Role of Shell Size in the Shell Selection Behavior of Hermit Crabs

(Coenobita clypeatus)

Staci Both, Sara Reszczynski, Erica Steen, and Emily Vogel

Spring 2007

BI 441


Abstract

††††††††††† Six land hermit crabs (Coenobita clypeatus) were tested to determine shell preference when presented with multiple empty shells. Each crab was given a choice of three shells varying in size and observed every 30 minutes over the course of a 120 minute trial.Two of the six crabs successfully changed shells during the trials.We hypothesized that if the crabs chose to exchange shells, they would move into a larger shell (allows for protection from predators and room to grow). We found that the two crabs that exchanged shells chose to move into a smaller shell rather than a larger one.However this was not a significant difference (p=.23).†††

Keywords: shell selection, Coenobita, shell size

Introduction

One of the unique characteristics of hermit crabs (Pagurus) is that they carry their home on their back, allowing them to have a mobile protection. According to Conover (1978), this distinctive form of habitat selection is one of the main reasons for the evolutionary success of hermit crabs. Their high level of joined mobility and protection allow them to live in almost all marine environments (Hazlett, 1981). The shells that hermit crabs use are from gastropods and they are acquired either by attending predation events and taking a shell as soon as the gastropod is eaten, or by finding an empty shell. In most cases, shells are in limited supply, which would require the hermit crabs to fight for a shell or settle for using another object as temporary protection. Hermit crabs will improvise and use broken or damaged shells, sponges and even pieces of bamboo if they can not find a high quality shell (Weir, 2001). Without a shell, a hermit crab is more vulnerable to predators and it can also die from desiccation or osmotic stress. Shell selection plays an important role throughout the hermit crabís life because it constantly grows and has to change shells (Rotjan et al, 2004). The hermit crab is in a constant battle, however, because even if it is able to find a good shell that protects from predation and allows room for growth, it still has a chance of losing that shell in a battle with another hermit crab (Vance, 1972).

Shell selection is a complex process revolving around many different shell characteristics including shell weight, shell volume, and shell size. As mentioned above, hermit crabs will occupy a damaged shell if there are no other options, but they spend more time investigating intact shells than damaged shells (Rotjan et al, 2004). The choice of shell will, in turn, affect the hermit crabís growth, clutch size, and survival (McClintock, 1985).There are positives and negatives to each shell choice. For example, larger shells reduce vulnerability to predators and allow the hermit crab more room to grow. If shell supply is limited, a crab will choose a larger shell so that it has room for growth and doesnít need to worry about finding another shell (Hazlett, 1981). However, the cost of having a larger shell is that it may be heavier, thus making it harder to carry. Energy expended on carrying the shell takes away from energy needed to get food or reproduce. Hermit crabs are not the only organisms that inhabit gastropod shells. As epifauna accumulates on top of the shell, the shell becomes heavier. If this is the case, crabs in these shells will compensate by inhabiting smaller and lighter shells (Conover, 1978).

In contrast, smaller shells are easier to carry and would allow greater mobility. However, if a shell is too small, it will inhibit the hermit crab from growing. Another cost of having an ill-fitting shell is that there is a greater risk of predation; it is easier for predators to reach the hermit crab. The overall fitness of a hermit crab is linked to shell utilization (Fotheringham, 1976).

The hermit crabs used in this experiment were purple pincher hermit crabs (Coenobita clypeatus). It is our hypothesis that with a limited number of shells to choose from, the hermit crabs will choose the larger shell because it allows more room to grow and will better protect the hermit crab against predators.

Methods and Materials

††††††††††† On Saturday, March 31st six land hermit crabs (Coenobita clypeatus) were purchased from Pet Supplies Plus (4505 S. 76th Street, Greenfield, WI 53220).This species of hermit crab is also commonly referred to as the Purple Pincher or Caribbean Crab.The six hermit crabs were placed into a standard 76 liter glass aquarium tank, with Zoo Med Laboratories Inc.TM Hermit Crab Sand substrate (1.3 centimeters deep).Two Zoo Med Laboratories Inc.TM Hermit Crab Bowls were placed in center of tank- one with food and the other with water (hermit crabs given ad libitum access to food and water).The hermit crabs were fed Zoo Med Laboratories Inc.TM Zoo Menu Hermit Crab Food Pellets and Soft Canned Hermit Crab Food.The water was treated with Zoo Med Laboratories Inc.TM Hermit Crab Drinking Water Conditioner as directed.Food and water was changed daily (mid-day)The tank was misted with water using a SpraycoTM 250 milliliter squirt bottle approximately four times a day to maintain humidity in the environment.A second standard 76 liter glass aquarium tank was set up for experiments, using the Zoo Med Laboratories Inc.TM Hermit Crab Sand substrate (1.3 centimeters deep). This experimental tank was also misted with water to maintain humidity, but contained no food or water.The experimental tank was also divided into three equal parts width-wise using cardboard dividers (which allowed for three individuals to be tested during a trial).After the hermit crabs were placed in the first/holding tank they were allowed to habituate to the new environment for about 4.5 hours.

††††††††††† We obtained 16 empty shells varying in sizes.The shells were weighed, measured (diameter of opening), photographed (See Appendix A), and then the photo was labeled with a letter for later identification (See Table 1 for empty shell info) .Hermit crabs were also photographed and weighed in their original shells, and given a number (on the photo) for identification (See Appendix B).Once all six hermit crabs and empty shells were identified and photographed the data collection was started.

Data were collected over the course of three days, roughly between 2000 hours and 0500 hours.Day one consisted of giving each hermit crab a choice between three shells: small, medium, large shell (chosen in comparison to their original shell).Hermit crabs 1, 2, and 3 were placed in the experimental tank in separate chambers, and then three empty shells were placed in the middle of the each of the three chambers.The experimental tank was kept in a dark room (hermit crabs are nocturnal), and the door was shut to keep out noise and other distractions which could disturb the hermit crabsí shell changing behavior.Every 30 minutes, for the entire 120 minute trial, the hermit crabs were observed to see if any shell changing behavior occurred.If a shell change occurred (in one of the early observation times) it was recorded, but the crab was left in experimental tank until the end of the 120 minute trial as to not disturb the other hermit crabs.After the 120 minute trial, the hermit crabs that did not change shells were placed back in the holding tank.If a crab changed shells, we weighed the crab in its new shell (new shell weight logged earlier-one of empty shells) and weighed the original shell (that the crab left behind) to determine how much the hermit crab weighs without a shell.The results were recorded and all empty shells were removed from the experimental tank and prepared for the next group of three hermit crabs.Hermit crabs 4, 5, and 6 were then placed in experimental tank chambers, and each given a choice of three empty shells.The 120 minute trial was conducted in the same manner as for hermit crabs 1, 2 and 3.The data was recorded and day one was complete.

Day two consisted of the same procedure as day one, except that a different assortment of three empty shells were used for each hermit crab.Crabs 4, 5, and 6 were tested in the first 120 minute trial on day two, and hermit crabs 1, 2, and 3 were then tested in the second 120 minute trial.This allowed the time of experiment to not be a factor in the results (earlier tested= less/more active).See Table 1 to determine shells available to each hermit crab during day two trials.

Day three was conducted to determine if the presence of conspecifics and the abundance of empty shells would cause shell changing behavior.All six hermit crabs were placed back into holding tank together.All of the remaining empty shells were then placed into the holding tank containing all six hermit crabs.Observers sat 2.5 meters away from the tank and observed the hermit crabís behavior for 120 minutes (Nightlight used as source of light to view tank).Any observations were recorded by observers.Observers decided that even after the 120 minute trial, they would leave the empty shells in the holding tank overnight.This would show if the hermit crabs would change shells if given more time.

The data was all entered into a Microsoft Excel spreadsheet and then analyzed using a paired two tailed T-test.

 

Table 1.Empty shells and how they were classified in Days 1 and 2 (three shells presented to hermit crabs)

Shell

Mass (grams)

Diameter of opening (cm)

Crabs 1, 2, & 4 (larger)

Crabs 3, 5, & 6 (smaller)

A

9.8

3.7

Large

N/A

B

17.9

3.9

Large

N/A

C

25.1

2.9

Medium

N/A

D

13.5

2.4

Medium

Large

E

17.7

2.3

Medium

Large

F

17.5

1.9

Small

Medium

G

17.4

1.7

Small

Medium

H

11.6

1.9

Small

Small

I

5.8

1.7

N/A

Small

J

2.5

1.8

N/A

Small

K

3

1.4

N/A

Medium

L

5.1

1.3

N/A

Large

M

2.3

1.3

N/A

N/A

N

2.1

1.3

N/A

N/A

O

1.9

1.4

N/A

N/A

P

1.6

1.2

N/A

N/A

Results

Only two of the six hermit crabs exchanged shells (crabs 2 and 4). During the first trial, these crabs chose shells that were larger than the original with an average difference in the mass of shells being 5.95 g for the first trial.During the second trial, the same two hermit crabs exchanged their shells for ones that weighed less than their first choice with an average difference between original shell mass and final shell mass being 0.9 g.These results were not significant as the p-value was 0.23 on a scale where .05 marks significance.See figures 1 and 2.

Crab

Hermit + original shell (g)

hermit w/o shell (g)

Trial

Shell Choices

Time to change (minutes)

DAY 1

 

 

 

 

 

1

10.2

 

1a

B, D, and H

No Shell Change (120 min trial)

2

22.6

6.7

1a

A, C, and F

< 30 (chose shell F)

3

7.2

 

1a

E, G, and I

No Shell Change (120 min trial)

4

13.7

6.3

1b

B, C, and E

> 90, <120 minutes (chose shell E)

5

4.4

 

1b

I, G, andP

No Shell Change (120 min trial)

6

2.5

 

1b

J, H, and O

No Shell Change (120 min trial)

 

 

 

 

 

 

DAY 2

 

 

 

 

 

1

10.2

 

2b

A, C,and G

No Shell Change (120 min trial)

2

N/A

6.7

2b

B, D, and H

No Shell Change (120 min trial)

3

7.2

 

2b

J, K, and L

No Shell Change (120 min trial)

4

N/A

6.3

2a

A, D, and H

No Shell Change (120 min trial)

5

4.4

 

2a

J, H, and N

No Shell Change (120 min trial)

6

2.5

 

2a

M, P, and I

No Shell Change (120 min trial)

 

 

 

 

 

 

DAY 3

 

 

 

 

 

1

10.2

 

 

ALL AVAILABLE

 

2

N/A

6.7

 

ALL AVAILABLE

>60, <90 (chose shell D)

3

7.2

 

 

ALL AVAILABLE

 

4

N/A

6.3

 

ALL AVAILABLE

>90, <120 (chose shell H)

5

4.4

 

 

ALL AVAILABLE

 

6

2.5

 

 

ALL AVAILABLE

 

 

 

 

Figure 1.Hermit Crab 2: Shell changing in relation to mass of shell (grams), over three trials

 

 

Figure 2.Hermit Crab 4: Shell changing in relation to mass of shell (grams), over three trials.

Discussion

Our hypothesis that hermit crabs would prefer a larger shell was not significantly supported by our data.We only had two hermit crabs that actually changed their shells and after the second shell change, they inhabited shells that were smaller than their original shell.Hermit crabs have been documented to choose shells that allow them to fully retreat into when in the presence of a predator, or in the case of reproductive females, enough room to hold eggs.It has not been precisely documented what is the deciding factor in hermit crab shell choice, although volume, mass, and opening width are all considered when choosing a shell (Lively, 1988).

While the crabs were offered a variety of shells, the exact reason they chose the shell they did is unclear based off of the data we collected.It can be argued that the reason why the crabs that did initiate a shell change chose larger shells the first time but reverted back to smaller shells with the second exchange may have been because the larger shell weighed too much and the energy cost was too great. In the case of crab 2, the second shell was significantly smaller than the original shell it was purchased in. According to Hazlett (1981), the mass of a shell is the conclusive factor in hermit crab shell selection. In both cases of shell exchange in our experiment, the first choice was exchanged for a lighter shell.

If repeating this experiment, both a larger sample size and more time between trials would be beneficial. If there was more time in between trials, perhaps the hermit crabs would have habituated to the heavier shell and rejected a second exchange.It would also be intriguing to observe shell selection behavior of reproducing female crabs and how their shell requirements would differ from those of males. According to Hazlett (1981), male hermit crabs are larger than females in most species, but the shell preference is supposedly similar. However, since larger shells allow a bigger clutch size, it could be hypothesized that female hermit crabs would focus more on volume of shell than weight.

While we didnít achieve the results we expected, we were able to gain insight into hermit crab shell selection behavior.While most crabs didnít even bother to participate in the trials, the two that did react differently than expected; as is to be anticipated when working with animals. If we were to repeat this experiment, both sample size and/or the type of shells we offered as choices might incur a different result.

 

Literature Cited

Bertness, M.D. (1981). Conflicting advantages in resource utilization: the hermit crab housing dilemma. The American Naturalist, 118 [3], 432-437. Retrieved February 18, 2007 from JSTOR database.

Conover, M.R. (1978). The importance of various shell characteristics to the shell-selection behavior of hermit crabs. Journal of Experimental Marine Biology Ecology, 32, 131-142.

Fotheringham, N. (1976). Population consequences of shell utilization by hermit crabs. Ecology, 57 [3], 570-578. Retrieved January 26, 2007 from JSTOR database.

Hazlett, B.A. (1981). The behavioral ecology of hermit crabs. Annual Review Ecol. Syst, 12, 1-22.

Lively, C. M. (1988). A graphical model for shell-species selection by hermit crabs. Ecology, 69 [4], 1233-1238. Retrieved January 26, 2007 from JSTOR database.

McClintock, T.S. (1985). Effects of shell condition and size upon the shell choice behavior of a hermit crab. Journal of Experimental Marine Biology Ecology, 88, 271-285.

Rotjan, R.D., Blum, J., & Lewis, S.M. (2004). Shell choice in Pagurus longicarpus hermit crabs: does predation threat influence shell selection behavior? Behavioral Ecology Sociobiology, 56, 171-176.

Vance, R. R. (1972). The role of shell adequacy in behavioral interactions involving hermit crabs. Ecology, 53 [6], 1075-1083. Retrieved February 14, 2007 from JSTOR database.

Weir, K.L. (2001). Home sweet fossil home. Hermit crabs use of fossilized marine snail shells. Natural History, 110, 22-23. Retrieved February 2, 2007 from Thomson Gale database.