To Match, or Not to Match: Does Substrate Reflectivity Influence the Choice of Shell Reflectivity by Pagurus maclaughlinae?

Caroline Capobianco (PI), Valerie Seadon (PI), Jamie Wagner, Brenda SwitzerDepartment of Biological Sciences, Florida Institute of Technology

Day 1• 88 shells paired up by size to create 44 pairs• One from each pair was painted white. The other was painted black.Day 2• Crabs removed from shells (Figure 3, 5)• Placed in tray with 2 shells each (Figure 4, 6)• Crabs were left for 24 hours to make their choicesDay 3• The selection of each crab was noted• Data was analyzed using the G-Test in SPSS 14.0• Frequency distribution was created using Excel 2003

Materials and MethodsFigure 8- “Naked” Pagurus maclaughlinae

Figure 3- Pliers being used to chipaway at apex of shell

Figure 5- Paint brush being used to entice crab out of shell

Figure 4- Hermit crabs and shells on black substrate

Figure 6- Hermit crabs and shells on white

substrate

Figure 1- Pagurus maclaughlinae in shell

G=0.876, df=2, P=0.645

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Shell Selection

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Figure 7- Frequency Distribution of Shell Selection by Pagurus maclaughlinae

Most species of hermit crab rely on empty gastropod shells to protect their soft, uncalcified abdomen from predation, dessication and physical stresses.6 The selection of a shell is very important because an adequate shell enhances growth, reproductive success and survivorship.5 Hermit crabs can use various characteristics of shells to evaluate the shells available to them such as shell species, shell size, and amount of shell damage.13 Finally, hermit crabs can consider the weight of the shell relative to their body weight and internal volume of the shell.8

Hermit crabs do not enter gastropod shells at random but select shells according to their species or other characteristics like those mentioned above.9 This shell preference can allow the coexistence of similar species because it may lead to habitat partitioning.12 There are two ways that crabs can develop preferences. First, the hermit crab can learn aspects of a particular shell by occupation and seek out those characteristics that were beneficial in the past.3 Second, during the period after ecdysis, the abdomen of the hermit crab can become molded to a particular shape by the shell in which it is occupying.3,11 Therefore the hermit crab could develop a preference for the shell shape to which its body is molded. Hermit crabs are able to determine the color of the substrate and the color or reflectivity of the shell.7 Reflectivity refers to the amount of light reflected from the surface. For example, white reflects more light than black so the reflectivity is higher. Shell selection varies with background coloration.8 Shells similar in color to the substrate can help camouflage the crabs from predators.7 It has been shown that predation pressure is an important selective pressure on hermit crabs.1

Introduction

Photo by: Sarah Rhodes Photo by: Sarah Rhodes

•More crabs chose white shells on the white substrate•Crabs chose about 50/50 on the black substrate •Nine crabs did not choose a shell•Two shell pairs were thrown out because one crab died, and the other shell pair had two crabs

Results

Partridge (1980) determined that Pagurus hirsutiusculus showed a preference for shells that blend in with its background. However, in our study background reflectivity did not influence the selection of shell reflectivity in Pagurus maclaughlinae. Hermit crabs may have needed more than 24 hours to select a shell, since 20% of the crabs did not choose a shell. The rationale for this study stated that that the hermit crabs would select a shell based on camouflage to avoid predation. Since there were no predators present, the hermit crabs did not have to base their choice on predator avoidance. The importance of shell characteristics in shell selection vary according to the species of hermit crab.2 Shell reflectivity may not be important to Pagurus maclaughlinae in its natural environment if the crab has a mechanism to escape predators or if there is not a significant predation pressure on them.Pagurus maclaughlinae may have based its selection on other parameters that were no kept constant in this experiment. For example, the shells used were not weighed but the sizes were kept relatively constant. Size is an important characteristic in selection of a shell.13 Pagurus longicarpus avoids relatively large shells more than small shells.13 The hermit crabs that were collected were all relatively small for the size shells that were offered to them. If they based their selection on size, there would be no preference for either shell since the shells in each pair were relatively the same size. The majority of Pagurus maclaughlinae that did not choose shells were ovigerous. It was noted that ovigerous Pagurus maclaughlinae females used more Cerithium muscarum shells than Nassarius vibex shells, which was the species that was offered to them. 10 It is possible that they were operating their selection based on shell species rather that shell reflectivity. Improvements that could be made to this study include: increasing selection time to 48 hours instead of 24 hours, using a natural predator to influence selection based on predator avoidance, conducting the study in the field instead of in the lab, and using a natural substrate such as black or white sand instead of construction paper placed underneath a clear tackle box.

Discussion

References1. Bertness MD (1981). Predation, physical stress, and the organization of a tropical rocky intertidal hermit crab community. Ecology. 62(2): 411-4232. Conover MR (1978). The importance of various shell characteristics to shell selection behavior. Journal of Experimental Marine Biology and

Ecology. 32: 131-1423. Elwood RW, McClean A, Webb L (1979). Development of shell preferences by the hermit crab Pagurus bernhardus. Animal Behavior. 27: 940-9464. Gravel BE, Wong PY, Starks, PT, Pechenik JA (2004). The use of artificial shell for exploring shell preference in the marine hermit crab Pagurus

longicarpus. Anneles Zoologici Fennici. 41: 477-4855. Hazlett BA (1981). The behavioral ecology of hermit crabs, Annual Review of Ecological Systematics. 12: 1-226. Hazlett, BA (1992). The effect of past experience on the size of shells selected by hermit crabs. Animal Behavior. 44: 203-2057. Partridge BL (1980). Background Camouflage: an additional parameter in hermit crab shell selection and subsequent behavior. Bulletin of Marine

Science. 30: 914-9168. Reese ES (1962). Shell selection behavior of hermit crabs. Animal Behavior. 10: 347-3609. Sato M and Jensen GC (2005). Shell selection by the hermit crab, Pagurus hartae (Mclaughlin and Jensen, 1996) (Decapoda, Anomura).

Crustaceana. 78: 755-76010. Tunberg BG, Nelson WG, Smith G (1994). Population ecology of Pagurus maclaughlinae Garcia-Gomez (Decapoda, Anomura, Paguridae) in the

Indian River Lagoon, Florida. Journal of Crustacean Biology. 14(4): 686-69911. Turra A and Leite FPP (2003). The molding hypothesis: linking shell use with hermit crab growth, morphology, and shell-species selection. Marine

Ecology Progress Series. 265: 155-16312. Vance RR (1972). The role of shell adequacy in behavioral interactions involving hermit crabs. Ecology. 53(6): 1075-108313. Wilber TP (1990). Influence of size, species and damage on shell selection by the hermit crab Pagurus longicarpus. Marine Biology. 104: 31-39

•Objective: To determine if the reflectivity of the substrate influences the selection of shell reflectivity in the hermit crab Pagurus maclaughlinae.

•Rationale: The selection of a shell that matches the background will serve as camouflage and aid in predator avoidance.

•Hypothesis: Pagurus maclaughlinae will choose low reflectance shells on a low reflectance substrate and high reflectance shells on a high reflectance substrate.

•Independent Variables: Substrate Reflectivity and Shell Reflectivity•Dependent Variable: Shell Choice

Objective and Hypothesis

Figure 2- Pineda Causeway, collection site for Pagurus maclaughlinae.

Photo by: Sarah RhodesPhoto by: Sarah RhodesPhoto by: Sarah Rhodes

Pagurus maclaughlinae