence the mechanical defenses the shell provides and may
indicate different predation styles and pressures in native
ranges. Overall, the shells of C. ariakensis may be less
durable and strong due to a greater proportion of chalky
material, though further studies are needed to test this hypothesis.
In summary, we found that the shells of six- and nineyear-old specimens of C. virginica withstood significantly
higher compression force than any age class of C. ariakensis
studied (4- and 6-year-old). The higher compressive
strength observed for C. virginica shells at age six is likely
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S. A. LOMBARDI ET AL.
due to thicker valves and denser shells compared to C.
ariakensis. We found no differences in hardness values
between any age class, foliated layer, or species; however,
differences in density and observed variations in hardness
within C. ariakensis (Appendix Fig. 1) may indicate different shell compositions between species. The findings of this
research can be applied to oyster management decisions, as
well as to oyster-inspired bioceramic technology. Moreover,
this work contributes to an understanding of shell mechanics and sheds light on similarities and differences between
two closely related species while identifying potential differences in predation vulnerability.
Acknowledgments
Support for this research was provided by grants to the
University of Maryland from the Oyster Recovery Partnership and the Howard Hughes Medical Institute Undergraduate Science Education Program. This project would not
have been possible without the encouragement, assistance,
and expertise of Peter Lucas, Erin Vogel, and Shannon
McFarlin from George Washington University. We thank
Brian Lawn from the National Institute of Standards and
Technology for his materials science expertise and guidance. Further, we are grateful to Adriane Michaelis, Karen
Kesler, Rebecca Kulp, Emma Weaver, and Drew Needham
for their assistance with shell polishing and data collection,
and to Robert Bonenberger, Vincent Politano, Jeffrey Jensen, and William Higgins for their academic and logistical
help. Additionally, we thank the Horn Point Oyster Hatchery program for supplying the Crassostrea ariakensis used
in this study. We also thank the editor and two anonymous
reviewers for constructive comments that helped strengthen
the manuscript.
Literature Cited
Block, J. D., and S. Rebach. 1998. Correlates of claw strength in the
rock crab, Cancer irroratus (Decapoda, Brachyura). Crustaceana 71:
468 – 473.
Bottjer, J. F., and J. G. Carter. 1980. Functional and phylogenetic
significance of projecting periostracal structure in the Bivalvia (Mollusca). J. Paleontol. 54: 200 –216.
Calvo, G. W., M. W. Luckenback, S. K. Allen, Jr., and E. M. Burreson.
2001. A comparative field study of Crassostrea ariakensis (Fujita
1913) and Crassostrea virginica (Gmelin 1791) in relation to salinity in
Virginia. J. Shellfish Res. 20: 221–229.
Carriker, M. R. 1996. The shell and ligament. Pp. 75–168 in The
Eastern Oyster, Crassostrea virginica, V. S. Kennedy, R. I. E. Newell,
and A. F. Eble, eds. Maryland Sea Grant Publication, College Park,
MD.
Chateigner, D., M. Morales, and F. M. Harper. 2002. QTA of prismatic calcite layers of some bivalves, a link to trichite ancestrals.
Mater. Sci. Forum 408: 1687–1692.
Checa, A. G., F. J. Esteban-Delgado, and A. B. Rodrı́guez-Navarro.
2007. Crystallographic structure of the foliated calcite of bivalves. J.
Struct. Biol. 157: 393– 402.
Chinn, R. E. 2002. Ceramography: Preparation and Analysis of Ce-
ramic Microstructures. ASM International, Materials Park, OH. Pp.
35– 43.
Ford, S. E., and M. R. Tripp. 1996. Diseases and defense mechanisms.
Pp. 581– 660 in The Eastern Oyster, Crassostrea virginica, V. S.
Kennedy, R. I. E. Newell, and A. F. Eble, eds. Maryland Sea Grant
Publication, College Park, MD.
Galtsoff, P. S. 1954. Recent advances in the studies of the structure and
formation of the shell of Crassostrea virginica. Proc. Natl. Shellfish
Assoc. 45: 116 –135.
Galtsoff, P. S. 1964. The American oyster: Crassostrea virginica (Gmelin). Fish. Bull. 64: 1– 480.
Geraldi, N. R., S. P. Powers, K. L. Heck, and J. Cebrian. 2009. Can
habitat restoration be redundant? Response of mobile fishes and crustraceans to oyster reef restoration in marsh tidal creeks. Mar. Ecol.
Prog. Ser. 389: 171–180.
Grabowski, J. H., A. R. Hughes, D. L. Kimbro, and M. A. Dolan. 2005.
How habitat setting influences restored oyster reef communities. Ecology 86: 1926 –1935.
Grubich, J. R. 2005. Disparity between the feeding performance and
predicted muscle strength in the pharyngeal musculature of black drum,
Pogonias cromis (Sciaenidae). Environ. Biol. Fishes 74: 261–272.
Harding, J. M. 2007. Comparison of growth rates between diploid
DEBY eastern oysters (Crassostrea virginica, Gmelin 1971), triploid
eastern oysters, and triploid suminoe oysters (C. ariakensis, Fujita
1913). J. Shellfish Res. 26: 961–972.
Harlan, N. P. 2007. A comparison of the physiology and biochemistry of
the eastern oyster, Crassostrea virginica and the Asian oyster, Crassostrea ariakensis. Master’s thesis. University of Maryland, College
Park, MD.
Lee, S.-W., Y.-N. Jang, K.-W. Ryu, S.-C. Chae, Y.-H. Lee, and C.-W.
Jeon. 2011. Mechanical characteristics and morphological effect of
complex cross structure on biomaterials: fracture mechanics and microstructure of chalky layer in oyster shell. Micron 42: 60 –70.
Lombardi, S. A. 2012. Comparative physiological ecology of the eastern
oyster, Crassostrea virginica, and the Asian oyster, Crassostrea ariakensis: an investigation into aerobic metabolism and hypoxic adaptations. Ph.D. dissertation, University of Maryland, College Park, MD.
Lombardi, S. A., N. P Harlan, and K. T. Paynter. 2013. Survival,
acid-base balance and gaping responses of the Asian oyster Crassostrea ariakensis and the eastern oyster Crassostrea virginica during
clamped emersion and hypoxic immersion. J. Shellfish Res. 32: 409 –
415.
Mackin, J. G. 1959. Mortalities of oysters. Proc. Natl. Shellfish Assoc.
50: 21– 40.
Maschner, R. P., Jr. 2000. Studies of the tooth strength of the Atlantic
cow-nose ray, Rhinoptera bonasus. Master’s thesis, California State
Polytechnic University.
Matsche, M., and L. Barker. 2006. Juvenile oyster mortality following
experimental exposure to anoxia/hypoxia—C. ariakensis vs. C. virginica. Report to the Maryland Department of Natural Resources. Available from the Maryland Department of Natural Resources, Annapolis.
Meyer, D. L., and E. C. Townsend. 2000. Faunal utilization of created
intertidal eastern oyster (Crassostrea virginica) reefs in the southeastern United States. Estuaries 23: 34 – 45.
Mount, A. S., A. P. Wheeler, R. P. Paradkar, and D. Snider. 2004.
Hemocyte-mediated shell mineralization in the eastern oyster. Science
304: 297–300.
Newell, R. I. E. 1988. Ecological changes in Chesapeake Bay: Are they
the result of overharvesting the American oyster, Crassostrea virginica? Pp. 536 –546 in Understanding the Estuary: Advances in Chesapeake Bay Research. Proceedings of a Conference, March 29 –31,
1988, Baltimore, MD, M. P. Lynch and E. C. Krome, eds. Chesapeake
Research Consortium Publication 129, Chesapeake Research Consortium, Solomons, MD.
CRASSOSTREA SHELL MECHANICS
Newell, R. I. E, V. S. Kennedy, and K. S. Shaw. 2007. Comparative
vulnerability to predators, and induced defense responses, of eastern
oysters Crassostrea virginica and non-native Crassostrea ariakensis
oysters in Chesapeake Bay. Mar. Biol. 152: 449 – 460.
NRC (National Research Council). 2004. Nonnative Oysters in the
Chesapeake Bay, National Academies Press, Washington, DC.
Porter, E. T., J. C. Cornwell, and L. P. Sanford. 2004. Effect of
oysters Crassostrea virginica and bottom shear velocity on benthicpelagic coupling and estuarine water quality. Mar. Ecol. Prog. Ser.
271: 61–75.
Qasim, T., C. Ford, M. B. Bush, X. Hu, and B. R. Lawn. 2006. Effect
of off-axis concentrated loading on failure of curved brittle layer
structures. J. Biomed. Mater. Res. Appl. Biomater. 76B: 334 –339.
Qasim, T., C. Ford, M. B. Bush, X. Hu, K. A. Malament, and B. R.
Lawn. 2007. Margin failures in brittle dome structures: relevance to
failure of dental crowns. J. Biomed. Mater. Res. Appl. Biomater. 80B:
78 – 85.
R Core Team. 2013. R: A Language and Environment for Statistical
Computing. R Foundation for Statistical Computing, Vienna. [Online].
Available: http://www.r-project.org/ [2013, September 6].
Rothschild, B. J., J. S. Ault, P. Goulletquer, and M. Héral. 1994.
183
Decline of the Chesapeake Bay oyster population: a century of habitat
destruction and overfishing. Mar. Ecol. Prog. Ser. 111: 29 –39.
Sayegh, A. F., and S. B. Dong. 1970. Analysis of laminated curved
beams. J. Eng. Mech. Div. 96: 471– 482.
Singh, G., J. D. Block, and S. Rebach. 2000. Measurement of the
crushing force of the crab claw. Crustaceana. 73: 633– 637.
Steimle, F. W., and C. Zetlin. 2000. Reef habitats in the Middle Atlantic
Bight: abundance, distribution, associated biological communities, and
fishery resource use. Mar. Fish. Rev. 62: 24 – 42.
Ullmann, C. V., U. Wiechert, and C. Korte. 2010. Oxygen isotope
fluctuations in a modern North Sea oyster (Crassostrea gigas) compared with annual variations in seawater temperature: implications for
palaeoclimate studies. Chem. Geol. 277: 160 –166.
White, M. E., and E. A. Wilson. 1996. Predators, pests, and competitors. Pp. 559 –580 in The Eastern Oyster, Crassostrea virginica, V. S.
Kennedy, R. I. E. Newell, and A. F. Eble, eds. Maryland Sea Grant
Publication, College Park, MD.
Yoon, Y., A. S. Mount, K. M. Hansen, and D. C. Hansen. 2009.
Electrolyte conductivity through the shell of the eastern oyster using a
four-electrode measurement. Bioelectrochemistry 156: 169 –176.
Appendix
Differences in density and observed variations in hardness within Crassostrea ariakensis
Appendix Figure 1. Two microindentations from the same Crassostrea ariakensis shell. We believe the
image on the left is from the chalky layer, and the image on the right is from the non-chalky foliated layer. The
indentation shown in Panel A is almost 4 times larger than the indentation shown in Panel B. These large
indentations, possibly indicating chalky deposits, were not observed in Crassostrea virginica.