Zooplankton are tiny marine animals that drift with ocean currents and the winds. Many zooplankton can swim up and down the water column, but most are so small that they can't swim strongly against the current. Zooplankton also include larvae, immature forms of larger marine animals. These ocean babies will eventually gain the ability to swim, to propel themselves through water, or settle on the ocean floor when they become adults. 

Image credits: main image, Sean Murtha.