CLADOCERANS: CHARACTERISTICS, CLASSIFICATION, HABITAT, FEEDING - BIOLOGY - 2025

The cladocerans or sea fleas are a group of small crustaceans belonging to the Branchiopoda class. They are characterized by presenting a univalve carapace, but with a bivalve appearance, since it is folded almost completely covering the body, with the exception of the head.

These organisms are almost exclusive to freshwater bodies, but a few species have managed to thrive in marine environments. They move in the water column as part of the plankton, using their antennae, although some species have adapted to living in benthic environments (underwater bottoms).

Cladocero. Taken and edited from Denis Barthel.

These microcrustaceans are a very important component of zooplankton and are a fundamental part of the trophic networks of the communities where they live. They have a wide worldwide distribution, both in freshwater and marine bodies, from tropical to boreal areas, and from the pelagic zone to great depths.

characteristics

The cladocerans are part of the branchiopods, which are characterized, among other aspects, by presenting the appendages of the trunk in the form of leaves or sheets (filopodia). The presence of gills at the base of these appendages, or legs, is what gives them the name of branchiopods (gills on the feet).

The cladocerans are also characterized by presenting a univalve carapace, without hinge, folded laterally, which covers part or almost all of the body, except for the head; this shell can sometimes be reduced.

In the cephalic region they have a single middle eye, not pedunculated, which can be compound or naupliar (simple). The buccal appendages are difficult to recognize because they are reduced and highly modified, the maxillae are sometimes absent.

The first pair of antennae is reduced and in some species it can become vestigial or modified in males. On the other hand, the second pair of antennas is quite evident and developed, in most cases it performs locomotion functions, either by swimming movements in the water column or by crawling along the bottom.

The somites of the trunk are not easily differentiated, the posterior portion of the body is curved and is known as the post-abdomen. The body usually ends in a pincer-shaped caudal furcation.

Taxonomy and classification

Cladocerans are currently considered a superorder of crustaceans. The first species were described in 1776 by OF Müller. However, the taxon was first erected in 1829, by the renowned French entomologist Pierre André Latreille.

More than 600 described species are known to science and taxonomists acknowledge that there are still many more to be described.

From the phylogenetic point of view (study of descendant ancestor relationships), several systematists agree that the current classification of cladocerans is artificial, since the groups are polyphyletic, that is, some representatives of the group do not share the same ancestor in common, and the similarities are due to evolutionary convergences.

The current classification groups cladocerans into six orders, of which 2 are under discussion:

Anomopoda

Group composed of 13 families of freshwater cladocerans. They usually present 5, rarely 6, pairs of thoracic appendages. The shell encloses the body, in which it is difficult to recognize the separation between the trunk and the postabdomen. They present a direct development, that is to say without larval stages.

Ctenopoda

Group of cladocerans represented by three families. Ctenopods are mainly freshwater, with very few marine representatives. They have six pairs of appendages on the trunk. The shell encloses the trunk. The development is direct.

Haplopoda

An order of cladocerans represented by a single family (Leptodoridae) and a single genus of Holoartic freshwater microcrustaceans. The carapace is very small compared to that of other groups. They have 6 pairs of appendages on the trunk. They have an elongated head and a compound eye. Development is indirect, with a larval stage.

Drawing of a postlarva of the cladocero Haplopoda Leptodora hyalina. Taken and edited from A. Milnes Marshall, via Wikimedia Commons.

Onychipoda

This group of cladoceros is composed of 3 families, with representatives in fresh and marine water bodies. They have 4 segments on the trunk. On the head they have a large and complex (compound) eye.

Cladocera

In the taxonomy of cladocerans, this order is considered as a momentary grouping or incertae sedis, which is the name used to indicate groups that are not locatable in a specific taxon. The group is made up of 4 freshwater genera.

Gymnomera

This order is not fully accepted among cladocerans taxonomists. Some researchers consider that the families and species contained in this group are part of the orders Haplopoda and Onychopoda.

Habitat

Cladocerans are microcrustaceans that generally inhabit fresh water bodies such as rivers, lakes and ponds; some species are of marine habits. They are cosmopolitan organisms, they inhabit tropical, subtropical and even arctic regions.

With regard to its vertical distribution, many species inhabit the water column as part of the plankton, swimming in the pelagic and demersal zones, a few others inhabit the benthic zone, where they move along the bottom.

They are organisms capable of living in fluctuating or dynamic environments, with variations in pH and temperature. They inhabit from relatively warm areas to very cold environments. They can be found inhabiting from the superficial littoral zone, to great depths.

Feeding

The cladocerans can feed on suspended particles (they are suspensivores) that they capture with their antennae and some trunk appendages, there are also filter feeders, scrapers and avid hunters (predators).

The genera Polyphemus and Bythotrepes, for example, have modified the anterior appendages to capture their prey. These prey are mostly protozoa, rotifers, and other microcrustaceans. Other cladocerans, such as Daphnia, include algae and even bacteria in their diets.

Reproduction

The reproductive system of cladocerans may consist of one or two gonads. The mature gametes are led to the exterior by gonoducts that, in females, open in the lateral or dorsal part of the postabdomen. In males, on the other hand, they open laterally or ventrally, generally near the anus.

Sexual

Males may or may not have copulatory organs. If they are absent, the gametes go out directly through a gonopore. When the penis is present, it originates as an extension to the exterior of the gonoduct.

During copulation the male holds the female with the antennae and turning the abdomen introduces the copulatory organ (if any) into the female openings, or the gonopores of both sexes come into contact. Sexual reproduction in these crustaceans is secondary and they use it alternately with asexual reproduction.

Asexual

Cladocerans reproduce asexually through a process called cyclical parthenogenesis, in which sexual and asexual reproduction alternate. Adverse environmental conditions can induce the appearance of males in the population, from parthenogenetic mothers.

In parthenogenesis, females produce fertile eggs that are not fertilized by the male, but which still hatch into viable individuals with the genetic makeup of the mother.

As a mechanism to ensure genetic variability during parthenogenesis, parthenogenetic eggs cross-over before entering anaphase. This phenomenon is known as endomeiosis.

Eggs and larvae

Cladocerans can produce from a few to hundreds of eggs. Its incubation time will depend on the taxonomic group, even the species.

Most species have direct development, which means that they do not have larval stages and when the organisms hatch from the egg they are quite similar to adults. On the other hand, a few other species show indirect development, which is why they experience at least one nauplius-type larval phase.

The cladocerans can produce dormant eggs or resistance eggs. These eggs can be washed away by currents, displaced by sight, or carried by other invertebrates and vertebrates such as birds and frogs.

Dormant eggs can go long periods without hatching, waiting for environmental conditions to be the most favorable for their development.

Importance

Ecological

Cladocerans are very important organisms within the communities where they live. They are part of the zooplankton that feed on phytoplankton. They are important in the transfer of energy in the trophic webs, being food for other organisms such as rotifers, other crustaceans and fish.

Aquaculture

In aquaculture, the genera Daphnia and Moina have proven to be of great importance for the culture of fish and other crustaceans. This is because their nutritional value is high and they have another series of characteristics that make them ideal organisms for use as food.

Among these features are:

• They are relatively accessible organisms to grow in large quantities.
• They have a high reproduction rate.
• Accelerated growth under controlled conditions.
• They turn out to be easy prey for the organisms that consume them.

The cladocerans are used to feed only the larvae of fish and crustaceans, but not adult organisms. Several studies indicate that in these phases there is a certain food preference for copepods and cladocerans rather than for other organisms, such as rotifers or protozoa.

There are several experiences of successful culture of freshwater fish using cladocerans to feed their larvae and postlarvae. Examples of this are the cultures of pacú, catfish, cachamas, bocachicos and the hybrid cachamoto (a cross between cachama and morocoto).

Cladocero Daphnia magna, a species used in aquaculture to feed larvae and postlarvae of freshwater fish and crustaceans. Taken and edited from Dieter Ebert, Basel, Switzerland, from Wikimedia Commons.

Environmental studies

An example of the importance of cladocerans in environmental impact studies is the Daphnia magna species, since it is one of the organisms most used as a bioindicator in this type of study.

Additionally, this and other species of cladocerans are easy to maintain and reproduce under laboratory conditions, so they can be used in toxicity bioassays.

These bioassays measure the tolerance levels of organisms to different concentrations of chemicals or contaminants. The results of these analyzes allow government entities responsible for environmental health to generate policies and establish maximum limits on chemical discharges into water.

References

1. Cladocera. Recovered from en.wikipedia.org.
2. FC Ramírez (1981). Cladocera. Atlas of zooplankton of the south-west Atlantic and working methods with marine zooplankton. Publication of the National Institute for Fisheries Research and Development (INIDEP, Ministry of Commerce and Maritime Interests, Undersecretariat of Maritime Interests, Argentine Republic. 936 pp.
3. JM Fuentes-Reines, E. Zoppi, E. Morón, D. Gámez & C. López (2012). Knowledge of the cladocera (Crustacea: Branchiopoda) fauna of the Ciénaga Grande de Santa Marta, Colombia. Marine and Coastal Research Bulletin.
4. Key to Australian Freshwater and Terrestrial Invertebrates. Recovered from keys.lucidcentral.org.
5. RC Brusca & GJ Brusca (1990). Invertebrates. Sinauer Associates: Sunderland. 922 pp.
6. WoRMS Editorial Board (2019). World Register of Marine Species. Recovered from.marinespecies.org.
7. J. Green. Branchiopod crustacean. Encyclopædia Britannica. Recovered from britannica.com
8. M. Prieto, L. De la Cruz & M. Morales (2006). Experimental culture of the cladocero Moina sp. fed Ankistrodesmus sp. and Saccharomyces cereviseae. Magazine MVZ Córdoba.
9. M. Núñez & J. Hurtado (2005). Acute toxicity bioassays using Daphnia magna Straus (Cladocera, Daphniidae) grown in modified culture medium. Peruvian Journal of Biology.