Here, the various categories of pelagic organisms are defined, and especially those categories of animals this volume is involved with.
Plankton is the term that is used for all aquatic organisms that live freely in the water and are not capable to displace themselves independently from the water currents or water movements, because they have none or only limited powers of movement. In other, less exact words, they drift or tend to drift by the sea (or any other type of water they are in).
Plankton is divided in phytoplankton and zooplankton, which are explained below. A third, in fact not less important group of planktonic organisms, is the co-called bacterioplankton, consisting of free-living, floating bacteria.
Opposite to the rather passive plankton, is the group of active and effective swimmers that are indicated with nekton, or micronekton in case of smaller sized pelagic animals; these terms are explained below also.
Phytoplankton or plant-like plankton, consists mainly of microscopic algae, and are really passively transported, though some of these species of algae have still the tiniest means of movement by ciliary hairs and they can migrate up in the water column for short distances.
Zooplankton or animal-like plankton, consists of a vast group of very different types of animals, ranging in size from microscopic small to about a few meters large (as in the case of jelly fishes or colonies of pyrosomid tunicates). Generally, they can be provided with all sorts of motile appendages or hairs by which they can perform some sort of swimming, or their body or the colony is capable of propulsion by (muscular) contraction. Despite these capacities, zooplankton is unable to escape from the effects of large water movements, however, on the other hand there are many of them which can migrate vertically in the water column over even very long distances. The so called diurnal vertical migration, performed by all sorts of different plankton species, is a good example of this, where vertical distances can be covered of hundreds of meters.
According to the different mesh sizes of the used plankton nets, plankton is subdivided into size classes, but without any true biological or systematical meaning, and there is no size standard either. This volume maintains 1 mm as the lower size limit for (metazoan) zooplankton to be included in the key. However, other size limits are used also, as for example, in the following devision (note that these terms may apply to both fyto- and zooplankton):
picoplankton (< 2µm)
nanoplankton (2-20 µm)
microplankton (20-200 µm)
mesoplankton (0.2-2 mm)
macroplankton (> 2mm).
Nekton is the term that is used for all marine animals sufficiently mobile to determine their own horizontal and vertical distribution. They have sufficient powers of locomotion and are therefore capable to displace themselves independently of the water movements; in other words, if necessary they can swim against the current effectively. Fish, squid, cuttlefish and marine mammals are examples of nektonic animals.
Micronekton is the name for an intermediate group between zooplankton and nekton; usually they are smaller pelagic organisms and caught in moderately sized trawls with mesh sizes of 4-5 mm. Micronekton is consisting mainly of decapod crustaceans, smaller cephalopods and small fishes.
It is obvious that zooplankton and micronekton are rather artificial groups to categorise pelagic animals — at least they are no taxonomic coherent entities and they contain many different kind of organisms.
Species that are planktonic all the time of their life, are holoplanktonic (belonging to the holoplankton) whereas species that spend just a part of their life cycle in the free water, are meroplanktonic (belonging to the meroplankton). The latter is in particular the case with benthic invertebrate species that have a planktonic larval phase.
Species that are found just above or near the bottom are called hyperbenthic; they might get caught with plankton nets, that is to say, when the net is rather close to the bottom or the animal is rather shallow. Species that occur swimming in the vicinity of the bottom are benthopelagic. A special case are species that normally live at the bottom but undertake occasionally (short) excursions into the free water, or migrate upwards at night, or go swarming in the pelagic during their reproductive period. These species are temporarely or even seasonally planktonic.