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Function
Essentially, the function of the antennules is to act as the lobster's "nose." As water passes over the antennules, chemicals dissolved in the water waft over small hairs along the structures. On these hairs are sensors that are activated when the chemicals hit them. This allows lobsters to sense food in the water as well as nearby predators.
Antennule Motion
Lobsters continuously swish their antennules up and down through the water. According to a 2001 issue of the journal "Science," researchers at the University of California, Berkeley, and Stanford University determined that the speed of the down-stroke relative to the upstroke was important. The down-stroke is very fast, which allows the water to penetrate the dense hairs on the antennules and reach the receptive sensors, while the slow upstroke does not allow the water to enter.
Reason for Different Speed Strokes
The speed of the downward stroke not only allows the water to reach the hairs but also does so without disturbing the pattern of turbulence as the antennules pass through it. Lobsters are able to capture a very accurate picture of what is going on in the water, allowing them, theoretically, to determine the direction and distance to the origin of the smell. On the upstroke, the chemicals are trapped in the hairs, allowing the lobster to hold onto the scent until the next down-stroke.
How the Function Was Determined
The researchers at Berkeley and Stanford filmed the movements of a lobster's antennules in slow motion. They then built a model of the lobster, which was able to replicate these movements mechanically. The researchers positioned the lobster in a current of water and introduced dye to the water to replicate the spread of odor-causing chemicals, then they created turbulence in the water and pumped it toward the model. Finally, by shining a sheet of laser light across the exact section of water they wanted to investigate, researchers filmed the effect of the antennules passing through the dye-colored water.