Shell Switches Hermit Crabs moved into natural shells more than painted shells (Fig. 3a: χ2 = 4.829, df = 1, p = 0.028). Typically, hermit crabs that switched shells during the experiment switched into natural shells as their final shell switch (Fig. 3b: χ2 = 7.2, df = 1, p = 0.007). In addition, animals trained in natural shells were just as likely to move into an empty natural shell as animals trained initially in painted shells ( Fig. 4: G = 0.001, df = 1, p = 0.99).

Shell Assessment Hermit crabs also assessed natural shells more often than painted shells (Fig. 5a: t = 3.531, df = 57, p = 0.001). Hermit crabs trained in painted shells assessed more shells than crabs trained in natural shells (Fig. 5b: t = 2.711, df = 62, p = 0.009). Finally, hermit crabs trained in natural shells assessed a higher proportion of natural shells than hermit crabs trained in painted shells (Figure 5c: t = 2.208, df = 56, p = 0.031).

Summary Hermit crabs appear to show a preference for natural shells. When given a choice, hermit crabs switched into natural shells and seemed to avoid painted shells. A hermit crab’s preference for natural shells appeared to also affect the way the animal assessed the different available shells. Hermit crabs flipped (our indicator of assessment) more natural shells than painted shells. Hermit crabs that were trained in painted shells flipped more shells than hermit crabs that were trained in natural shells. This suggests that hermit crabs in painted shells may have wanted to switch more readily than those in natural shells.

Further research should be conducted to determine why hermit crabs have a preference for natural shells.

Short Term Study Detrimental effects such as mortality should be researched further to see if those hermit crabs carrying painted shells have a higher mortality rate than those that carry natural shells.

Long Term Study Sexual selection may also play a role in shell use and should be further studied. Female hermit crabs may prefer natural colored shells and therefore the male hermit crabs are choosing to carry natural shells as opposed to painted ones.

Diaz, H.; Orihuela, B.; Rittschof, D.; & Forward Jr., R. B. 1995. Visual orientation to gastropod shells by chemically stimulated hermit crabs, Clibanarius vittatus (BOSC). Journal of Crustacean Biology, 15(1), 70-78.

Gherardi, F. 2005. Fighting Behavior in Hermit Crabs: The Combined Effect of Resource-Holding Potential and Resource Value in Pagurus longicarpus. Behavioral Ecology and Sociobiology, 59(4), 500-510.

McClintock, T. S. 1985. Effects of shell condition and size upon the shell choice behavior of a hermit crab. Journal of Experimental Marine Biology and Ecology, 88(3), 271-285.

Tricarico, E. & Gherardi, F. 2007. Resource Assessment in Hermit Crabs: The Worth of Their Own Shell. Behavioral Ecology, 18(3), 615-620.

We would like to thank the Animal Behavior Research Group at FGCU for great input in the process of this research and presentation and the FGCU Biology Department for the lab space.

Figure 1

6 Empty Shells

2 Hermit Crabs

Shell type is critical to hermit crabs because the shell provides the crab shelter with protection. Hermit crabs assess empty shells by comparing the resource value of the shell that they occupy presently to the resource value of other shells that they encounter (Tricarico and Gherardi 2007; Gherardi 2005). Hermit crabs in the pet trade are exposed to unusual conditions, such as painted shells, which may affect the resource value of different shells and welfare. In this study, we will determine whether hermit crabs assess painted and natural (i.e. unpainted) shells differently. To determine how hermit crabs evaluate painted and natural shells, hermit crabs carrying natural and painted shells were placed into separate aquaria with natural-empty shells and painted-empty shells. Hermit crabs preferred natural shells to painted shells. Hermit crabs were more likely to move into empty-natural shells than empty-painted shells. In addition, hermit crabs trained in painted shells assessed a higher proportion of empty shells than hermit crabs trained in natural shells. A hermit crab’s shell also affected the type of empty shell assessed: hermit crabs in natural shells were more likely to flip empty-natural shells than empty-painted shells, while hermit crabs in painted shells flipped empty-natural or -painted shells equally. Since hermit crabs prefer natural shells to painted shells the pet trade should discontinue the use of painted shells. However, further research is needed to determine if there are any detrimental effects of painted shells to hermit crabs beyond preference.

Cosmetic Procedures in Companion Animals Many cosmetic alterations have clear deleterious effects to companion animals. By identifying the animal’s preference we might be able to determine how the companion animal evaluates the costs and benefits of the cosmetic procedure to their physiology, morphology, and behavior.

Hermit Crab Shell Preference Hermit crabs are expected to show strong preferences among different shell types because of the effect that shells have on individual crabs. The shell provides shelter and protection for the crab. Hermit crabs might be expected to move to a new shell when the quality of the empty shell is greater than their present shell. Hermit crabs assess a variety of characteristics when comparing alternative shells, including the condition (e.g. cracked or chipped), mass, shape, and size of the shell (McClintock 1985; Diaz et al. 1995). Shell assessment and selection of available shells should have a strong effect on the welfare of hermit crabs.

Hermit Crabs Response to Painted Shells Many hermit crabs in the pet trade are sold with paint-covered shells that vary in color and design for cosmetic purposes. It remains unclear how hermit crabs respond to the shells used in the pet trade. In this study, we determined whether hermit crabs assess painted and natural (i.e. unpainted) shells differently. This test helps us determine how hermit crabs evaluate the cosmetically altered shells used in the pet trade. We expected the hermit crabs to avoid painted shells if painted shells adversely affect the welfare of the animal.

Figure 1: The hermit crabs were acclimated to their present shell type during an initiation period that lasted 1 – 3 weeks (Fig. 1). During this training, 50 hermit crabs were placed into one of two 18.9 liter aquariums with similarly treated hermit crabs: 25 hermit crabs with natural shells were placed into one aquarium with the other natural shell hermit crabs and 25 hermit crabs with painted shells were placed into one aquarium with the other painted shell hermit crabs.

Figure 2: There were eighteen replicates for each of two treatments. Each replicate consisted of a 9.5-liter aquarium that contains two hermit crabs and six empty Turbo argyrostoma shells (Fig. 1). One treatment contained two hermit crabs carrying natural shells and the other treatment contained two hermit crabs carrying painted shells. The hermit crabs then had the option of switching to six empty shells, three of the shells being painted and three of the shells being natural (Fig. 2a & 2b).