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What are Zooplankton

calanoid copepod

The name plankton is derived from the Greek word planktos meaning to wander, and refers to the weak swimming movements of organisms in this category. Plankton can be subdivided into animals, or zooplankton, and plants, or phytoplankton.

There are advantages in being small in aquatic environments: in the first place the base of the food chain, i.e. the phytoplankton, consist of microscopic plants from 1/1000 of a mm to 2 mm in size. These cannot be efficiently harvested by large animals.

Secondly, the phytoplankton occur abundantly in the upper 200 metres of the water column where there is adequate light for photosynthesis; being small helps planktonic animals to maintain buoyancy and keeps them close to their food source.

Thirdly, being small reduces the number of predators that are able to feed on them.

crab megalopa larva

Fourthly, small size means that generation time is short and this means that these animals can rapidly colonise new habitats and exploit new food sources.

Zooplankton can be further subdivided into holoplankton, i.e. permanent members of the plankton, and meroplankton, i.e. temporary members of this category.

Meroplankton consist of larval and young stages of animals that will adopt a different lifestyle once they mature. For example bottom-living animals such as crabs and lobsters enter the plankton as larvae for the purpose of dispersion. Also many fish are planktonic in the early stages of their development.

It is not entirely true that zooplankton are at the mercy of ocean currents. Many organisms in this category undergo vertical migrations over the course of every 24 hour period (diel vertical migration). The most common pattern is to migrate deeper in the water column during daytime and ascend towards the surface at night.

scyphozoan jellyfish

The most likely explanation for this behaviour is to escape predators that are feeding in the upper, lighted layers during daytime and to exploit the food sources which are most abundant near the surface when it is too dark for successful capture by visual predators.

However, vertical migration may also remove organisms from faster moving currents near the surface into deeper slower currents that may even be travelling in the reverse direction.

Thus some degree of control over distribution is possible by varying the time spent at different depths.

Herbivorous zooplanktonic organisms are faced not only with large vertebrate predators such as fish, but also with invertebrate carnivores not much bigger than themselves.

As well as being small and migrating vertically, they have adopted a range of other protective strategies, e.g. being transparent and aggregating into schools or swarms.

Some jellyfish, although still regarded as planktonic, can be very large i.e. more than a metre across the bell.

However, they are very well defended with thousands of tiny stinging cells distributed mostly on tentacles.