PLANKTON: CHARACTERISTICS, TYPES, FEEDING AND REPRODUCTION - BIOLOGY - 2025

The plankton is the set of pelagic aquatic organisms that are at the mercy of water currents. They are organisms unable to swim or, although they can swim, they do not have enough strength to oppose these currents.

The term comes from the Greek and means wanderer or wanderer. Planktonic organisms share pelagic life with nekton. The latter is represented by organisms capable of swimming and opposing currents.

Collage, plankton diversity. Taken and edited from: Kils via Wikimedia Commons.

The plankton is made up of a great diversity of organisms, ranging from bacteria to vertebrates, through protists and algae. Virtually all large groups of organisms have at least one planktonic species.

Some species are planktonic throughout their lives, others only spend part of their lives in plankton. The size of these organisms can range from less than 5 microns (bacteria) to more than 2 meters (jellyfish).

Types

Plankton has traditionally been divided into groups depending on different criteria, the most common being the type of diet (autotrophic or heterotrophic).

-Depending on the type of food

Phytoplankton

Phytoplankton diversity. Taken and edited from: Prof. Gordon T. Taylor, Stony Brook University, via Wikimedia Commons.

Made up of autotrophic planktonic organisms. It includes algae and photosynthetic protists. They are indispensable not only for marine life but also for terrestrial life. Phytoplankton produce about 50% of the O ₂ present in the atmosphere. It is also the basis of most aquatic food chains.

Phytoplankton depend mainly on the contribution of terrigenous nutrients that are carried by rivers. Other contributions come from bottoms and deep waters during upwelling times. Because of this, phytoplankton are more abundant in coastal waters than in ocean waters.

Zooplankton

It is composed of heterotrophic forms. Among these are protists and animals. They are the primary consumers in most aquatic food chains. They can feed on phytoplankton or other members of zooplankton.

Most organisms are small in size, but some species can exceed 2 meters, such as jellyfish.

Depending on the aquatic environment where they are located

Dulceacuícola

As their name implies, they live in fresh waters. These can range from large rivers and lakes to temporary pools. Even in the phytotelmatas planktonic forms can be found. Phytotelmatas are containers of water of plant origin, such as bromeliad leaves, or holes in tree trunks.

Marine

Marine plankton live in seas, oceans, and intertidal pools. Plankton that inhabit brackish water bodies are also called in this way.

-According to its size

There are discrepancies between the classifications used by the different authors. A classic division, separates them into:

Ultraplankton

It is comprised of organisms smaller than 5 microns. Some authors separate this group into Femtoplankton and Picoplankton. Bacteria and flagellated microalgae (prasinophyceae) fall into this group.

Nanoplankton

Planktonic organisms with sizes between 5 and 50 microns. Some authors limit the maximum size in this group to 20 microns. Others argue that it can go up to 63 microns.

Nanoplankton is represented by coccolithophores. These are exclusively marine unicellular algae (protists).

Microplankton

Its size is between 50 microns and 0.5 mm, an example of this type of organisms are dinoflagellates; Unicellular protist algae that present two flagella.

Dinoflagellate Noctiluca scintillans. Known for exhibiting bioluminescence. Taken and edited from Maria Antónia Sampayo, Institute of Oceanography, Faculdade Ciências da Universidade de Lisboa, via Wikimedia Commons

Mesoplankton

The size is greater than 0.5mm and less than 5mm. This group includes small crustaceans called copepods. These are one of the most abundant members of plankton, especially marine ones. Cladocerans, other Mesoplanktonic crustaceans, are more abundant in fresh waters.

Crustaceans, Harpacticoides copepods. Recovered from Frank Fox, via Wikimedia Commons.

Macroplankton

The organisms that make up this group have a size range that ranges from 5 to 50 mm. Some ctenophores and some thalliaceae are included. The larvae of numerous fish also fall into this category.

Megaloplankton

They are the plantonic organisms of more than 50 mm in length. Numerous species of jellyfish are included, some of which can have umbrellas of more than 2 m in diameter. Until a few years ago, some authors also included in this line the sunfish, the heaviest bony fish.

-According to the permanence in the plankton

Meroplankton

Also known as temporary plankton. They are only part of plankton during one phase of their development. Examples of meroplankton are the larvae of fish, crustaceans and other benthic or pelagic organisms.

Holoplankton

They remain all their lives in plankton, as occurs with copepods, cladocerans, diatoms, among others.

-According to its bathymetric distribution

Epiplankton

Also known as epipelagic plankton. It is located in the surface water layer, up to 200 m deep.

Mesoplankton

It corresponds to the mesopelagic zone (200 - 1,000 m.).

Batyplankton

It is the plankton of the bathypelagic zone. It is located between 1,000 and 3,000 m deep.

Abisoplankton

Also called abyssal plankton. It occupies the area below the bathypelagic, up to 6,000 m deep.

Hadal plankton

It is the plankton of the deepest areas. It is located more than 6,000 meters deep.

-According to its horizontal distribution

Neritic plankton

It is the plankton that is located in the body of water on the continental shelf; the marine area near the coast, with a maximum depth of 200 m.

Ocean plankton

It is the plankton found in oceanic waters; Waters further from the coast, with a depth greater than 200 m.

-According to the amount of light they receive

Photoplankton

Located in water bodies where sunlight penetrates. In these, phytoplankton can actively photosynthesize.

Scotoplankton

Plankton located in completely aphotic waters.

Cnefoplankton

Plankton located in the twilight zone. This zone is located between the photic zone and the aphotic zone.

Feeding

Feeding by planktonic organisms can be autotrophic or heterotrophic.

Autotrophic

Autotrophic feeding is carried out by phytoplankton. The main representatives of phytoplankton are diatoms and dinoflagellates. Because they need to photosynthesize, these organisms are located in the photic layer, that is, in the layer where sunlight can penetrate.

Heterotrophic

Heterotrophic feeding is that carried out mainly by zooplankton. Zooplankton can be herbivores (they eat phytoplankton) or carnivores. In the case of carnivores, these can be primary, secondary or teciary consumers.

Primary consumers feed off producers. The secondary of the primary and the tertiary of the secondary. Some copepods are primary consumers and others secondary. Some species of jellyfish can be considered tertiary consumers.

Reproduction

Virtually all possible forms of asexual and sexual reproduction can be observed in plankton. Some species present only one form of reproduction, others may present alternation of asexual and sexual generations.

Asexual

Asexual reproduction is that which is carried out without the intervention of sex cells or gametes. In all forms of asexual reproduction only one parent is involved, but in some cases there may be two.

The main types of asexual reproduction are fragmentation, fission, budding, and sporulation. All of these types of asexual reproduction are present in plankton.

For example, cyanophytes or cyanobacteria, may present bipartition (binary fission), fragmentation, or sporulation depending on the species. Ctenophores can divide by fragmentation and it is believed that they can also divide by fission and budding.

Sexual

Sexual reproduction involves the participation of sex cells or gametes. These gametes can come from two different parents or from a single parent. This type of reproduction involves reductional (meiotic) division during gametogenesis.

Meiosis reduces the genetic load of daughter cells by half. These cells are haploid. The fusion of two haploid cells gives rise to a diploid zygote that will develop into a new organism.

Sexual reproduction can occur in both phytoplankton and zooplankton members. Copepods, for example, reproduce sexually.

-Alternance of asexual and sexual reproduction

In some planktonic groups there may be generations that reproduce sexually and others that reproduce asexually. Both generations develop as part of the plankton.

In other cases, some generations develop in the plankton, while the other is part of the benthos.

Diatoms, for example, reproduce asexually for several generations. One of the daughter cells, the one that inherits the maternal mortgage, will be smaller in each generation. This is because the maternal mortgage will serve as your epitheca. Upon reaching the minimum size, these diatoms will reproduce sexually.

In the other case, the scyphizoan jellyfish alternate planktonic generations (jellyfish) with benthic generations (polyps). The reproduction of jellyfish is sexual, forming male or female sexual gametes.

Fertilization gives rise to a larva called planula, which will migrate to the benthos, where it will fixate and give rise to a polyp. This polyp is called a sciphistoma or scifopolyp.

Scyphistoma can cause other budding polyps. It can also give rise to jellyfish by a division process called strobilation. These jellyfish thus created are small in size and called mayflies. The efirae migrate to the water column where they develop to form adult jellyfish.

Gelatinous plankton

Gelatinous plankton is a particular group of plankton made up of jellyfish (Cnidaria) and ctenophores. It receives this name from the consistency of the body of its members, which are made up of more than 90% water.

Currently this type of plankton has increased its abundance in some regions, although the causes are unknown. Some authors suggest that it is due to the increase in the global temperature of water bodies, others think that it is due to marine eutrophication.

Regardless of the cause, this increase has become a problem for humans. Its effects include interference with commercial fisheries and coastal power plant operations.

References

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5. Plakton. On Wikipedia. Recovered from en.wikipedia.org
6. G. Thorson (1971). Life in the sea. Introduction to marine biology. Guadarrama Editions.