Locomotion in the Plankton
Most larvae and other plankton are heavier than seawater, so they depend on buoyancy and/or locomotion to stay suspended in the water column. Some larvae are buoyant from lipids stored in the egg or from the accumulation of lipids during feeding.  Thus, plankton may become more buoyant throughout their larval stage. Barnacle nauplii larvae are a good example because they feed throughout this stage and store lipids in specialized oil cells. At the time of metamorphosis, these larvae settle in shallow water, so buoyancy may be an important factor for finding the right adult habitat.

Plankton move through the action of  cilia (see the ciliary bands, which appear on the edges of body lobes in the sea cucumber larva to the left), antennae (as seen in the copepod), jointed appendages (as in the crab larva), muscle contractions in the body wall (as in annelids) and/or jet propulsion (as seen in this jellyfish movie).  Because the plankton are heavier than seawater, they depend on these forms of locomotion just to stay in the water column (the density of water is a constraint on the distance these small organisms are able to move).  Movement of flagella works by the flagellum beating back and forth or in a helical rotary pattern.  Cilia often occur in distinct tracts and move together in waves, as seen in this ctenophore movie. The ciliary movement provides a uniform pulsing force that has forward (power) and recovery strokes.

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Invertebrates 2000