Division Crustacean. Class Crustacean. Body length 0.1 mm to 3 cm. The antennae are long, serve as sensory organs, and are used for swimming. Crayfish move with the help of thoracic limbs. Reproduction is sexual. More than 1800 species. Inhabit marine and freshwater reservoirs. In freshwater reservoirs species of the genus Cuclors are common. They make up the main part of the zooplankton. The copepods are the main food of many planktonic fish. Some of the copepods are intermediate hosts for helminths.

Сopepods (koʊp means 'copepods') are a group of small crustaceans that live in almost all fresh and salt water bodies. Some species are planktonic (living in marine waters), some are benthic (living on the sea floor), some species have parasitic phases, and some continental species can live in wet land areas such as swamps, under leaf litter in wet forests, marshes, springs, temporary ponds, puddles, wet moss or water-filled plant depressions (phytotelmata). Many live underground in marine and freshwater caves, sinkholes or streambeds. Сopepods are sometimes used as biodiversity indicators. Like other crustaceans, copepods are larval. In paddle clams, a larva hatches from an egg with a head and a tail, but no real thorax or abdomen. The larva molts several times until it resembles an adult and then, after several more molts, reaches the development of an adult. The shape of the larva is so different from that of the adult that it was once considered a separate species.

Copepods vary greatly, but are usually between 1 and 2 mm long, with a tear-shaped body and large antennae. Like other crustaceans, they have an armoured exoskeleton, but they are so small that in most species this thin armour and the whole body are almost completely transparent. Some polar copepods can reach up to 1 cm in length. Most copepods have a central compound eye, usually bright red, in the centre of the transparent head. Subterranean species may be eyeless, while members of the genera Copilia and Corycaeus have two eyes, each with a large anterior cuticular lens paired with a posterior inner lens forming a telescope. Like other crustaceans, copepods have two pairs of antennae; the first pair is often long and prominent.          Free-living copepods of the classes Calanoida, Cyclopoida and Harpacticoida usually have a short cylindrical body with a rounded or beaked head, although there is considerable variation. The head is fused to one or two first thoracic segments, while the remainder of the thorax consists of three to five segments, each with limbs. The first pair of thoracic appendages are modified to form maxillary limbs to assist in feeding. The abdomen is usually narrower than the thorax and contains five segments without appendages, except for a few tail-like 'branches' at the tip. The parasitic copepods (the other seven classes) vary greatly in morphology and cannot be generalised. Because of their small size, copepods do not need a heart or circulatory system (members of the order Calanoida have a heart but no blood vessels), and most lack gills. Instead, they take in oxygen directly through their bodies. Their excretory system consists of maxillary glands.

The second pair of head appendages in free-living copepods is usually the main source of time averaged movement, beating like oars to pull the animal through the water. However, different groups have different feeding and movement patterns, ranging from almost motionless for a few minutes (e.g. some harpacticoid copepods crustaceans), to intermittent movements (e.g. some cyclopoid copepods crustaceans), to continuous movements with some escape reactions (e.g. most calanoid copepods crustaceans). Slow motion macro video (50%) taken with an ecoscope showing juvenile Atlantic herring (38 mm) feeding on paddlefish - fish coming up from below and catching each paddlefish individually. In the centre of the image the copepods successfully escape to the left. Some copepods have an extremely fast escape response when a predator is detected, and can jump at high speed over a distance of a few millimetres. Many species have neurons surrounded by myelin (to increase conduction speed), which is very rare in invertebrates (other examples include some ringworms and small crustaceans such as palaemonid shrimps and penaeids). Even more rarely, myelins are highly organised, resembling the well-organised shell of vertebrates (Gnathostomata). Despite their fast escape response, copepods are successfully hunted by slow swimming seahorses, which approach their prey so slowly that it does not feel the turbulence, and then suck the copepod into their mouth too suddenly for the paddlefish to escape. Finding a mate in the three-dimensional space of open water is not easy. Some female copepods solve the problem by emitting pheromones that leave a trail in the water for the male to follow.

Most free-living copepods feed directly on phytoplankton, capturing cells one at a time. A single copepod can consume up to 373,000 phytoplankton per day. They typically need to clear about a million times their body volume of water each day to meet their nutritional needs. Some of the larger species are predatory towards their smaller relatives. Many benthic copepods feed on organic detritus or bacteria that grow in it, and their mouthparts are adapted for scraping and biting. Plant-eating copepods, especially those that live in colder seas, store energy from their food in the form of oil droplets and feed on blooming plankton in spring and summer. In polar species, these droplets can take up more than half of their body volume. Many copepods (e.g. fish lice such as Siphonostomatoida) are parasitic and feed on the organisms of their hosts. In fact, three of the 10 known copepod species are entirely or largely parasitic, while another three make up the majority of free-living species.

All copepods are sexually differentiated. Males are usually smaller than females and, in free-living species, differ from females in having curved (one or both) anterior antennae and a different structure of the fifth pair of thoracic legs. The males and females of many parasitic species are particularly different. In some, females attached to the host lose their segmentation and limbs completely, while small males retain the appearance of late larvae and swim freely. In others (family Lernaeopodidae), small males, also resembling late larvae, attach themselves to large worm-like parasitic females with their pincer-like jaws and foot jaws. During mating, the male holds the female with the fifth pair of thoracic legs and the first antennae and uses the same fifth pair of legs to attach a sausage-like spermatophore near the genital foramen, i.e. on 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 pincers that grasp the spermatophore and carry it to the desired location (Figure 204). From the spermatophore, the sperm enters the female's spermatophore. When the eggs hatch, they are fertilised. Most marine planktonic species lay their eggs directly into the water, but in all freshwater and parasitic species, as well as bottom-dwelling and coastal species, the eggs are glued together by a special secretion and attached near the female genitalia. Some paddlefishes form one or two egg sacs in this way, which the female carries with her until the larvae emerge from the eggs. A nauplius larva emerges from the egg. The larva molts several times and gradually takes on the characteristics of an adult crayfish. There are 12 larval stages of the paddlefish. The first two stages, ortonauplius, are characterised by the presence of only both pairs of antennae and one pair of stomata, while the next four stages, metanauplius, have the remaining oral appendages established and developing, but the body remains unsegmented. The last 6 stages are called copepodite stages and are characterised by the segmentation of the hind end of the body and the gradual development of the thoracic legs. Different paddlefish take different amounts of time to complete metamorphosis and the larval biology is not the same in all species.