An exploration into the chelae force generation of Green crabs, Carcinus
maenus, and their crushing power on the shells of the intertidal snails
Littorina obtusata and Littorina littorea.
James Carrolla,d , Jessica Roegerb,d , Ryan Walkerc,d
aBiology Department, University of New Brunswick, 100 Tucker Park Road, E2L 4L5, Saint John, New Brunswick
bMarine Sciences Department, James Cook University, James Cook Drive, QLD 4811, Townsville, Australia
cScience Department, Nova Scotia Agricultural College, Cox Road, B2N 5E3, Truro, Nova Scotia
dMarine Semester, Huntsman Marine Science Centre, 1 Lower Campus Road, E5B 2L7, St. Andrew’s, New
Brunswick
Introduction:
Crabs are a highly successful organism that has developed the use of a claw for feeding,
defence and offense (Claussen et al., 2008). This has allowed them to become a formidable
predator in the intertidal zone and their interactions with other organisms are a
fundamental process in this area. The functions of crabs rely on jointed appendages and a
lever system operating within the appendages (DeMont, 1996). The lever system is
controlled by muscle and joint interactions and utilizes a beam and fulcrum to transmit
force (DeMont, 1996). The force generated by these actions is an important factor in the
lifestyle of crabs because it determines their specialisations within the environment
(Claussen et al., 2008).
The green crab, Carcinus maenus, demonstrates the forces that can be generated through
their claws (chelae) and the impact that their chelae have on the crabs lifestyle and
interactions. C. maenus is an invasive species to the Canadian coastline. It was introduced in
the mid 1800’s from Europe but is now very well established, especially in the Bay of Fundy
area, New Brunswick (Mitchell et al., 2003). It is a relatively small crab but there are prolific
numbers that can be found all through the intertidal zone and this has had severe impacts
on the existing flora and fauna (Mitchell et al., 2003). One of the main prey items of green
crabs are the sea snail molluscs Littorina littorea and Littorina obtusata. The introduction of
C. maenus has altered the distributions and morphology of these snails significantly and
there is strong interest into the ongoing effects of these interactions (Singh et al., 2000;
Block & Rebach, 1998).
C. maenus are very effective at consuming L. Littorea and especially L. Obtusata (Rochette et
al., 2007). They use a variety of techniques to break into the shell such as by crushing,
inserting the tip of an arm into the shell and pulling out the snail (winkling), as well as
chipping around the mouth of the shell to better reach the snail body (Rochette et al.,
2007). This has meant that crabs have had to develop varied chelae morphologies to deal
with hard-shelled snails. Green crabs have dimorphic chelae that are developed for different
functions (Mitchell et al., 2003). They have a major chelae, often called the “crusher” and a
minor chelae, or “cutter”. The crusher is bigger and thicker so that it can generate more
force. It has a higher mechanical advantage (MA) and more slow muscle that both
contribute to more power in the chelae (Warner & Jones, 1976). The cutter chelae is
thinner, longer and sharp-toothed. It has a lower MA and more fast muscle that allows it to
catch and manipulate fast moving prey (Warner & Jones, 1976; MacPhail et al., 1955). The
claws themselves consist of a movable finger called a dactyl that opposes an extension of
the propodus called the pollex (Claverie & Smith, 2007) (see Appendix A.). Force varies
depending on where the prey is clamped onto within the claw grip (Warner et al., 1982).
There is more force delivered closer to the pivot point (Warner et al., 1982) so it is in the
crabs’ best interest to manoeuvre their prey further into the claw.
The strength delivered by crabs is further dependent on the internal structure of the crab.
There are numerous muscles connected to the outer shell of the claw and the apodemes
(Taylor, 2000; Schenk & Wainwright, 2001). There are two apodeme’s within the propodus.
The extensor apodeme is involved in opening the pollex and is set above the flexor
apodeme which closes the pollex. The flexor apodeme is much larger than the extensor
apodeme to allow for the extra stress that is applied when the claw closes (DeMont, 1996).
The flexor apodeme area, as well as the position of the prey within the claw, is used in
conjunction with muscle stress values, angle of pennation where the muscle fibres are
situated relative to the central tendon and the distance of muscle force from fulcrum to
calculate total force generated (DeMont, 1996). This is derived from the equation by
Alexander (1969) which has been used in many previous studies with valid effects (DeMont,
1996; Mitchell et al., 2003; Taylor et al., 2000; Rochette et al., 2007; Warner & Jones, 1976).
The purpose of this study is to further investigate the interactions between Green crabs C.
maenus and the snails L. obtusata and L. littorea by looking at the force needed to crush the
snails shell and whether or not C. maenus is capable of doing this. The calculations for snail
shell breaking resistance are being taken from Edgell & Rochette’s (2008) study into the
predator-prey relationships of C. maenus, L. littorea and L. obtusata. This developed into the
hypothesis that the estimates of force generation by C. maenus are consistent with
published data on L. littorea and L. obtusata snail shell strength and their susceptibility to
green crab crushing predation is correctly explained. It is therefore predicted that
calculations of green crab force generation will fit within the range of snail shell breaking
resistances. This range should display L. littorea as the highest shell strength indicator and L.
obtusata as the lowest shell strength indictor as described by Edgell and Rochette (2008).
This study will give unique insights into relationship that has developed between the snails
and the green crab and whether or not there is significant co-evolution occurring.
Materials and Methods:
The specimens collected for this experiment, green crabs Carcinus maenas, were collected
from Point L’etete, New Brunswick in late fall October 31, 2011 at low tide. Ten male green
crabs ranging from 4.2 to 5.8 centimetres in carapace width were collected, and placed in
Ziploc bags to prevent individuals from fighting and for transport to the lab. . Crabs with
claws intact and inter-moult were chosen. The male crabs upon arrival to the lab were
placed in a freezer to euthanize them.
After 12 hours of freezing the crabs were removed and let out to thaw so that effective
dissection could be done. The crushing claws were removed from the body to begin
calculations. A cylindrical piece of wood, 6.5 millimetres in diameter, was used to represent
a small sea snail shell of Littorina obtusata and Littorina littorea for exact comparisons with
the study by Edgell and Rochette (2008). This was placed in the gape of the claw to simulate
where the snail shell would be crushed within the claw which, in turn, determined where
the L2 value could be measured along the dactyl. Once the exact position of crushing was
determined it was marked with red nail polish along the biting edge so it could be measured
under the dissecting scope.
Once the nail polish was dry the entire dactyl with flexor (closer) apodeme still attached was
removed. This was done by cutting out the front panel of the manus shell (see Appendix A),
cleaning away white muscle tissue so the apodemes were exposed then gently pulling the
dactyl and apodeme away from the rest of the claw (see Appendix B).
The L1 and L2 readings could then be conducted under a dissecting scope with a graduated
scale in the ocular to measure them. The true lengths of the graduated scale were measured
using a ruler. The dactyl with the flexor (closer) apodeme was placed under the dissecting
scope and the L1 and L2 were then measured to determine mechanical advantage (see
Appendix C).
The next step was to determine the area of the apodemes which was done on the computer
program, “Image J”. The apodeme was first cut away from the dactyl without damaging it,
and then pictures of the apodemes were taken and placed into the program. The outline
was traced on the computer and the area was calculated and given in square millimetres
(mm2).
Calculations were then used to determine the crushing power of the claw:
F = A σ sin 2α
G = F x MA
σ = 6.67 x 105 N/M2
α = 0.532225
(A) is the area of the apodeme and mechanical advantage (MA) is equal to the ratio of L 1/L2.
L2 is the distance from the pivot point to the spot on the dactyl where the snail would touch
the claw and L1 is the distance between the pivot and the point of connection of apodeme
to dactyl. The longer L1 is the more force it produces and the longer L2 is the less force it
produces.
Results:
To be able to accurately calculate the force generation of the crushing claw, L1, and L2
values needed to be computed in order to find the mechanical advantage of each crushing
claw. These values are then placed into the muscle force equation used to compute the crab
strength values.
Table 1:
L1 and L2 lengths, in millimeters, with the calculated mechanical advantage
for all ten male green crabs.
Length L1 (mm)
4.55075846
4.667444574
3.09218203
4.95915986
3.967327888
6.884480747
4.842473746
4.375729288
3.818181818
3.792298716
Length L2
(mm)
4.784130688
5.834305718
4.900816803
6.067677946
4.375729288
7.001166861
4.375729288
4.667444574
4.363636364
4.55075846
L1:L2
0.951219512
0.8
0.630952381
0.817307692
0.906666667
0.983333333
1.106666667
0.9375
0.875
0.833333333
As described above, the equation of F = Aσ(sin2α) was used to be able to calculate the
muscle force of each male green crab. Apodeme area from each crab claw was represented
by A in the equation and the sigma value was a constant at 6.67 x 10 5 N/m2. Alpha was the
angle of pennation and varies slightly in different papers, but for this experiment, the
constant angle of 0.532225 radians was used. Each muscle force was then calculated and
placed into the formula Gf = F X MA where F is the muscle force calculated, and MA the
mechanical advantage, which is used to yield the generating force (Gf) of the crushing claw.
Table 2:
Apodeme areas with muscle force and generated force of each male green
crab in respective units to show the force of each crab crushing claw.
Apodeme Area
(mm²)
Apodeme
Area (m²)
Muscle Force (N)
Force Generated (N)
58.74
76.94
30.99
34.89
49.06
107.25
53.68
75.69
56.65
53.83
0.00005874
0.00007694
0.00003099
0.00003489
0.00004906
0.00010725
0.00005368
0.00007569
0.00005665
0.00005383
34.26341714
44.87959337
18.07666492
20.35155982
28.61701132
62.55960995
31.31188683
44.15046039
33.04430679
31.39938278
32.59203094
35.9036747
11.40551477
16.63348639
25.94609026
61.51694978
34.65182142
41.39105662
28.91376844
26.16615232
After computing the values of the generated force for each crab crushing claw, snail shell
strengths were then calculated to demonstrate the main focus of the study to determine if
crusher claw strength coincides with snail shell strength. From the methods, the size of snail
shell to be used was determined at 6.5 mm length for the representation of L. littorea and L.
obtusata. To obtain the force that the snail shells could withstand, the values were taken
from previous literature.
Figure 1: Graphs showing the breaking resistance (force) of L. littorea (black dot) and
L. obtusata (white dot), of 6.5mm length, are able to withstand, taken from Edgell
and Rochette (2008).
From Figure 1, values of 60.34N were taken for the snail L. littorea, and 21.12N for L.
obtusata by anti-logging the breaking resistances. These figures are independent of
carapace length and were plotted against the claw force of C. maenas and those findings are
represented in Figure 2.
In Figure 2, it was found that crab crusher claw strength was able to crush most snails of L.
obtusata, but could not crush snails of L. littorea.
Figure 2:
Graph of ten male C. maenas claws of different carapace width
plotted against forces generated by crush claws with snail shell strengths of L.
obtusata and L. littorea overlaid.
Discussion:
Some conclusions that can be drawn from this experiment are that the results found are
similar to results that have been found in the previous literature. The hypothesis and
predictions that were outlined for this study were that findings are to coincide with
published literature in the respect that the crab crusher claws are able to crush snails in L.
obtusata but not in L. littorea. For this experiment, tests were run in order to investigate
this hypothesis, with results finding that in fact C. maenas with an average carapace width
of 48.87mm SD±4.42mm are able to crush snails of 6.5mm shell length in L. obtusata, but
are not able to crush snails of the same size in L. littorea.
The reason that L. littorea was able to withstand the crushing force capabilities of crabs in
this size range is that they possess thicker shell walls than L. obtusata, thus giving them
more antipredatory abilities. In previous studies, it has been discussed that a possible
reason for the stronger shell is that contemporary populations of L. littorea only, relatively
recently, invaded America from Europe (Chapman et al., 2007). If this prediction is true,
than it is highly plausible that L. littorea has already undergone a co-evolutionary pattern
with C. maenas and that has resulted in thicker shell walls and better antipredatory tactics
(Edgell & Rochette, 2008).
In the case of L. obtusata there is ongoing evolution that is the result of phenotypic
plasticity. There are two populations of L. obtusata within the Gulf of Maine (GoM)
described in the literature, and each population of northern and southern snails have
different shell strengths due to the different stress factors that the two populations face. It
has been determined that snails of the southern population in the GoM have thicker shell
walls and smaller aperatures (to defend from winkling) than those conspecifics of the
northern population (Rochette et al., 2007). This shows that here in the Southern Bay of
Fundy (BoF), snails belong to the northern population, and are assumed to be easier to
crush due to thinner shell walls and larger aperatures than snails found in more southern
regions.
The findings of this experiment have shown that snails of L. obtusata were able to be
crushed by C. maenas but not those of L. littorea. Some problems that could be addressed
are that the crab carapace widths used could have more variation, because for this study
there was a narrow range, and larger crabs possess a greater force generation than smaller
ones. Also, other factors to include are that crabs were taken from one location, only males
were used, and crusher claw size was not a major factor in selection of crabs for this
experiment. Another major factor that this study was based upon was the assumption that
from previous literature, crabs are able to crush snails belonging to L. obtusata and not L.
littorea, meaning that we have a complete understanding of how the biology functions. If
this assumption is true, than our results in Figure 2 would all be more or less in the middle
of the two snail shell strengths, and would not have any outliers as we encountered for our
experiment. With the assumption in mind, it brings attention to other factors that should be
made aware of, as some smaller crabs have a larger force generation than others, and that
different techniques to kill the snail can be employed for the lower force generating crabs of
various sizes.
One technique that should be explained with more detail is that the snail can be placed at
different areas along the dactyl. This is important as the force at the tip of the dactyl is less
than the force generated at the base, closer to the pivot point where the force is greatest
(Warner et al., 1982). The reason that this is a potential factor is that the shell for this
experiment was placed inside the chelae near the base close to the pivot point where
perhaps the shell is broken outright instead of near the tip where repeated loading may
eventually break the shell. Repeated loading is the repeated crushing of the shell to induce
stress areas that will eventually allow the shell to break. This is another factor that should
be monitored, as it can account for snail deaths of larger snails from smaller crabs (Boulding
& Labarbera, 1986).
In conclusion, the experiment conducted did follow previous patterns of published
literature. From our experiment, it might be useful that for further studies, other factors of
how crabs can capture snails should be looked at in more detail with respect to repeated
loading, placement in the chelae, winkling, and chemical cues. Also a variety of gender and
sizes of green crabs could give a better understanding of the techniques employed in
hunting of sea snails as it allows for more variety and a better understanding of the two
interactions in the natural environment. From our experiment, it can be stated that green
crab claw strength has the ability to crush, outright, snails of the species L. obtusata but not
L. littorea, rendering the hypothesis correct that with the assumption of crab crusher
strength following the same pattern as derived from previous data recorded in the
literature.
References:
Alexander, R. McN. (1969). Mechanics of the feeding action of a cyprinid fish. Journal of
Zoology London. 159: 1-15.
Block, J. D. & Rebach, S. (1998). Notes and news: Correlates of claw strength in the rock
crab, Cancer irroratus (Decapoda, Brachyura). Crustaceana. 71(4): 468-473.
Boulding, E.G., Labarbera, M. (1986). Fatigue Damage: Repeated Loading Enables Crabs to
Open Large Bivalves. Biological Bulleti. 171: 538-547.
Chapman, J. W., Carlton, J. T., Bellinger, M.R., and Blakesee, A.M.H. (2007). Premature
refutation of a human-mediated marine species introduction: the case history of the
marine snail Littorina littorea in the Northwestern Atlantic. Biol. Invasions. DOI
10.1007/s10530-007-9098-9.
Claussen, D. L., Gerald, G. W., Kotcher, J. E., & Miskell, C. A. (2008). Pinching forces in
crayfish and fiddler crabs, and comparisons with closing forces of other animals. J.
Comp. Physiol. B. 178: 333-342
DeMont, E. 1996. Measuring how muscles function in levers. The American Biology Teacher.
58(8): 490-492.
Edgell, T. C. & Rochette, R. (2008). Differential snail predation by an exotic crab and the
geography of shell-claw covariance in the Northwest Atlantic. Evolution. 62: 12161228.
MacPhail, J.S., Lord, E.I., & Dickie, L.M. 1955. The green crab – a new clam enemy. Fish Res.
Board Can. Progr. Rep. Atl. Coast Stn. No. 63. Pp. 3-11
McLain, D. K., Pratt, A. E., & Berry, A. S. (2003). Predation by red-jointed fiddler crabs on
congeners: interaction between body size and positive allometry of the sexually
selected claw. Behavioral Ecol.14: 741-747.
Mitchell, S.C., Kennedy, S.M., Williams, P.J., & DeMont, M.E. 2003. Morphometrics and
estimates of force generation by the chelae of a North American population of the
invasive green crab, Carcinus maenas (L.) Can. J. Zool. 81: 203-215.
Taylor, G. M., Palmer, A. R., & Barton, A. C. (2000). Variation in safety factors of claws within
and among six species of Cancer crabs (Decapoda: Brachyira). Biological Journal of
the Linnean Society. 70: 37-62.
Rochette, R., Doyle, S. P., & Edgell, T. C. (2007). Interaction between an invasive decapod
and a native gastropod: predator foraging tactics and prey architectural defences.
Mar. Ecol. Prog. Ser. 330: 179-188.
Schenk, S. C. & Wainwright, P. C. (2001). Dimorphism and the functional basis of claw
strength in six brachyuran crabs. Journal of Zoology. 255: 105-119.
Singh, G., Block, J. D., & Rebach, S. (2000). Measurement of the crushing force of the crab
claw. Crustaceana. 73(5): 633-637.
Taylor, G. M. (2000). Maximum force production: why are crabs so strong? Proc. R. Soc.
Lond. 267: 1475-1480.
Warner, G. F., Chapman, D., Hawkey, N., & Waring, D. G. (1982). Structure and function of
the chelae and chela closer muscles of the shore crab Carcinus maenas (Crustacea:
Brachyura). J. Zool., Lond. 196: 431-438.
Warner, G. F. & Jones, A. R. (1976). Leverage and muscle type in crab chelae (Crustacea:
Bracyura). Journal of Zoology. 180: 57-68.
Appendix:
A.
Diagram of crab claw structures.
B.
Image demonstrating the point where the shell meets the dactyl when it is crushed
further inside the chelae.
C.
Image representing the physics of the lever with L1 and L2 depicted on the dissected
claw.