Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic, but more are benthic, and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds and puddles, damp moss, or water-filled recesses (phytothelmata) of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Some copepods are parasitic and attach themselves to fish, sharks, marine mammals, and many kinds of invertebrates such as molluscs, tunicates, or corals.

Copepods form a subclass belonging to the subphylum Crustacea (crustaceans). Some authors consider the copepods to be a full class. The group contains ten orders with some 14,000 described species. A scientist that studies copepods is a copepodologist.

Planktonic copepods are hugely important food organisms for small fish, whales, seabirds and other crustaceans such as krill in the ocean and in fresh water. They are typically 1-2 mm long, with a teardrop shaped body and large antennae. Although like other crustaceans they have an armoured exoskeleton, they are so small that in most species this armour, and the entire body, is almost totally transparent. Copepods have a single eye, usually bright red and in the centre of the transparent head. Some polar copepods reach 1 cm. Most of the smaller copepods feed directly on phytoplankton, catching cells singly, but a few of the larger species are predators of their smaller relatives. Herbivorous copepods, particularly those in rich cold seas, store up energy from their food as oil droplets in the while they feed in the spring and summer plankton blooms. These droplets may take up half of the volume of the body. Some scientists say they form the largest animal biomass on earth. They compete for this title with Antarctic krill (Euphausia superba). Because of their smaller size and relatively faster growth rates, however, and because they are more evenly distributed throughout more of the world's oceans, copepods almost certainly contribute far more to the secondary productivity of the world's oceans than krill do.

Many planktonic copepods feed near the surface at night, then sink into deeper water during the day. Their moulted exoskeletons, faecal pellets and respiration at depth all bring carbon to the deep sea, and copepods are abundant enough to have an impact on the carbon cycle, and be significant to climate change.

Copepods are sometimes found in the public mains water supply, especially systems where the water is not filtered, such as New York City and Boston, Massachusetts. This is not usually a problem in treated water supplies. In some tropical countries, such as Peru and Bangladesh, a correlation has been found between copepods and cholera in untreated water, because the cholera bacteria attach to the surfaces of planktonic animals. The risk of cholera from infected water can be reduced by filtering out the copepods (and other matter), for example with a cloth filter.

Many species have neurons surrounded by myelin, which is very rare among invertebrates (other examples are some annelids and malacostracan crustaceans like palaemonid shrimp and penaeids). Even rarer is the fact that the myelin is highly organized, resembling the well-organized wrapping found in vertebrates (Gnathostomata).

Some copepods are very evasive and can jump with extreme speed over a few millimeters (warning: takes some time to load to the correct speed):

This scene was scanned with the ecoSCOPE, an underwater high speed microscope. Because clupeids (herrings) and copepods are both significant in terms of global biomass, this is the first record of what is one of the largest carbon flows of any animal food-chain transition in the oceans. It is a predator / prey relationship running at extreme speeds, with chances for both sides. Very little is known about the details, in spite of its importance for global processes, because copepods are difficult to keep in the laboratory and lose most of their escape capacity, and herring are very fast, alert and evasive organisms and flee normal camera systems or SCUBA divers. Oceanographers point out the importance of learning more about the influences of physical parameters, such as light, pollution or temperature, or the effects of hunting in a swarm. This copepod-herring relationship has implications for the fate of carbon in marine systems. If the copepod 'wins' (escapes), much carbon will sink with its fecal pellets into the depth of the oceans (see biological pump), sequestering carbon dioxide in the benthos. If the herring 'wins', much carbon will remain in pelagic foodchains and return to the atmosphere via respiration.

For the use of copepods as bioindicators, see particle (ecology).

External links

• Diversity and geographical distribution of pelagic copepoda
• Rudi Strickler copepod videos
• The virtual copepod page
• Copepod World
• Copepods and cholera in untreated water
• CNN report: Kosher water

Crustaceans

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