TULIPANI, D.C.; BOUDRIAS, M.A.; University of San Diego; University of San Diego: The functional morphology and fluid dynamics of swimming in the pelagic red crab, Pleuroncodes planipes.

Pleuroncodes planipes, the pelagic red crab, feeds on planktonic organisms in the surface waters of the eastern Pacific Ocean at certain early stages of it life, alternately swimming rapidly to the surface and slowly sinking down in the water column. The crab�s body and limbs have evolved in shape and structural details to reduce drag and increase streamlining while swimming. Yet in the passive sinking posture, the crabs are able to increase surface area by extending all appendages away from the cephalothorax forming a parachute-shape, increases drag, thereby slowing its sinking rate. The functional morphology of the crab in both swimming and sinking postures has been examined using photo analysis, scanning electron microscopy and light microscopy. Limb design, body flexion, and the arrangement of setae and grooves on the body and limbs all enhance streamlining and drag reduction while swimming. Fluid dynamic parameters, such as Reynolds number, drag coefficient, and drag illustrate the differences of the two body postures with the sinking posture having a surface area 3 times greater than swimming posture. Velocities (in cm/s and body lengths/s) were determined for both the swimming and the passive sinking segments; average swimming velocity is more than 3 times faster than sinking velocity. The functional morphology of the body and limbs, as well as the overall shape of the crab, have enabled them to be streamlined swimmers while allowing for efficient feeding during the much slower sinking phase.