Rutgers, The State University of New Jersey
Department of Mechanical and Aerospace Engineering
DMS I Project
Hermit Crab Shell Design
Coenobita Clypeatus
Team 10
Abraham Diaz (Leader)
Santiago Moreno
David Chuang
Chris Graziadio
Project Advisors: Dr. Hao Lin ME Dept.
Dr. Elizabeth Demaray Rutgers Camden
December 12, 2005
Fall 2005
1
Table of Contents
Title ……………………..…………………… ………………………….1
Table of Contents …….…………………… …………………………...2
Introduction ………….…………………… …………………………....3
Design Description ……..……………… ……………………………....4
Conceptual Designs …………………… ……………………………….6
Evaluation and Optimal Design………………………………………..10
Material Selection and Manufacturing ………………………………12
Conclusions and Further Work ……………………………………….13
References ……………………………………………………………....14
2
Introduction
Hermit crabs are unique in that as they grow, they become too large for their
shells. When this happens, the hermit crab must search for a new dwelling. They insert
their abdomen into abandoned gastropod mollusk shells that they carry with them for
protection, if they do not find a natural shell to live in. Their abdomens are soft and
asymmetrical, flexed and twisted to fit into the borrowed shells. Their abdominal
appendages are especially modified for keeping the shell firmly supported on the body.
The challenge here is that there are currently not enough shells left on the
beaches for hermit crabs to use. Approximately 30 percent of all hermit crabs are living
in shells that are too small, and this skyrockets to 60 percent when the animal goes
through its growth spurt. Hermit crabs need properly fitting shells for protection, mating
success, and reproduction. The housing shortage is assumed to be caused by pollution
and the collection of seashells by humans, although scientists are unsure if these are the
only causes.
Our solution for this problem will involve creating artificial housing. Since there
are several varieties of hermit crabs, the team will focus on designing a shell for a species
known as “Coenobita Clypeatus”. We are working together with Dr. Elizabeth Demaray,
from Rutgers Camden and Dr. Hao Lin from the M.E. department.
We will need to research different materials to be used for these housings. Crabs
like shells with high volume-to-weight ratios, thus, they will have ample room to grow
and will be light to carry. The material must also be strong and long-lasting so that many
generations of hermit crabs will eventually use these houses.
The designs of the new houses will need to be specially tailored to the animals
needs. The new design contains an internal flange attached to the front opening for the
crab to clutch with its holding claw. This will decrease the weight and increase the
volume compared to a real shell. The design also has an overhang for additional
protection when the body of the hermit crab must be extended outside the shell.
The houses must be manufactured without separate parts, so no glue or solvents
will be used that will harm the animals. Previous research by the Hand Up Project used a
stereo lithography process, where a laser deposits thin layers of plastic to create the
structure in one piece. Also, the possibility of using rapid-prototyping is high since one
of the RP specialties is to create parts which are composed of only one piece. We will
also consider different materials that are friendly to the environment like biodegradable
materials. However, biodegradable materials are not able to be used with the rapid-
prototyping machines.
3
Design Description
We conducted research to obtain the most important parameters for creating the
optimal shell for the hermit crab. The most important parameters taken into
consideration are: the ratio of depth to aperture width, shell volume to weight ratio, shell
size, aperture length, and shell strength. These features will help the hermit crab decide
to pick the shell instead of suboptimal shells. Suboptimal shells are those found at the
beach like cans, bottles, and coconut shells.
Hermit crabs of different sizes will obviously need to use different size shells.
Trying to create an optimal fit for the crabs is one of the most important objectives in
creating a new shell. If the shell is too small, the hermit crab will not be able to fully hide
within its shell, leaving it easy prey for predators. If the shell is too large, the hermit crab
will have a difficult time moving it and expend more energy than it should. Research
shows that shell shape is not as important as shell size, but when shell sizes are similar
crabs seem to prefer shell shapes that they are familiar with [1].
Small crabs were found to weigh between 0.4 and 1.2 g, while large crabs
weighed between 21.0 and 27.0 g. Research found small crabs to use shells 42x22 mm or
30x30 mm in size. Larger crabs used shells 70x28 mm, 110x68 mm, or 200x50 mm in
size [2]. Our shell design will try to take these sizes into account when creating the new
shell. Larger crabs tend to use proportionally lighter shells than do smaller crabs [3].
Larger ones may not need as much protection as smaller crabs because they are stronger
and will be able to fend off predators better. Smaller crabs need proportionally heavier
shells to give as much protection as possible.
Another important parameter to take into account is shell volume to weight ratio.
Crabs tend to maximize shell volume in relation to shell weight. Hermit crabs would like
to have a large volume for protection, and they would also like to have a low shell weight
for easy maneuverability. Research found an equation relating crab mass to total mass of
the crab and the shell to be crab mass = 0.52 + 0.41 * total mass [4]. From this equation
we find that the ratio of crab mass to shell mass is 0.6949. An exception to this is that
hermit crabs that live in high flow areas prefer heavier shells so they will not get bounced
around too much [1]. The high flow rates also reduce movement needed by the crab, so
the heavier shell will not cause too much energy loss. Also, land hermit crabs prefer
lighter shells than do marine hermit crabs [5]. This is because the buoyant force of the
water helps the crabs to support the shell.
We must also take into account the shell aperture length as well as the depth to
aperture width ratio. The depth to aperture width ratio is important because the hermit
crab needs a large space to withdraw to, away from the aperture, to avoid predators [6].
It will also help the crab to keep its shell if it gets into a shell fight with another hermit
crab. If the aperture is too large, predators will be able to get inside the shell, and if the
aperture is too small the hermit crab will not be able to fit inside. Also, if the aperture is
too large the shell may be damaged more easily. Another problem with an aperture that
is too large is that the water the hermit crab needs to prevent desiccation may leak out,
4
leading to death. Two research papers found equations relating shell aperture size to crab
mass. Equation 1 is crab mass = 1.31 * aperture width^1.64 [6]. Equation 2 is log shell
aperture length = .2623 * (log crab mass) + .4342 [7].
The shells should be made as strong as possible to stop attacks from predators.
Known predators of the hermit crab include other crustaceans, fish, and birds [8]. Shell
peeling is one way predators get hermit crabs out of their shells to eat them. Some
predators can also drill holes in the shells, making the hermit crabs vulnerable to attack.
Stronger shells will help to prevent predators from eating the hermit crabs. We do not
want any holes to appear in our shells because they increase the risk of predation, make it
easier for the shells to be broken, and allow the water inside the shell to leak out causing
the crab to become dehydrated.
When we created our conceptual designs, we attempted to take into account these
parameters that were found to be important. These parameters include the ratio of depth
to aperture width, shell volume to weight ratio, shell size, aperture length, and shell
strength. By creating a shell according to these parameters, we will increase the chances
of survival for the hermit crabs. It will decrease the amount it time it takes for a hermit
crab to choose a shell, increase the strength of the shell to stop predators from breaking it,
and make the hermit crab more comfortable which may decrease the amount of shell
exchanges between hermit crabs.
5
Conceptual Designs
Our conceptual designs are based on the land hermit crab, Coenobita Clypeatus.
The first conceptual design is pictured below in Figure 1.
Figure 1. Default View
Figure 2 shows a side view of the model. This view illustrates the important criteria that
led to the selection of this particular design. Given the small size of hermit crabs and the
irregularity of sandy beaches where these creatures may move around, it is important to
design a shell that does not cost too much energy and effort to carry. As seen in Figure 2,
the bottom is a smooth arc shape that will reduce friction as it slides on the sand. The
bottom can be thought of as being aerodynamic in order to reduce drag force. Another
important feature is the semicircle at the front. This may prevent sand from piling up in
front as the shell moves forward and also may prevent sand from getting inside the
housing.
Figure 2. Side View
6
For the second conceptual design there were some important features added to the
original artificial shell design. These features can be seen in Figure 3.
Figure 3. Isometric View of Conceptual Design.
The zigzag patterns on the bottom of this shell design were implemented to decrease
friction. Compared to the original design, this design has less surface contact with the
original artificial shell created. Less surface contact with the shell means less friction,
which makes it easier for the hermit crab to move by exerting less energy and thus
conserving energy for other life enduring processes. The zigzag patterns on the bottom
of this shell were also used for more stability. When the crab is near the ocean tidal
waves the zigzag pattern will allow the water to go through under the shell instead of
pushing against the shell so that again the hermit crab will need less energy to hold its
position. The other main feature is the pointy shell structure at the top of the shell is used
to prevent predators from biting it and trying to crack the shell to eat the hermit crab.
The thickness of the shell is increased compared to nature’s shell so that birds or other
predators cannot try to pry open the shell by ripping apart the aperture. From all of these
added features, the hermit crab will be safe and will increase the chances of survival due
to less energy usages and better shell protection features.
7
An isometric perspective of the third conceptual design can be seen in Figure 4.
Figure 4. Isometric View of Conceptual Design.
One of the features of this conceptual design is the concave bottom. It is shaped like that
to help reduce friction. This is important because reducing friction will allow the crab to
use less energy to move around. The only contact lines will be in the front and the rear of
the shell. Having two contact points may make it more stable as well. Another feature is
the overhang above the aperture. This overhang is used for protection to make it tougher
for predators to reach the crab. These features can be seen in the side view in Figure 5.
Figure 5. Side View of Conceptual Design.
This shell design also has a slightly rounded aperture to help the crab block it with
its claws more effectively. Inside the shell there is a grip for the crab to grab onto with its
claw to hold the shell on.
8
The final conceptual design contains three important features: a protective
overhang, an angled aperture, and curved contact with the ground. These features are
shown in Figure 6.
Protective
Overhang Angled
Aperture
Curved
Ground
Figure 6. View of conceptual design features. Contact
The combination of an angled aperture and overhang is a protection against hermit crabs’
predators. The old model included an aperture at the front face; however, this design
lacks protection against predators since the aperture is located at the front face of the
design. As the crab hides inside the shell, the aperture location gives access to predators
to attack. It is ideal to have the aperture at an angle, as shown by Figure 6 followed by
the protective overhang. As the crab hides, the entire shell will shift forward, thereby
occulting the aperture against the floor. This idea was noted when Coenobita Clypeatus
hides inside its shell. After hiding, the face of the aperture was facing down which makes
it much more difficult for predators to attack effectively.
The third consideration for this preliminary design is transforming the old
model’s bottom shape. The bottom shape needs to be changed from a flat surface
ground-contact to a curvature-line contact. This smooth curvature will give the new
design the option of reducing contact area with the ground. The objective of this design
is to eliminate flat geometries and use the crab’s morphology to try to match nature’s
design. Another advantage of the curved shape is in the carriage and transportation of
salty water. Salt water is important for the hermit crab survival. Hermit crabs need a
certain water salinity concentration to carry around inside the shell. The reason behind
this is to keep the correct humidity and salinity inside the hermit crab’s shell. The old
model only had a flat surface at the bottom, which did not help the shell to retain water.
The new concavity of the bottom surface will help the hermit crab carry and transport
salty water around.
9
Evaluation and Optimal Design
All of our conceptual designs had good points and certain features appeared
multiple times, so the final optimal design combined the best features of our conceptual
designs. Our final design takes into account the morphology of the hermit crab to create
an optimal shell. Figure 7 shows our final optimal design.
Figure 7. Isometric View of Preliminary Design.
We attempted to make our shell similar to how shells are found in nature, thus we
attempted to eliminate all sharp faces found on the old design. This shell should also
have a good volume-to-weight ratio and depth-to-aperture width ratio. This shell will be
stable because two of the crab’s legs and the shell will form a triangular base which is
very stable. This stability is called the tripod position, similar to how triangular stands
are used to hold cameras steady. We want the shell to be stable so the crab will not be
falling or rolling. A front view of the optimal design is shown in Figure 8.
10
Figure 8. Front View of Final Optimal Design.
This shell has a protective overhang to stop predators from reaching the crab easily. It
also has a grip inside of the shell for the crab to hold onto to carry the shell with it. The
bottom of the shell is curved to help reduce friction and to keep sand from piling up in
front of it. The curvature of the shell should also help to keep the water the hermit crab
needs from leaking out. Figure 9 shows a side view of the optimal design.
Figure 9. Side View of Final Optimal Design.
.
11
Material Selection and Manufacturing
Creating the shell with rapid-prototyping will be feasible for this project. Rapid-
prototyping gives the advantage of creating the final design with one piece which is
suitable for the hermit crab needs. The optimal shell shall be created by using plastic as
our primary material. There are several plastics and resins used in industry today,
however, for the purpose of the course, rapid-prototyping machines used at Rutgers
University are limited to some materials. The following are the materials used with the
rapid prototyping machines at Rutgers University:
Resin: Accura SI 40 ND
Material: Plastic PB 400 ABS MODEL
For the purpose of the course, we will use plastic as our material for the final
design. However, it would be an ecological disaster to go and spread many of these
shells on beaches around the world using this material. Environmental Regulations from
the EPA revoke the use of plastics as the primary material for the deployment of the
entire program.
As an alternative, biodegradable plastics can be use to promote the housing for the
hermit crab. Yet another problem which is beyond the scope of this course is that
Biodegradable materials are currently not usable for rapid prototyping. Biodegradable
materials are mainly done by molding.
There are several companies today that are currently developing biodegradable
plastics for their products. Reported by a Japanese society, Biodegradable Plastic
Society, biodegradable plastics are GreenPla plastics which can be used for various
applications. Their disposal consists of decomposing to water and carbon dioxide by an
action of microorganisms commonly existing in a natural environment. Thus, they return
to nature without harming the environment.
Below there is a table for different companies that are using this new technology.
Product Name Application Green (Pla) Company
Bag for terminal device Daily necessities PBSA Fujitsu
Mouse Pad Stationary PLA Office Media
Electronic Case Packaging material PLA Sony
Food Container Cup, ball, tray PLA Tohcello
Cellphone case Communications ****** Motorola
Table 1. Biodegradable Materials design applications.
Also, rapid-prototyping would not be used for mass production of shells. More research
must be done on biodegradable materials to try to find one suitable for creating the hermit
crab shells. The material must be strong enough and last long enough for the crab to
make use of it, but they can’t sit on beaches for hundreds of years either
12
Conclusions and Further Work
A shortage of available shells is a big problem facing the hermit crab. Without
adequate protection, this animal species will become extinct. To prevent this from
happening we must help to create a shell that can serve as the protection needed by the
crabs. To design this shell correctly, we needed to take into account the many parameters
that make a shell desirable to a hermit crab. These parameters include the ratio of depth
to aperture width, shell volume to weight ratio, shell size, aperture length, and shell
strength. We believe that our final preliminary design fills these parameters correctly and
will appeal to a homeless hermit crab. Since humans have caused the shell shortage
facing hermit crabs, we must try to help them survive.
The team will simulate predators attacking hermit crab shells. We want to know
what are the stresses and forces developed, assuming a canine as our predator. Also, a
person walking down the beach that accidentally steps on the crab’s shell shall be taken
into consideration. In order to do this, we have found a literature article paper related to
the jaw forces of several canines [9]. This paper will justify the input forces to be used
with ANSYS. We will use the same analogy for a person stepping on the shell
Finally, we want to give our most sincere thanks to our advisors, Professor Hao
Lin and Professor Elizabeth Demaray for taking the time during those meetings and
advising us on how to approach this project.
13
References
[1] D. Hahn, “Hermit crab shell use patterns: response to previous shell experience and to water
flow”, Journal of Experimental Marine Biology and Ecology, Vol. 228, pp. 35-51, 1998.
[2] M.J. Kaiser, H. Hinz, R.M. Callaway, A. Nall, C.L. Biles, “Resource degradation: a subtle
effect of bottom fishing”, Marine Biology, Vol. 146, pp. 401-408, 2005.
[3] A. Turra, F.P.P. Leite, “Shell-size selection by intertidal sympatric hermit crabs”, Marine
Biology, Vol. 145, pp. 251-257, 2004.
[4] C.F. Herreid, R.J. Full, “Locomotion of hermit crabs on beach and treadmill”, Journal of
Experimental Biology, Vol. 120, pp. 283-296, 1986.
[5] R.B. Garcia, F.L.M. Mantelatto, “Shell selection by the tropical hermit crab Calcinus tibicen
from Southern Brazil”, Journal of Experimental Marine Biology and Ecology, Vol. 265, pp. 1-14,
2001.
[6] C.M. Lively, “A Graphical Model for Shell-Species Selection by Hermit Crabs”, Ecology,
Vol. 69, pp. 1233-1238, 1988.
[7] J.E. Angel, “Effects of shell fit on the biology of the hermit crab Pagurus longicarpus (Say)”,
Journal of Experimental Marine Biology and Ecology, Vol. 243, pp. 169-184, 2000.
[8] J.A. Pechenik, J. Hsieh, S. Owara, P. Wong, D. Marshall, S. Untersee, W. Li, “Factors
selecting for avoidance of drilled shells by the hermit crab Pagurus longicarpus”, Journal of
Experimental Marine Biology and Ecology, Vol. 262, pp. 75-89, 2001.
[9] S. Wroe, C. McHenry, J. Thomaso, “Comparative bite force in big biting mammals
and the prediction of predatory behavior in fossil taxa”, Proceedings of the Royal
Society, Vol 272 pp. 619-625, 2005
14