What are Zooplankton?

Zooplankton are microscopic animals that eat other plankton. Some zooplankton are larval or very immature stages of larger animals, including molluscs (like snails and squid), crustaceans (like crabs and lobsters), fish, jellyfish, sea cucumbers, and seastars (these are called meroplankton).

Other zooplankton are single-celled animals, like foraminifera and radiolarians.

Other zooplankton are tiny crustaceans, like Daphnia. Megaplankton includes the larger organisms such as jellyfish. There are numerous types of organisms that are considered zooplankton, below are some examples:

Protozoans

These are single celled animals. Protozoans comprise a large and diverse group of microscopic organisms that live as single cells or in simple colonies. Some of them can produce elaborate shells that eventually sink and form large deposits at the bottom of the sea. They are divided into five protist phyla: Mastigophora (the flagellates), Sarcodina (the amebas), Ciliophora (the ciliates), Opalinida, and Sporozoa. Most are motile and most ingest food.

The 26,000 living species are found in freshwater and at all depths in the ocean. Some are even parasites in the bodies of humans or other animals, sometimes causing diseases

Jellyfish (megaplankton)
Bell shaped jellyfish are gelatinous animals with a simple body structure. When viewed from above, they are round and their organs radiate out from a central stomach, giving them radial symmetry. Jellyfish are planktonic and have little locomotory ability. Some may produce some movement by pulsing their bodies and jetting water from beneath their bells. Most jellyfish are carnivores. They stun their prey with stinging cells (nematocysts) located on their tentacles. The stunned prey is passed into the frilly mouth, hanging down from the centre of the bell. Jellyfish themselves are an important food source for some turtle species.

Note – marine pollution, such a plastics, can cause serious damage and even death to turtles as they mistaken this rubbish for their food source and can becomes tangled or choke to death.

Chaetognath Worms (holoplanktonic)
The only true planktonic worms are the chaetognaths, or arrow worms, because they are shaped liked arrows. Often they are called glass worms due to their transparency. They have fins and a pair of hooked, chitinous, grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. All species are hermaphroditic, carrying both eggs and sperm. A few species are known to use neurotoxins to subdue prey. These worms are carnivores and are found amidst large concentrations of plankton.

Marine Gastropods
These include muscles, cockles, clams, oysters, sea slugs, cuttlefish, etc
The larvae of benthic molluscs usually found in coastal waters, such as marine gastropods including heteropods or pteropods are another type of zooplankton. There are also molluscs known as pteropods that are permanently planktonic.

Arthropods
Small crustaceans can be no larger than a pin head, but may reach one centimetre in length in deep ocean.

Copepods (Phylum Arthropoda, Subphylum Crustacea, Order Copepoda)

 Most macrozooplankton are copepods found in marine and freshwater ecosystems. Copepods swim using an antenna and frontal structures on their bodies. They eat phytoplankton and detritus, and occasionally other zooplankton smaller in size.

Larger crustaceans are the "malacostracan", whose bodies are soft just after they have shed their old shell. This, as a group, includes shrimps, lobsters and crabs, living on the bottom of the sea when large; but they are planktonic during the early stages of their life.

Euphausids are the most common planktonic crustaceans in many waters. They can glow in the dark. One of the better known of these species is Euphuusia superba commonly known as krill. The an important source of food for many types of whales. Krill can reach 60 mm in length and is common in the southern ocean, near Antarctica. Krill sometimes occurs in large shoals covering an area of over 10 square km, and several hundred metres deep.

Planktonic Eggs & Larvae
Many marine animals discharge eggs and sperm to float in seawater, mix, and eventually become fertilised (i.e. fish, tunicates, molluscs. etc). One animal may produce millions of eggs in a single spawning season. Because of the action of predators (and other environmental hazards), however, the mortality rate is usually high with only a few surviving to adulthood.

Fish eggs contain yolk, and when the eggs hatch into larvae, some yolk remains and are carried in a yolk sac under the body of the larvae. The larvae subsist on this yolk as they are drifting in the ocean until they have developed their mouths sufficiently to feed on small phytoplankton. To survive, the larvae depend upon developing to a stage at which they can catch prey of the current type and size to eat, before the yolk in the sac is used up.

Behaviour & Adaptations
Many types of zooplankton migrate deeper into the water during the day and come up at night (this is often known as diel migration). The migration is not regular, and reverse migration has taken place, when the plankton accumulates near the surface in daylight instead of the night. All plankton migrate differently based on factors like age, sex and the season. At certain stages in the life cycle, migration can occur, while at other stages it would appear that light is the prime factor in migration. Food is usually close to the surface. It has been suggested that these animals feed at night and sink deeper into cooler water during the day, to digest the food -but these processes are not fully understood.

Zooplankton has limited power of movement relative to currents in the water. By travelling vertically up and down, through the different layers of water at different speeds & directions, the plankton could end in different surface water than that of the previous night. Del migration "might" be a method of moving to new food supplies.

Some predators, for example pelagic anchovy and pilchard, migrate downwards simultaneously with the plankton. These fish could, themselves be following the plankton, or may be fleeing other predators such as birds or larger fish. Other fish rise from the sea bottom at night to feed on plankton; hence the zooplankton run a gauntlet every time they move.

Zooplankton have a range of protective devices to help lessen their chances of being eaten. The most common of these is being invisible to predators. This is achieved by either being transparent, or merging into the background colour.