Download The scent of a predator: the effects of chemical threats on shell

The scent of a predator: the effects of chemical threats on shell selection in
the hermit crabs, Pagurus samuelis and Pagurus hirsutiusculus
1Department
Joanelle Hernández1 & Eric Sanford2
of Biology, Brown University; 2Section of Evolution and Ecology, University of California, Davis
• Predators can affect prey by: 1) reducing the prey population through consumption and 2) inducing phenotypic changes in their prey.
• Changes arising in prey populations from chemical predator threats can be morphological, physiological, behavioral or life-historical.
• Indirect or chemically signaled trait-mediated impacts have been studied less. However, their implications are all-encompassing within
ecological communities because their effects are transferred across other species which come into contact with the prey.
• Hermit crabs, because they exhibit complex behaviors and respond to chemical cues, are an ideal model to study trait-mediated
behavioral plasticity. Shell selection in hermit crabs is a compromise between shell traits and different aspects of fitness. For example,
hermit crab growth, survival, and brood size depend on the characteristics of chosen shell species. In Horseshoe Cove, two Pagurid
species occupy up to 12 different species of gastropod shells.
1) Do hermit crabs respond to predation threat by selecting shells that are better at protecting them from
their predators?
2) Do hermit crabs differentiate between, and respond to, chemical signals across multiple phyla of
predators?
3) Do Pagurus hirsutiusculus and Pagurus samuelis respond differently to these cues?
Pycnopodia helianthoides
Cancer antennarius
• Field surveys of tidepools in Horseshoe Cove quantified patterns of shell
use.
In each laboratory trial, individual hermit crabs were given a choice of two species
of gastropod shells matched as closely as possible in internal volume. Hermit crabs
were exposed to predator effluent (n=6), either C. antennarius (Cancer crab) or P.
helianthoides (sea star), or a no-effluent control (n=6). Naked hermit crabs were
placed in a culture dish halfway between the two shells, observations were taken
for the first five minutes, and time to first shell choice was noted (latency). Shell
selection was recorded at fifteen minutes, two and six hours.
• For each species, crabs were grouped into small (shell length less than
12mm) and large (shell length greater than 12mm) categories.
Small
Littorine
Nucella
Tegula
Other
Predation
Pagurus hirsutiusculus
Pagurus samuelis
150
120
Effluent
Control
A
90
AB
60
30
B
B
n=12
Latency (seconds)
Latency (seconds)
150
Effluent
Control
120
N.S.
90
?
?
Pagurus samuelis
Cancer
30
P.hirsutiusculus
n=12
Cancer
Pycnopodia
1) Only Pagurus samuelis showed sensitivity to Cancer effluent. Still, they did
not select the thicker Nucella lamellosa shell that may offer more crab
protection.
Pycnopodia
Predator Treatment
Predator Treatment
• Pagurus samuelis responded to Cancer effluent and took longer to make the first
shell choice. Individuals exposed to the sea star Pycnopodia selected shells faster,
although this trend was not significant.
Shell Choice
Shell combinations
P. samuelis P. hirsutiusculus
Cancer
Pycnopodia
(Effluent and (Effluent and
control)
control)
TF vs NO
NO vs TF
TF vs NL
NO vs NL
TF vs NO
NO vs TF
TF vs NL
NO vs NL
Studies suggest that Pycnopodia is able to give off an active or inactive
chemical cue. Further experiments are needed to test how these states affect
hermit crabs’ ability to sense these predators.
Legend
TF= Tegula funebralis
NO= Nucella ostrina
Activity level or frequency of shell switching may better indicate sensitivity to
predator cues. Alternatively, hermit crab species may differ in their ability to
detect predators.
2) Although there was a trend in Pagurus spp. response to Pycnopodia cues, it
was not statistically significant. This suggests that hermit crabs may be
unable to detect some predators.
• Pagurus hirsutiusculus’ time to first shell choice did not differ between individuals
exposed to predator effluent and control water in either predator treatment.
= shell
selected
Pagurus hirsutiusculus
Large
60
0
0
• Large P.samuelis is
most commonly in Tegula,
whereas P.hirsutiusculus
often use Nucella
Pagurus samuelis
Large
Small
Tegula funebralis
Nucella ostrina Nucella lamellosa
NL= Nucella lamellosa
Acknowledgments: This research was supported by grant
#DBI-0453251 to S.L. Williams from the National Science
Foundation. Thank you to the BML community and all people
who contributed to this project.
• Choice made after 15 min seldom changed after 2 hrs or 6 hrs.
• P. samuelis chose Tegula funebralis, regardless of other shell options or
presence of predator cues.
References:
Bertness, M. D. 1981. The American Naturalist 118, 432.
Preisser, E. L., et al. 2005. Ecology 86, 501.
• P. hirsutiusculus chose Nucella ostrina shells.
McClintock. 1985. J. Exp. Mar. Biol. Ecol. 88, 271.
Rotjan, R. et al. 2004. Behav. Ecol. Sociobiol 56,
171.
3) Shell preference was species-specific: 100% of Pagurus samuelis chose
Tegula funebralis while 100% of P. hirsutiusculus picked Nucella ostrina. Shell
selection in the laboratory matched patterns seen in the field.
Shell choice may reflect trade-offs among crushing resistance, shell weight
influence on escape speed, and room for embryo brooding. Considering
these factors together could explain crabs’ failure to pick a thicker shell.
Although hermit crabs appear to respond to predator cues in some cases (by
changing speed of shell selection), they appear remarkably consistent in their
shell preferences. These choices may reflect the optimal balance of multiple
costs and benefits conferred by different shell types.