ZOOPLANKTON: Free-drifting Animals

Although zooplankton are truely planktonic, i.e. the freely drift through the ocean, they are capable of movement. Some slowly cruise through the water, while others make sharp jumps from one location to another. This is because water at their scale is very VISCOUS. It is as if they are swimming in syrup. It is therefore more efficient to jump and then glide.

Although they are typically very small, approximately 1 mm or less, some zooplankton are capable of impressive swimming feats. Many forms vertically migrate to adjust their ambient light environment. This daily, or diel migration involves a round trip from several hundred meters (300 to 1000 m) deep at sunset, a night of feeding in the then dark photic zone, and then a return trip back into the depths at sunrise. This would be the same as one of us walking to Atlanta for the night and then back for classes in the morning!

How do zooplankton live in the ocean?

To observe zooplankton in the ocean, Drs. Cabell Davis and Scott Gallager from Woods Hole Oceanographic Insititution developed a towed video camera that allows us to photograph zooplankton as they live in the water column.


The zooplankton are typically spread out in the water column, but when they are found they occur in groups or PLANKTON PATCHES. We are finding out new things about how these organisms orient in their environment.


Heterotrophs - Organisms that live off carbon fixed by primary production.

Herbivores - Direct users of the primary producers, i.e. the phytoplankton.

Detritivores - Consumers of dead organic matter produced by the senesence of pytoplankton, egestion of material from other zooplanktors (fecal material), or the remains of other zooplankton.

Carnivores - Predators feeding on other zooplankton.

Omnivores - Zooplankton that use a combination of food sources.

There are a variety of feeding stratagies in the zooplankton. Some such as COPEPODS feed through a range of methods including setting up feeding currents with their legs that then pull phytoplankton cells past their mouths. CHAETOGNATHAS or arrow worms typically stay still in the water column and then lunge at a passing copepod to capture it with their sharp grasping spines. These saber-like spines are driven into the copepod during the ambush. Other organisms filter feed by pumping water through themselves or by the construction of large amounts of MUCUS upon which particles (phytoplankton, microzooplankton and detritus) become stuck. An example, is the chordate SALPS that produce lots of mucus.


Much of the effort in studying the zooplankton involves the laborious, but exciting problem of indentifying all of the different species. A single tow off the RSMAS dock  might have hundreds of different species! The problem is that there are between 30 and 40 phyla (major groups after kingdoms) that are now recognized in animals.

General groups:

PROTOZOA - Single-celled animals.

Heterotrophic dinoflagellates - switch hitting in some cases chlorophyll plus predation to get nutrients.

Foraminifera - Calcium carbonate shelled protozoan grazers and predators in the microplankton. Of major geological importance because of the shell.

METAZOA - Multi-celled animals.


Holoplankton - Zooplankton that spend their entire life in the plankton from birth to reproduction as an adult.

Meroplankton - Forms that spend only a portion of their life cycle in the plankton. Many BENTHIC (bottom dwelling organisms) and most NEKTON (free swimming fishes and squids) have meroplanktonic larval forms.