COPEPODA: CHARACTERISTICS, HABITAT, LIFE CYCLE AND APPLICATIONS - BIOLOGY - 2025

The copepods (Copepoda) are small crustaceans, usually water (class Maxillopoda), who live in saltwater and freshwater. Some species can inhabit very humid terrestrial places such as mosses, mulch, litter, mangrove roots, among others.

Copepods are generally a few millimeters or less in length, have elongated bodies, narrower at the back. They constitute one of the most numerous groups of metazoans on the planet with around 12,000 described species. Its collective biomass exceeds billions of metric tons in the global marine and freshwater habitat.

Figure 1. Calanoid copepod (ovigerous sacs are seen in blue). Source: flickr.com/photos//3390084439

Most are planktonic (they inhabit superficial and intermediate areas of water bodies), while others are benthic (they inhabit the bottom of water bodies).

General characteristics

Size

Copepods are small, with dimensions generally between 0.2 and 5 mm, although exceptionally some can measure up to a few centimeters. Their antennae are often longer than their other appendages and they use them to swim and fixate on the water-air interface.

The largest copepods are often parasitic species, which can measure up to 25 centimeters.

Figure 2. Diversity of copepods, image illustrated by the eminent zoologist Ernst Haeckel. Source: Ernst Haeckel

Male copepods are generally smaller than females and appear less abundantly than females.

Body shape

An approximation of the basic shape of most copepods, it conforms to an ellipsoid-spheroid in the anterior part (cephalothorax) and a cylinder in the posterior part (abdomen). The anténula is roughly cone-shaped. These similarities are used to calculate the body volume of these crustaceans.

The bodies of most copepods are clearly divided into three tagmata, whose names vary between authors (tagmata is the plural of tagma, which is a grouping of segments in a morphological-functional unit).

The first region of the body is called the cephalosome (or cephalothorax). Includes the five fused head segments and one or two additional fused thoracic somites; in addition to the usual appendages and maxillipeds of the head.

All other limbs arise from the remaining thoracic segments, which together constitute the metasoma.

The abdomen or urosome has no limbs. The regions of the body that carry appendages (cephalosome and metasome) are often collectively referred to as the prosoma.

Copepods with a parasitic habit usually have highly modified bodies, to the point of being practically unrecognizable as crustaceans. In these cases, the ovigerous sacks are usually the only vestige that reminds them that they are copepods.

Basic taxonomic forms

Among the free-living copepods, three basic forms are recognized, which give rise to their three most common orders: Cyclopoida, Calanoida and Harpacticoida (they are normally called cyclopoids, calanoids and harpacticoides).

Calanoids are characterized by a major flexion point of the body between the metasome and the urosome, marked by a distinctive narrowing of the body.

The flexion point of the body in the orders Harpacticoida and Cyclopoida, is located between the last two segments (fifth and sixth) of the metasoma. Some authors define the urosome in harpacticoids and cyclopoids, as the region of the body posterior to this point of flexion).

Figure 3. Basic forms of the most important copepod orders, the point of flexion is highlighted in red. (A) Cyclopoida (B) Calanoida (C) Harpacticoida. Source: self made.

Harpacticoids are generally vermiform (worm-shaped), with the posterior segments not much narrower than the anterior ones. Cyclopoids generally taper steeply at the main flexion point of the body.

Both the antennas and the anténules are quite short in harpacticoids, medium in size in cyclopoids and longer in calanoids. The antennas of the cyclopoids are uniramias (they have one branch), in the other two groups they are birramos (with two branches).

Habitat

About 79% of the described species of copepods are oceanic, but there are also a large number of freshwater species.

Copepods have also invaded a surprising variety of continental, aquatic and humid environments and microhabitats. For example: ephemeral water bodies, acidic and hot springs, subterranean waters and sediments, phytotelmata, wet soils, litter, man-made and artificial habitats.

Most calanoids are planktonic, and as a group they are extremely important as primary consumers in food webs, both freshwater and marine.

Harpacticoids have dominated all aquatic environments, are usually benthic, and are adapted to a planktonic lifestyle. In addition, they show highly modified body shapes.

Cyclopoids can inhabit fresh and salt water, and most have a planktonic habit.

Lifecycle

Reproduction

The eggs develop giving rise to a non-segmented larva called nauplii, very common in crustaceans. This larval form is so different from the adult, that formerly it was thought that they were different species. To discern these problems, one must study the entire development from egg to adult.

Figure 4. Nauplius larva of a copepod. Source: Lithium57, via Wikimedia Commons

Molting cycle

Copepods can present a state of arrested development, called latency. This state is triggered by unfavorable environmental conditions for their survival.

The state of latency is genetically determined, so that when adverse conditions arise, the copepod will enter this state necessarily. It is a response to predictable and cyclical changes in habitat, and begins at a fixed ontogenetic stage that depends on the copepod in question.

Latency allows copepods to overcome unfavorable times (low temperatures, lack of resources, drought) and reappear when these conditions have disappeared or improved. It can be considered as a life cycle “buffer” system, allowing survival in unfavorable times.

In the tropics where periods of intense drought and rain often occur, copepods generally present a form of dormancy in which they develop a cyst or cocoon. This cocoon is formed from a mucous secretion with attached soil particles.

As a life history phenomenon in the Copepoda class, latency varies considerably in relation to taxon, ontogenetic stage, latitude, climate, and other biotic and abiotic factors.

Ecological paper

The ecological role of copepods in aquatic ecosystems is of utmost importance, as they are the most abundant organisms in zooplankton, having the highest total biomass production.

Nutrition

They come to dominate the trophic level of consumers (phytoplankton), in most aquatic communities. However, although the role of copepods as herbivores that basically feed on phytoplankton is recognized, most also present omnivory and trophic opportunism.

Nutrient cycling

Copepods often make up the largest component of secondary production at sea. It is believed that they can represent 90% of all zooplankton and hence their importance in trophic dynamics and carbon flux.

Marine copepods play a very important role in nutrient cycling, as they tend to eat at night in the shallower area and descend to deeper waters during the day to defecate (a phenomenon known as “daily vertical migration”).

Figure 5. Diversity of forms in parasitic copepods. Source: Scott, Thomas; Ray Society; Scott, Andrew, via Wikimedia Commons

Parasitism

A large number of copepod species are parasites or commensals of many organisms, including porifers, coelenterates, annelids, other crustaceans, echinoderms, mollusks, tunicates, fish, and marine mammals.

On the other hand, other copepods, mostly belonging to the Harpacticoida and Ciclopoida orders, have adapted to permanent life in subterranean aquatic environments, in particular interstitial, spring, hyporheic and phreatic environments.

Some species of free-living copepods serve as intermediate hosts for human parasites, such as Diphyllobothrium (a tapeworm) and Dracunculus (a nematode), as well as other animals.

Predators

Aquaculture

Copepods have been used in aquaculture as food for marine fish larvae, because their nutritional profile seems to match (better than the commonly used Artemia), with the requirements of the larvae.

They have the advantage that they can be administered in different forms, either as nauplii or copepodites, at the beginning of feeding, and as adult copepods until the end of the larval period.

Their typical zigzag movement, followed by a short glide phase, is an important visual stimulus for many fish that prefer them to rotifers.

Another advantage of using copepods in aquaculture, especially of benthic species, such as those of the genus Thisbe, is that nonpredated copepods keep the walls of fish larval tanks clean by grazing algae and debris.

Several species of the calanoid and harpacticoid groups have been studied for their massive production and use for these purposes.

Pest control

Copepods have been reported as effective predators of mosquito larvae associated with the transmission of human diseases such as malaria, yellow fever and dengue (mosquitoes: Aedes aegypti, Aedes albopictus, Aedes polynesiensis, Anopheles farauti, Culex quinquefasciatus, among others).

Some copepods of the family Cyclopidae systematically devour mosquito larvae, reproducing at the same rate as these and thus maintaining a constant reduction in their populations.

This predator-prey relationship represents an opportunity that can be exploited to implement sustainable biological control policies, since by applying copepods the use of chemical agents, which can have adverse effects on humans, is avoided.

It has also been reported that copepods release volatile compounds into water, such as monoterpenes and sesquiterpenes, which attract mosquitoes to oviposit, which constitutes an interesting predation strategy for use as an alternative for biological control of mosquito larvae.

In Mexico, Brazil, Colombia and Venezuela some species of copepods have been used for mosquito control. Among these species are: Eucyclops speratus, Mesocyclops longisetus, Mesocyclops aspericornis, Mesocyclops edax, Macrocyclops albidus, among others.

Bioaccumulators

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