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Probing the evolutionary biomechanics of elastic energy storage in mantis shrimp.
The striking appendages of mantis shrimp are capable of predatory strikes faster than 20
m/s with impact forces over 400 N. Though these fast and powerful strikes occur outside the
range of muscle contraction, mantis shrimp are able to generate such movements by slowly
loading energy into a biological spring and releasing that energy in very short amounts of time.
In November of 2008, I started my research experience as an undergraduate at UC Berkeley with
professor Sheila Patek by performing a series of materials tests to reveal any patterns in the
evolutionary history of elastic energy storage systems. We took our first steps towards
understanding the evolution of springs by assessing any changes in spring properties across
different species of mantis shrimp. Specifically, we were interested in 2 aspects of the spring
properties. First of all, how do the springs change in both length and stiffness within species as
mantis shrimp grow larger? Then, after determining these relationships, how do these patterns
vary across different species?
Previous research on the a single mantis shrimp species suggested that energy storage is
possible due to a highly mineralized structure known as the ventral bars. The first step then
became to assess whether or not these ventral bars were conserved throughout the phylogeny of
mantis shrimp. In order to address this, micro computed tomography (micro-CT) scans were
created from voucher specimens of all animals in our study. With these high resolution microCT scans, we were able to reconstruct a 3 dimensional representation of each appendage and
determine whether or not these calcified structures were present. After analyzing the scans of all
10 species of our study, we determined that the ventral bars were present throughout the entire
evolutionary history of mantis shrimp.
Once we determined the presence of the ventral bars in all of the species of mantis shrimp
in our study, collection of data to determine spring properties began. In order to measure and
calculate spring properties such as maximum force production and spring stiffness, mantis
shrimp appendages were dissected and mounted to a materials testing machine. Collecting data
at 250 Hz, we were able to simulate spring compression with the materials testing machine and
record force as a function of compression distance. With this data, we were able to calculate the
maximum force, stiffness of the spring, and the work done during compression.
When looking across all species, there was a clear pattern that as a spring becomes longer
in length, it is capable of storing more elastic energy. However, when looking at spring stiffness,
the results were not as straight forward. When looking across species, there was no significant
relationship between stiffness and spring size. Even after performing a phylogenetic
independent contract to account for possible similarities due to relatedness of different species, a
significant pattern did not emerge. These results support the previous study on mantis shrimp
elastic energy storage which demonstrated that across the size range, the spring stiffness of
Gonodactylus falcatus remained constant. This suggests that perhaps as animals become larger,
the mechanism of increasing force production is not compressing a spring that is more stiff, but
rather compressing the same spring a further distance.
Hydrodynamic benefits of snail fish curling.
Animals that live in turbulent fluid environments expend an enormous amount of energy
to remain stationary. This becomes problematic for animals such as some species of snail fish
which latch onto craps and parasitize their bodies with their eggs. One strategy of snail fish
which reduces the energy required to remain stationary involves the modification of the pelvic
fins into suction cups. However, in order to reduce their energy expenditure even more, I
proposed that a whole-body curling behavior might also help decrease drag. While attending the
functional morphology and ecology of fish class at the Friday Harbor Laboratories of the
University of Washington in the summer of 2010, I observed this curling behavior in snail fish.
As the snail fish swam around, they would eventually attach themselves to a surface and wrap
their tails around the anterior part of their body.
I began the project by studying the behavior of the fish in a flow tank in order to observe
data such as relevant flow speeds and orientations of the snail fish. After allowing the snail fish
to acclimate in the flow tank for half an hour, I determined the minimum speed which the snail
fish would no longer maintain a curled position. At a flow speed of 33.84 cm/s, the snail fish
would detach from the bottom of the flow tank and swim against the direction of flow. I
measured the minimum flow speed of the tank (21.89 cm/s) and determined an intermediate flow
speed of 25.44 cm/s which fell in between these two measure flow speeds.
Additionally, I tested the behavior of snail fish to different orientations of flow. By
forcing the snail fish to adhere to a plexiglass plate, I was able to manipulate the orientation of
the fish in relationship to the direction of water flow in the flume. I allowed the specimens to
acclimate in the flow tank and initiate a curling posture before attempting to rotate the plexiglass
plate. In the majority of my trials, while rotating the plate 360 degrees, I observed the snail fish
reconfiguring its posture by wrapping its tail around the body with the direction of the flow
stream.
In order to determine whether or not this change in posture could confer a hydrodynamic
benefit to the snail fish within this regime of flow speeds, I created models, both in a straight
posture as well as a curled posture. After creating casts of snail fishes using dental wax, I was
able to inject these casts with resin and create models with anatomically accurate shape and size.
By attaching these models to a strain gage and place these models into the flow tank, I was able
to calculate the coefficients of drag for many permutations of flow speeds, flow orientation, and
posture. With this data, I was able to discern some trends about the effect of reconfiguring
morphology on drag.
As expected, the drag was lowest when the snail fish was straight and positioned exactly
against the flow. However, for any given flow speed, there was a significant reduction in drag
when the curled model was positioned so that the tail curled in the direction of the flow. Given
that the snail fish were observed to physically configure their tails into this orientation at
intermediate flow speeds, it is likely that that this curling behavior can be used by the animal to
reduce the amount of drag experienced in some situations.
Publications:
Rosario, M.V., Taylor, J. R. A., and Patek, S. N. Probing the evolutionary biomechanics of
elastic energy storage in mantis shrimp. Presented at the 2010 Annual Meeting for the Society
for Integrative and Comparative Biology.
Rosario, M.V., Taylor, J. R. A., and Patek, S. N. Probing the evolutionary biomechanics of
elastic energy storage in mantis shrimp. (In prep.)