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 Switzer
Department of Biological Sciences, Florida Institute of Technology
Photo by: Sarah Rhodes
Figure 1- Pagurus maclaughlinae in shell
Introduction
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
Materials and Methods
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 choices
Day 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
Figure 3- Pliers being used to chip
away 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
Photo by: Sarah Rhodes
Figure 8- “Naked” Pagurus maclaughlinae
Discussion
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.
Results
Figure 2- Pineda Causeway, collection site for Pagurus maclaughlinae.
G=0.876, df=2, P=0.645
14
12
•Objective: To determine if the reflectivity of the substrate influences the
selection of shell reflectivity in the hermit crab Pagurus maclaughlinae.
10
Frequency
Objective and Hypothesis
Photo by: Sarah Rhodes
8
white
black
6
References
2
0
white
•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
Photo by: Sarah Rhodes
Pagurus maclaughlinae
4
•Rationale: The selection of a shell that matches the background will
serve as camouflage and aid in predator avoidance.
Photo by: Sarah Rhodes
black
no choice
Shell Selection
Figure 7- Frequency Distribution of Shell Selection
by Pagurus maclaughlinae
•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
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