ZOOPLANKTON: CHARACTERISTICS, FEEDING, REPRODUCTION, CULTIVATION - BIOLOGY

The zooplankton is a part of the plankton found in water bodies such as seas and rivers. Its main characteristic is that it is made up of living beings that do not have the ability to synthesize their nutrients through photosynthesis, but must feed on other living beings, such as plants or small animals.

The classification of zooplankton according to size is as follows: protozooplankton (very small organisms, such as protozoria) and metazooplankton (slightly larger organisms). It is important to note that the animal is not always part of the zooplankton throughout its life, but is often part of it only during a period of it.

Zooplankton. Source: Mª. C. Mingorance Rodríguez / Public domain

This is how meroplankton then exist, which is made up only of larvae and eggs of some animals such as fish, mollusks, crustaceans or worms; and holoplankton, which is made up of animals that form part of zooplankton throughout their lives.

From an ecological point of view, zooplankton is very important in marine ecosystems, since it constitutes the base of the food chain, together with phytoplankton. Zooplankton is the food of larger animals like some fish and mammals like whales.

characteristics

Zooplankton is made up of a wide variety of heterotrophic organisms, which have managed to colonize both fresh and brackish aquatic environments.

Likewise, they move thanks to the movement of the marine current. Specialists claim that they are bad swimmers. Sometimes some organisms travel through pseudopods.

His behavior is quite peculiar. At night they tend to come close to the surface, in order to feed, while during the day they prefer to be located in deeper areas so as not to receive sunlight.

It is generally accepted that some of its members are the juvenile forms of some species of fish. Once these mature they abandon the zooplankton.

They reproduce asexually and sexually. In the latter case, fertilization can be internal or external and development in the vast majority of organisms is indirect, with the presence of larval stages until they become adults.

Zooplankton is made up of several types of animals, so its variety is impressive. For example, the so-called holoplankton is made up of unicellular organisms such as protozoa, while meroplankton are made up of mollusk larvae, echinoderms and crustaceans.

Feeding

The animals that are part of the zooplankton have heterotrophic eating habits. This means that they cannot make their own nutrients, so they need to feed on other living beings. In this sense, the member organisms of zooplankton feed mainly on phytoplankton.

Within the zooplankton there is a certain variety in terms of food. That is, there are some organisms that only feed on phytoplankton, while there are others that tend to feed on animals as members of the smaller zooplankton.

Likewise, more than 70% of the zooplankton is made up of crustaceans called copepods. According to many specialists, copepods are among the most devouring animals in the world, since approximately each one is able to eat half their weight each day.

Copépodo specimen. Source: Andrei Savitsky / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Reproduction

Due to the wide variety of organisms that are integrating zooplankton, it is possible to observe the two types of reproduction that exist: asexual and sexual.

Asexual reproduction

This type of reproduction does not involve the fusion of gametes (sex cells), so the descendants will always be exactly the same as the parent.

There are many types of asexual reproduction. However, in zooplankton the asexual reproduction method that is appreciated is bipartition.

Bipartition is a process of asexual reproduction that consists of obtaining or developing two individuals from the division of the parent organism. It is common in most of the protozoa found in zooplankton.

During this process, the first thing that must happen is that the DNA of the organism is duplicated so that there can be an equitable distribution between the two resulting cells after division. Subsequently, a process similar to mitosis occurs, with the result that two individuals are formed, each with the same genetic information as the progenitor cell that originated them.

Sexual reproduction

Sexual reproduction is a much more elaborate process than asexual. Its main characteristic is that it involves the union or fusion of two sex cells, a process known by the name of fertilization.

Fertilization

In most organisms that make up zooplankton, a type of internal fertilization is observed, which occurs through a structure known as the spermatophore. This is nothing more than a kind of bag, in which the sperm is kept or stored.

During the copulation process, the spermatophore is introduced into the female's body and remains attached to an organ called the seminal receptacle. This is where fertilization finally occurs.

Development

When the eggs have been fertilized, the eggs are formed. After a time has elapsed in which the new being is formed, a larva hatches from the egg, which must go through a series of molts until the adult individual is finally formed.

In other zooplankton organisms, such as some members of the phylum Echinodermata and Mollusca, fertilization is external. This means that the gametes, male and female, are released into the aqueous medium and there they meet and fuse, giving rise to larvae that must undergo a series of transformations until they reach adulthood.

Worldwide distribution

Zooplankton is widely distributed in all bodies of water on the planet, both brackish and freshwater. However, the variety in each location may be different, since there are certain variations between one aquatic ecosystem and another, which influences the presence of certain organisms there.

Taking this into account, in each of the oceans there will be zooplankton, but made up of different species, depending on the characteristics of the environment. An example of this is the Atlantic Ocean, where the Valella valella species of siphonophores is abundant, while in the Pacific Ocean there are also siphonophores, but this time of the Valella lata species.

In this sense, it is important to emphasize that zooplankton is present in all the oceans of the planet. What varies are the species of organisms that will integrate it. Likewise, the seasons of the year also seem to play an important role in the constitution and distribution of zooplankton worldwide.

Culture

According to specialists, zooplankton constitutes the best food for fish, since it has all the nutritional elements that they require to be able to survive and develop properly.

This is why there are those who have dedicated themselves to its cultivation, in order to use it in raising fish to feed them.

Now, there are some species of organisms, members of the zooplankton, that are farmed more frequently than others. These include:

Brachionus plicatilis, from the phylum Rotifer
Artemia salina, from the class of crustaceans Branquiopoda
Daphnia sp and Moina sp. Both members of the crustacean suborder Cladocera
Tigriopus japonicus, of the crustacean subclass Copepoda.

Some cultivation examples

Brachionus plicatilis

The cultivation of this rotifer can occur through three mechanisms:

Chlorell's culture transfer pond method This is a microalgae that serves as food for the rotifer. In the cultivation of Brachionus plicatilis using this technique, it is transferred through several ponds in which there is a high concentration of the Chlorella microalgae. However, this technique is not the most appropriate or efficient, since it depends on its concentration.
Feedback system: it is the system that is used most frequently today. In this, the formation of a microecosystem made up of pseudomonas bacteria is promoted. This method is the one that has proven to be the most efficient in the production of large quantities of Brachionus plicatilis.

Artemia salina

This is a particularly abundant organism in tropical and subtropical regions. Its cultivation is a fairly common process that involves several important steps:

Obtaining cysts. These can be obtained in crops or in rural areas. All the cysts that are collected are not viable, so they are subjected to a series of processes such as sieving, centrifugation and several washes to be able to select the most suitable ones to continue in the culture.
Hydration of the egg. In order to provide the necessary resources for its development.
Pass them through a decapsulant solution, waiting for the cyst to take on an orange color.
Washing with running water, to remove chemical residues
Hydrochloric acid soak
Rinse under running water
Place the eggs in seawater and incubate them under optimal conditions, until they hatch.

At the end of this procedure it is possible to obtain large quantities of Artemia salina to be used in specialized aquaculture.

References

Boltovskoy, D. (1981). Atlas of southwestern zooplankton and working methods with marine zooplankton. National Institute for Fisheries Research and Development, Mar del Plata, Argentina
Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
Longhurst, A. and Pauly, D. (1987). Ecology of Tropical Oceans. Academic Press. San Diego.
Thurman, H. (1997). Introductory Oceonography. Prentice Hall College.
Villalba, W., Márquez, B., Troccoli, L., Alzolar, M. and López, J. (2017). Composition and abundance of zooplankton in the El Morro lagoon, Isla de Margarita, Venezuela. Peruvian Journal of Biology. 24 (4).

ZOOPLANKTON: CHARACTERISTICS, FEEDING, REPRODUCTION, CULTIVATION - BIOLOGY - 2025