Order: Cyclopoida = Copepods. Copepods (Coreroda) Meaning in nature

Herbivorous copepods - Calanoida

Copepods (Copepoda) - the largest and most diverse group of crustaceans. Currently, they include 3 orders (kalanoids, cyclops, harpacticides), 210 families, 2300 genera and more than 14000 species, and this, of course, is not the full number of organisms that inhabit the seas and continental waters, transitional zones between water and land, or living in symbiotic relationships with other animals. They are the most numerous group of multicellular animals on earth, outnumbering even insects, which include more species but fewer individuals!

Copepoda are mobile, frisky and relatively large planktonic organisms. With the help of antennae and pectoral legs, striking them like oars, they "fly" in the water column. Their body is spindle-shaped with a clear division into two parts: the cephalothorax and the abdomen, which ends with a furka resembling a fork. An unpaired eye is located on the head, for which one of their most well-known groups to aquarists bears the name of the Cyclopes - after the mythical one-eyed giants. Most copepods are predators, attacking even smaller animals. But there are also herbivorous forms - kalanoids ( Calanoida), which have a larger cephalothorax and a shortened abdomen (see photo). Their anterior antennae are very long (sometimes more than the length of the body) and serve as the main organ of locomotion. They feed mainly on algae.

Some species of cladocerans are characterized by cyclomorphosis. Many species are found only in the period of open water, laying resting eggs for the winter - ephippia, from which in the spring, when the water temperature becomes acceptable, juveniles appear. They also use this during their life in water bodies that dry up: they are in the form of an embryo in the epippia until it rains.
Zooplankton lives in all water bodies. In stagnant waters, the zooplankton community - zooplanktonocenosis - is richer both in terms of the number of species and abundance. Cladocerans, as a rule, do not tolerate the current, therefore they prefer lakes, ponds, puddles, reservoirs, but rotifers better withstand dizzying pirouettes in the flow of water, therefore, in rivers, springs, plankton consists mainly of them.

Zooplankton is of particular importance in lake ecosystems, where its abundance and biomass reach significant values. In rivers, communities of planktonic invertebrates are formed in deep-water sections of the channel with a slow current, in kuryas, and floodplain reservoirs. On stretches and rifts where there is no zooplankton as such, planktonic invertebrates are found in drift and benthos.

Zooplankton and benthos are the main communities of invertebrates that ensure the normal functioning of aquatic ecosystems, their self-purification, and are the food base for many fish species. Zooplankton usually consists of three systematic groups of invertebrates: rotifers (Rotatoria), cladocerans (Cladocera), copepods (Copepoda). The same taxa of invertebrates are also present in the benthos, but due to the specifics of the generally accepted selection of benthic samples, as a rule, rotifers are not taken into account in benthic communities. Most species of crustaceans live both in the water column, being an integral part of zooplankton, and at the bottom of reservoirs, in benthos. So most kalanoids ( Calanoida) throughout their lives, except for the stage of resting eggs, lead a planktonic lifestyle; cyclops (Cyclopida) also inhabit the water column, and are a component of microzoobenthos; harpacticides ( Harpacticoida) are considered exclusively benthic animals, but are quite common in plankton. Therefore, speaking about the biodiversity of zooplankton organisms and their knowledge, we mean the diversity and knowledge of planktonic and benthic rotifers, cladocerans and copepods.

Order: Cyclopoida Burmeister, 1834 = Copepods

A detachment of copepods - Cyclopoida - is represented by the largest number of species in fresh waters.

Freshwater cyclops live in all kinds of water bodies, from small puddles to large lakes, and are often found in a very large number of specimens. The main zone of their habitat is the coastal strip with thickets of aquatic plants. At the same time, in many lakes, certain types of cyclops are confined to thickets of certain plants. So, for example, for Lake Valdai in the Ivanovo region, 6 groups of plants with their corresponding groups of cyclops species are described.

Relatively few species can be considered true planktonic animals. Some of them, belonging mainly to the genus Mesocyclops, constantly live in the surface layers of water, others (Cyclops strenuus and other species of the same genus) make regular daily migrations, descending to a considerable depth during the day.

Cyclopes swim a little differently than Calands. Simultaneously waving four pairs of pectoral legs (the fifth pair is reduced), the crustacean makes a sharp jump forward, up or sideways, and then, using the front antennae, can soar in the water for some time. Since the center of gravity of his body is shifted forward, while hovering, his front end tilts and the body can take a vertical position, and the dive slows down. A new swing of the legs allows the cyclops to rise. These swings are lightning fast - they take 1/60th of a second.

Most Cyclopes are predators, but there are also herbivorous species among them. Such common, widespread species as Macrocyclops albidus, M. fuscus, Acanthocyclops viridis and many others swim quickly above the bottom or among thickets in search of prey.

With the help of their antennae, at a very short distance, they sense small oligochaetes and chironomids, which they grab with their front jaws armed with spikes. The hind jaws and mandibles are involved in the transfer of food to the mandibles. The mandibles make quick cutting movements for 3-4 seconds, followed by a minute pause. Cyclops can eat oligochaetes and chironomids larger than themselves. The rate at which victims are eaten depends on their size and the hardness of their coverings. It takes 9 minutes to crush and swallow a 2 mm long bloodworm, and a 3 mm long larva is destroyed within half an hour. The more delicate, though longer (4 mm) oligochaete worm Nais is eaten in just 3.5 minutes.

Herbivorous cyclops, in particular the common Eucyclops macrurus and E. macruroides, feed mainly on green filamentous algae (Scenedesmus, Micractinium), capturing them in much the same way as predatory ones capture worms and bloodworms; in addition, various diatoms, peridiniums, and even blue-green algae are used. Many species can only eat relatively large algae. Mesocyclops leuckarti quickly fills its intestines with Pandorina colonies (colony diameter 50-75 microns) and almost does not swallow small Chlamydomonas at all.

Freshwater cyclones are very widespread. Some species are found almost everywhere. This is facilitated primarily by adaptations to endure unfavorable conditions, in particular, the ability of crustaceans to endure the drying up of water bodies and passively spread through the air in the form of cysts. The skin glands of many cyclops secrete a secret that envelops the body of the crustacean, often along with egg sacs, and forms something like a cocoon. In this form, crustaceans can be subjected to drying and freezing into ice without losing viability. In Kamerer's experiments, the cyclops were quickly eliminated by soaking dry sludge, which had been stored for about 3 years. Therefore, there is nothing surprising in the appearance of cyclops in spring puddles that occur when snow melts, in freshly flooded fish ponds, etc.

The second reason for the wide distribution of many species of cyclops should be considered the resistance of crustaceans, which are in an active state, in relation to the lack of oxygen in the water, its acidic reaction, and many other factors that are unfavorable for other freshwater animals. Cyclops strenuus can live for several days not only in the complete absence of oxygen, but even in the presence of hydrogen sulfide. Some other species also tolerate adverse gas conditions well. Many cyclops exist excellently in acidic water, with a high content of humic substances and extreme poverty of salts, for example, in reservoirs associated with upland (sphagnum) bogs.

Nevertheless, species and even genera of Cyclopes are known, limited in their distribution by some specific conditions, in particular temperature and salt conditions. For example, the genus Ochridocyclops lives only in Lake Ohrid in Yugoslavia, the genus Bryocyclops lives in Southeast Asia and equatorial Africa. Close to the latter genus is the exclusively subterranean genus Speocyclops, species of which have been found in caves and groundwater in southern Europe, Transcaucasia, the Crimea, and Japan. These blind small crustaceans are considered to be the remains of a once more widespread heat-loving fauna.

All V. are separate sexes; males usually differ from females in the structure of the front antennae, which serve them for grasping and holding females during copulation. The fifth pair of swimming legs also serves as an auxiliary organ during fertilization, specifically for gluing spermatophores, and then has a different shape than in females. The reproductive organs, both male and female, usually consist of an unpaired gland located in the cephalothorax, above the stomach, and for the most part of two more or less long and tortuous excretory canals, which open outward on the first abdominal segment. Spermatozoa come out in special capsules (spermatophores), which the male sticks to the so-called with the help of the hind legs. genital segment of females, sticking them into the genital openings or into the openings of special seed receivers (receptaculum seminis). The eggs are fertilized by spermatozoa as they are released. In the last part of the oviduct, a light and viscous mass hardens in water, which envelops the eggs that come out and sticks them together. Thus, two sacs with eggs are formed, attached to the right and left sides of the abdomen, which the female constantly carries with her (Fig. 1, Fig. 4 b); sometimes one unpaired pouch is formed. Inside these sacs, sticking out in the genital openings, the development of eggs takes place, and only with the release of the embryos are the sacs themselves destroyed. Development is accompanied by a rather complex and in many V. regressive metamorphosis. The embryos come out in the form of that larvae, typical of lower crustaceans, which is called the nauplius (see this next); they have an oval body with a small unpaired ocellus and three pairs of limbs, of which both posterior pairs terminate in two branches (Fig. 2).

I. Free-living V., to which, in fact, the previous description refers, with mouthparts that serve for chewing food, belong to the number of very ordinary animals. About 60 freshwater species are known in Europe; they are found in shallow stagnant or slowly flowing waters, sometimes in great numbers, and constitute the main food of some fish. Among the free-living marine copepods, some stick to the coast, others live on the high seas; sea ​​waters are sometimes found in myriad masses, which over a long distance give the surface of the sea a red or yellow color and serve as food for fish, for example. for herring and mackerel. The food of free V. is made up of small living animals. They are divided into 6 families containing about 500 species. Most freshwater V. belongs to the family Cyclopidae, to various species of the genus Cyclops; cyclops are up to 3 mm long (males are smaller), greenish, brown, and other colors (Fig. 1).

From the family Calanidae a species of Cetochilus septentrionalis, in European seas, often colors the sea reddish. Diaptomus castor - in fresh waters.

B. The second suborder, the tail-gills (Branchiura), differs in many respects from true copepods and contains only one family of karpoeds, Argulidae. Common carp (Fig. 6) (Argulus foliaceus) greenish in color, 5-6 mm long, lives on the skin of carps and other freshwater fish, sucking blood out of them; during the breeding season, karpoeds leave their host and swim freely.

1. Cyclops Cyclops coronatus, female, from the dorsal side: A′, A″ - first and second pair of antennae, In - intestines, Ov - egg sacs. 2. Cyclops larva in the Nauplius stage: A′, A″ - first and second pair of antennae, M - third pair of legs of the larva, future mandibles. 3. Perch eater Ac htheres percarum: a - nauplioid larva, b - female from the ventral side, c - male from the side, Mxf′, Mxf″ - first and second pair of jaw legs, In - intestines, Ov - ovaries, Kd - glands that secrete sticky liquid. 4. Chondracanthus gibbosus: a - female from the side, b - female from the ventral side, c - male from the side, An′ - first pair of antennae, An″ - second pair turned into hooks. F′, F″ - legs, A - eye, M - oral parts, Oe - esophagus, In - intestines, T - testes, Vd - their excretory duct, Sp - sac with spermatophores, Ov - egg sacs, ♀ - male, attached to the body of the female. 5. Lernaea branchialis: a - male (2-3 mm long), b - female (during the fertilization period, 5-6 mm long), c - female after fertilization, d - the same with egg cords, naturally. size, Oc - eyes, M - stomach, T - testicles, F′ - Fiv - swimming legs, Sp - bags with spermatophores, A′, A″ - 1st and 2nd pair of antennae, R - proboscis, Mxf - jaw legs. 6. Argulus foliaceus, male.

Subfamily Eucyclopinae
These include cyclops inhabiting ponds and other shallow water bodies, the littoral part of reservoirs and lakes. Their favorite biotype is a zone of thickets of coastal aquatic vegetation. Representatives of this family are usually absent in the nonlagial of large water bodies. In rivers, they are found in the coastal part. Animals lead a benthic lifestyle (climbing macrophytes, substrate), which is especially characteristic of representatives of the genera Ectocyclops and Paracyclops. In this regard, the body of the Cyclopes is strongly flattened in the dorso-abdominal part, and the antennae are shortened. In the water bodies of Moldova, the subfamily includes nine species and three subspecies. One is the most widespread.

Eucyclops serulatus - Eucyclops serrulatus
It is characterized by the presence of a group of small hairs on the sides of the last thoracic segment, a thin abdomen, the structure of the genital segment and the receptaculum seminis. The genital segment, for example, in the anterior section, is greatly expanded, in the posterior section it is almost cylindrical. The seed receptacle consists of two wide sections, in the middle part communicating with a narrow duct. The outer edges of the furcal branches are equipped with a number of small, well-defined sawtooth spines. Usually they do not reach the base of the branches.
A characteristic feature of the species is the presence of an awl-shaped bristle on the outer edge of the furca. Middle apical setae well developed.
Anterior antennae 12-segmented, short; three distal segments very long and thin. The legs of the fifth pair are made in the form of an angular plate bearing a powerful knife-like spike on the inside. The color of the cyclops is usually straw yellow, sometimes brown.
The egg sacs of the female are strongly divergent, elongated-oval. The male is distinguished by the absence of spines on the outer edge of the furca.
The length of the female is 0.8-1.5, the male is 0.6-0.8 mm. E. serrulatus forms two varieties - proximus and speratus. They differ from the typical form in the structure of the furcal branches and the armament of their outer edge. Both varieties are found in the reservoirs of Moldova.
E. serrulatus can be reliably attributed to cosmopolitans. It lives in Europe, Asia, North America, New. Zealand and Africa. In the USSR, it is distributed within all landscape zones, from the Arctic islands, the Far East, mountain water bodies of the Caucasus to steppe and lowland water bodies of the European part and Central Asia. The view is benthic. Inhabits the most diverse and flowing water bodies, in thickets of aquatic micro- and macrophytes, avoiding open places with strong currents. Found in terrestrial spring waters, as well as caves and deep wells.
This crustacean is eurythermic. The amplitude of its adaptability to temperature changes is very wide: 4-30°C. The boundaries of the optimal development of the species are 6-25°C. In relation to the degree of mineralization of water, E. serrulatus manifests itself to be highly euryhaline. Common in water bodies with a pH of 4.6-9.8. They are not found in dystrophic reservoirs with a pH below 4.5. However, a change in pH to the alkaline side causes a sharp reduction in the fecundity of this species.
In Moldova, it occurs in a variety of water bodies, often year-round. It does not reach high quantitative indicators of development. In small reservoirs, its abundance ranges from 4.0-10.0, in Lake Cahul - 18.0-22.0, in the left-bank tributaries of the Prut - 0.5 thousand ind./m?. Its density is somewhat higher in the Yagorlyk backwater - 3.5-6.2 thousand ind./m?. In the Dubossary reservoir, the crustacean is rare (3.0 thousand ind./m?).
The number of litters in E. serrulatus varies widely depending on the temperature of the habitat, the amount of food and its composition, the age of the female, etc. °С - 60 clutches (total number of eggs - 1270, 1489 and 2248, respectively). The same relationship has been established with respect to the embryonic development of the crustacean. At a temperature of 16 ° C, it lasts four, and at 30 ° C - one day. During starvation, the number of clutches is reduced from 24 to 14. Under optimal feeding conditions, the clutch averages 38.6-39.4 eggs;
The individual fecundity of the female varies between 25-60 eggs. The number of eggs in egg bags is rarely the same. E. serrulatus reaches sexual maturity at a water temperature of 18-20°C on the 15th-16th day of life. The naupliar period under these conditions lasts three to four days, and the copepodite period lasts five days. The total lifespan of a cyclops is 44-57 days. The length of newborn nauplii averages 0.088, daily growth is 0.020 mm.
Cyclops is included in the food spectrum of juveniles and older age groups of fish feeding in the coastal zone among macrophytes.

Subfamily Cyclopidae
Within Moldova, it is represented by four genera (Eucyclops, Acanthocyclops, Cyclops, Mesocyclops),
21 species and one subspecies. Cyclopes, belonging to this subfamily, are well distinguished due to the absence of a group of setae on the sides of the last thoracic segment and a sawtooth row of spines on the outer margin of the furcal branches. The armament of the distal segment of the fifth pair of legs consists of two appendages, among which the inner spine is sometimes strongly reduced.
In Moldova it is difficult to find a reservoir where representatives of the subfamily would not be found.

Cyclops vicinus - Cyclops vicinus
Widespread species. In the USSR, it is known within all landscape zones, excluding the subtropics. In the east it reaches the river basin. Amur. It occurs in the tundra, desert, semi-desert, estuaries of the Black and Azov Seas. In recent years, it has been found in the reservoirs of the Ferghana Valley.
In Moldova, it occurs everywhere, reaching mass development in some reservoirs.
The body of the crustacean is slender. The first three thoracic segments are smooth, the fourth with sharp, drawn-out angles. The genital segment is expanded in the upper part, with small protrusions. The first antennae of the female are 17-segmented, reaching the middle of the second thoracic segment. The legs of the fifth pair are two-segmented. Distal segment with lateral spine on inner margin and long apical seta with 4-5 small spines at base. The furcal branches are long, slightly diverging, with a continuous row of hairs on the inner margins. Of the outermost apical setae, the inner furca is nearly twice as long as the outer one (Pl. 3).

Tab. 3. Representatives of copepods and cladocerans: 1a - Daphnia magna female; 1b - male; 2 - Moina macrocopa; 3 - Chidorus sphaericus; 4 - Eudiaptomus gracilis; 5 - Cyclops vicinus; 6a - Limnocletodes behningi, female; 6b - male.

The length of the female is 1.2-2.2, the male is 1.1-1.5 mm.
The species is characteristic of the pelagic zone of lakes, reservoirs, ponds, and other small, mainly meso- and eutrophic water bodies. Cyclops avoids dystrophic reservoirs. Found in slow flowing rivers. The distribution in the water column is uneven and depends on the physicochemical and biological characteristics of the reservoir.
Cyclops vicinus tolerates high mineralization of water well. Indicated for brackish waters. It occurs at a Cl content of 0.31 - 5.52 g / l. It adapts well to fluctuations in CaO content in water. MgO, chlorides. In relation to the temperature factor, C. vicinus manifests itself as a eurythermal species, which is also confirmed by the example of water bodies in Moldova. Sexually mature cyclops in the water bodies of the republic are found throughout the year, the maximum number occurs in winter and spring, and winter females are larger than summer ones and contain more eggs in egg bags. For example, in Komsomolskoye Lake (Kishinev) Cyclops was recorded throughout the year, with an average frequency of occurrence of 80%. It reached the highest numerical development (70.5-73.6 thousand ind./m?) in December and January. The number of eggs per female during this period varied on average from 50 to 62 pieces, which is two, two and a half times more than in the summer population.
In the ponds of the republic, the number of adult individuals of C. vicinus ranges from 2 to 110 thousand ind./m? and more, and together with the larval stages reaches 250-300 thousand ind./m?. In small reservoirs, its level increases until March-April, amounting to 63-68 thousand ind./m?, and by summer it steadily decreases to 1.5-10.7 thousand ind./m?. Cyclops is constantly found in Lake Cahul and the Lower Dniester, but there are only a few hundred, less often thousands of specimens per cubic meter. In the Kuchurganskiy estuary-cooler of the Moldavian State District Power Plant and the Dubossary Reservoir, C. vicinus has been occasionally found in recent years at a low abundance.
The life expectancy of a crustacean under experimental conditions is a maximum of 83 days. During this time, it forms up to nine to ten pairs of egg sacs. Fertility varies widely: 30-101 eggs per female.
Under natural conditions, the average fecundity of 220 females of C. vicinus is 49, with a maximum of 111 eggs. In young females, the formation of the next pair of bags occurs every three to four, and in old females - eight to eleven days. Nauplius at birth reaches a length of 187 microns. At a temperature of 23-25°C, the transition to the copepodite stage occurs on the fourth or fifth day with a body length of 215 microns. Cyclops becomes sexually mature on the 25th day with a body length of 1.4 mm.
Cyclops vicinus is included in the food spectrum of many fish. In ponds, for example, in March, in the food boluses of yearlings of carp and crucian carp, there were respectively 9.0 and 6.9, and two-year-old crucian carp up to 29.9 thousand specimens of cyclops
Even in summer, in the presence of other nutritional components, their number in the food boluses of two-year-old carp and crucian carp reached 1.8-2.2 thousand copies. At that time, C. vicinus was more intensively consumed by the current-year-old fry of these fish (4.7-6.3 thousand ind./individual).

Acanthocyclops vernalis - Acanthocyclops vernalis
This is the most widespread species of the genus Acanthocyclops found in Moldova. It is widely distributed in the Holarctic. Indicated for the whole of Europe, Asia, North America, etc. In the USSR, it is recorded from the extreme north to the extreme south, including the mountainous region of the Caucasus, in the Asian part, in the north of Siberia and the Far East.
It lives in a wide variety of water bodies: snow and rain puddles, swamps, ponds, the coastal part of slowly flowing rivers and streams, lakes and reservoirs. Included in the fauna of the caves.
The species is characterized by significantly outwardly elongated posterior corners of the last thoracic segment, angular outlines of the genital segment, slightly divergent furcal branches, 17–18-segmented antennae, and armament of the distal leg segment of the fifth pair with a thick spine and short setae. Of the apical setae of the furcal branches, the innermost one is nearly as long as the outermost one.
Egg sacs are elongated, oval, slightly divergent. The color is yellowish or reddish to bright red.
In relation to water temperature, A. vernalis is eurythermic. Registered at a temperature of 1-32°C. Lives in water bodies with a pH of 4.4-8.2. Known in waters with CaO content reaching 100 mg/l with oxidizability up to 115 mg/l O2.
This species is mainly freshwater, rarely recorded in saline waters. It is polycyclic, in some shallow reservoirs it has two sexual periods per year, in permanent ones - one of them occurs in winter. In the dormant stage, it can tolerate the drying up of the reservoir.
The life span of individuals under experimental conditions reaches 76 days. The maximum fecundity was established in individuals from the Pridnestrovian fish farm (146 eggs) with a female body length (without furcal branches) of 1.56 mm. The average fecundity of the species, bred on the basis of counting the eggs of 82 females taken from the reservoirs of the republic at different times of the year, is 61 eggs.
At a temperature of 26–27°C, the duration of the naupliar period is two to three days; a mass transition to the copepodite stage is observed on the fourth day. Sexual maturity occurs on the 26th day with a body length of 1.14 mm. By way of feeding, C. vernalis is a typical predator.
In the water bodies of the republic, the species is distributed everywhere. It occurs all year round, but reaches its greatest abundance in the warm period. In ponds, at times it counts 125.0, in small reservoirs - 230.0, and in the Maramonov reservoir on the Kubolta River in July - 506.0 thousand ind./m². The mass appearance of males is observed in July.
Included in the food spectrum of fish of different ages.

1. Copepods. Harpacticoida available in the literature data should be attributed to the eurythermal complex with...
2. Crustacean Maxillopods. Copepods. Within Moldova, there are two species of Maxilopod crustaceans - these are ...
3. Copepods. Calanoida Copepods Calanids are exclusively planktonic, predominantly marine animals....

Characteristics of the detachment Copepods -Copepoda

The body of free-living copepods is divided into cephalothorax, thorax, and abdomen. The head is fused, without any traces of segmentation, fuses with the first thoracic segment, forming the cephalothorax. The front end of the head is often extended into a beak curved down, or rostrum. The absence of paired compound eyes is very characteristic; only the naupliar ocellus is located on the frontal part of the head. It was this circumstance that allowed the Danish naturalist Müller to call ordinary freshwater copepods "cyclops" in honor of the one-eyed giant of Greek mythology.

The head is equipped with 5 pairs of appendages. The anterior antennae are often very long, sometimes longer than the body, and are involved in swimming and soaring of crustaceans. In addition, they also perform the functions of the sense organs: sensitive bristles and cylindrical sensitive appendages sit on them. The posterior antennae are short, usually biramous. Mandibles are powerful and have a two-branched palp. Their chewing, highly chitinized part has sharp teeth that help break up food. A close examination of the teeth of the chewers of some marine copepods revealed that these teeth are covered with flint crowns, which increase their strength. The opening of flint crowns is interesting in two respects. First, it indicates the ability of copepods to absorb and concentrate silicon; almost all higher invertebrates - worms, molluscs, and other arthropods - are deprived of such an ability. Secondly, one can hope to find in geological deposits flint crowns of ancient copepods, almost completely not preserved in a fossil state.

The anterior jaws of copepods are very complex, as they are equipped with internal and external lobes and numerous feathery setae. The hind jaws have only internal lobes and also numerous setae. The head appendages are joined by a pair of single-branched mandibles belonging to the anterior thoracic segment merged with the head.

The posterior antennae, mandible palps, and forelimbs of filtering copepods make frequent and continuous strokes, creating water cycles that bring suspended food particles. These particles are filtered out mainly by the setae of the hind jaws.

The thoracic region consists of 5 segments with clearly visible boundaries between them. All 5 pairs of pectoral legs in primitive copepods are arranged in the same way. Each leg consists of a 2-segmented main body and two usually 3-segmented branches armed with spines and setae. These legs make simultaneous strokes, acting as oars and pushing the body of the crustacean from the water. In many more specialized species, the male's fifth pair of legs has been modified into an apparatus adapted to hold the female during mating and attach spermatophores to her genital openings. Often the fifth pair of legs is reduced.

The abdominal region consists of 4 segments, but in females their number is often less, since some of them merge with each other. A paired or unpaired genital opening opens on the anterior abdominal segment, and in the female this segment is often larger than the others. The abdomen ends in a telson with which the furcal branches are articulated. Each of them is armed with several very long, sometimes feathery setae. These bristles are especially strongly developed in planktonic species, in which they are adapted for soaring in water, as they prevent the crustacean from sinking.

Respiration of copepods is carried out by the entire surface of the body, there are no gills. Possibly, the weak development or even the absence of the circulatory system is also associated with this. Only representatives of the suborder Calanoida have a heart, and even they have a small one, although it beats very often: for example, in the marine crustacean Labidocera, it makes more than 150 beats per minute. In other copepods, the abdominal fluid is set in motion by contractions of the intestine.

During mating, the male holds the female with the fifth pair of thoracic legs and the first antennae and, using the same fifth pair of legs, glues the sausage-shaped spermatophore near her genital openings, i.e., to the underside of the first abdominal segment. In some species, one of the branches of the male's fifth pair of legs is equipped at the end with tweezers that capture the spermatophore and transfer it to the right place. From the spermatophore, the sperm enters the female's seminal receptacle. When eggs are laid, they are fertilized.

In some copepods, one is formed in this way, in others - two egg sacs, which the female wears on herself until the larvae leave the eggs.

A larva emerges from an egg nauplius. The larva molts many times and gradually approaches in its characteristics to an adult crustacean. There are 12 larval stages of copepods. The first two stages - orthonauplius - are characterized by the presence of only both pairs of antennae and a pair of mandibles, in the next four stages - metanauplius - the remaining oral appendages are laid and develop, but the body remains unsegmented. The last 6 stages are called copepodite and are distinguished by segmentation of the posterior end of the body and the gradual development of the pectoral legs. Different copepods require different times to complete metamorphosis, and the biology of larvae is far from being the same for all species.

The way of life, way of feeding and habitat of copepods are so diverse that it is better to consider this order not as a whole, but each of its suborders separately.

Free-living copepods belong to the suborders Calanoida, Cyclopoida, and Harpacticoida. Representatives of all three suborders inhabit both the seas and fresh waters.