What are plankton?.

Plankton , Greek for "drifter", are small plants (phytoplankton) and animals (zooplankton) that drift with the ocean's currents. They form the bottom of the food chain in the sea and are very important in ocean food webs.

Collecting plankton

Nets
Oceanographers use nets to catch these small creatures and study them. The nets are of a much finer mesh than fish nets, as the mesh openings must be small enough to concentrate the plankton while still allowing water through. Phytoplankton nets have a very small mesh opening (about 36/1000 of a mm) and zooplankton nets have larger meshes (about one 1/3 to 1/2 of a mm). The nets are attached to the hydrowire and towed behind the ship. Plankton tows can be done at any depth or time of day, and can be used with opening/ closing mechanisms to enable them to collect at a desired depth.

SCUBA Diving
Several decades ago a group of oceanographers decided that the best way to study some types of plankton was to get down in the water with them! These scientists developed a form of SCUBA diving called "blue-water diving" in order to observe plankton directly in the field. In this form of research diving, a group of divers are attached to a small boat via a "down-line".  This leaves them free to collect and observe without the danger of floating too far from the boat or sinking too deep. Many new, interesting things have been discovered about plankton (especially some of the more delicate, gelatinous forms that get mashed up in plankton nets) by using this technique

Submersibles
Most of the world's oceans are too deep for SCUBA divers to safely go.   Researchers can now use high technology submersibles to study creatures that live in the deep sea.  These submersibles are either "manned" (with people on board) or are remotely operated vehicles (ROV's).

Virtual Plankton Tow

Diatoms
These microscopic plant plankton or "phytoplankton" are a food source for the zooplankton (animal plankton) in the sea.  Diatoms have cell walls made of silica, and many form long chains of cells.

Coccolithophorids
These microscopic plants or "phytoplankton" have a cell wall made up of calcium carbonate plates.  Blooms of these phytoplankton can be seen from space with satellites, showing up as a milky white color in the surface ocean.

Copepod
"Copepod" means oar (or paddle) foot in Greek. These tiny crustaceans the size of a rice grain are estimated to be most numerous multicellular animals on earth. These are the "insects of the sea" and are a source of food for many other zooplankton and fish.

Krill
These zooplankton are important source of food for many marine mammals and seabirds.  They are especially well known in the Antarctic where they are an important part of the food web.

Crab zoea
We call this baby crab "meroplankton" which means an organism that only spends part of its life cycle in the plankton (as opposed to holoplankton which spend their entire life in the plankton).  The larvae of many bottom-dwelling invertebrates like crabs, starfish, mussels, clams, and lobsters all begin as larvae in the plankton before they settle on the bottom and become the more familiar adult creatures.

Pteropod or "sea butterfly"
These gelatinous zooplankton are relatives of snails.  Most of the pelagic (open water) snails have evolved to live without their shells, however, so that they do not sink too quickly.  Pteropods feed using a mucous web that they trap food particles with.

Ctenophore or "comb jelly"
These jelly plankton have 8 rows of "combs" made up of fused cilia.  They are the largest animals on earth to move by cilia.  Ctenophores are also highly bioluminescent, meaning they can produce and emit their own light.  Although bioluminescence occurs in many shallow-living marine species (the so-called phosphorescence of the sea is due in large part to near-surface bioluminescent phytoplankton called dinoflagellates), it becomes increasingly important in the deep sea where it is the only source of light below about 1000 meters.

Amphipod
These crustaceans are carnivorous, and often live attached to jelly plankton.

Medusae (jellyfish)
This jellyfish contains stinging threads, called "nematocysts", in its tentacles which are used to stun its prey.  You might wonder, what in the world eats jellyfish?  Other gelatinous zooplankton, sea turtles, and large fish called mola molas, are a few examples of creatures that capture and eat medusae.  We just have to look at the medusae to know that not all plankton are small.  Cyanea capillara- the "lion's mane jelly" can get up to 3 meters in diameter, with 150 ft. tentacles!

Creatures of the deep!!
Light is one of the factors that controls where organisms are found in the ocean. In this virtual net tow, we explore some of the creatures you might find in the deep sea.

Deep Sea shrimps
These beautiful shrimp migrate from below 400 meters to the surface waters each night to feed! As the sun sets over the Sargasso Sea, these shrimp and millions of other sea creatures migrate up from the depths to feast on microscopic plants growing in sunlit surface waters.

At dawn, these animals reverse course, sinking or swimming down to spend another day in the darkness.  Oceanographers call this "vertical migration". By almost any measure, this daily migration rivals the great seasonal movements of caribou on land or arctic terns in the air.

Deep Sea Jellyfish
 The deep red color of this jellyfish is common amongst many deep sea zooplankton.  This is because red wavelengths of light are quickly scattered by water near the surface of the ocean.  The only light that penetrates to depth is in the blue-green part of the spectrum, thus red bodies are difficult to see in blue light (looks black, and blends in with the background) and helps the animal avoid predators.

Assorted Deep Sea Fish
These are examples of fish you might find in the deep sea.  Many look like monsters, but are in reality just a few inches long! Many deep sea fish are black, red, or silvery. This is mostly a protective mechanism against being seen by their predators. Black blends in against the dark, deep waters, silver reflects any available light, and red looks black in deep water because this end of the light spectrum disappears with increasing depth (see explanation above for jellyfish).

Saber-Toothed Viper Fish

 The viperfish is one of the fiercest predators of the deep, living at depths greater than a mile.  Individuals average less than 12 inches long but can grow to 2 feet.  The viperfish has hundreds of  tiny light organs in its mouth that it uses to attract its prey.

Flashlight fish
Like many deep-sea organisms, these fish are "bioluminescent," meaning  they can produce their own light.  Notice the headlights on this fish and the glowing patches under its eyes.