Copepod

Copepods (meaning "oar-feet") are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic (drifting in sea waters), some are benthic (living on the ocean floor), 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 (phytotelmata) of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Copepods are sometimes used as bioindicators (see particle (ecology)).

Ecology
Planktonic copepods are important to global ecology and the carbon cycle. They are usually the dominant members of the zooplankton, and are major food organisms for small fish, whales, seabirds and other crustaceans such as krill in the ocean and in fresh water. 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, and to the global ocean carbon sink than krill, and perhaps more than all other groups of organisms together. The surface layers of the oceans are currently believed to be the world's largest carbon sink, absorbing about 2 billion tons of carbon a year, the equivalent to perhaps a third of human carbon emissions, thus reducing their impact. Many planktonic copepods feed near the surface at night, then sink (by changing oils into more dense fats) into deeper water during the day to avoid visual predators. Their moulted exoskeletons, faecal pellets and respiration at depth all bring carbon to the deep sea.

About half of the estimated 13,000 described species of copepods are parasitic and have strongly modified bodies. They attach themselves to fish, sharks, marine mammals, and many kinds of invertebrates such as molluscs, tunicates, or corals. They live as endo- or ectoparasites on fish or invertebrates in fresh water as well as in marine environments.

Characteristics


Copepods are typically 1 to 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 thin armour, and the entire body, is almost totally transparent. Some polar copepods reach 1 cm. Copepods have a compound, median single eye, usually bright red and in the centre of the transparent head; subterranean species may be eyeless. Like other crustaceans, copepods possess two pairs of antennae; the first pair are often long and conspicuous.

Copepods typically have a short, cylindrical body, with a rounded or beaked head. The head is fused with the first one or two thoracid segments, while the remainder of the thorax has from three to five segments, each with limbs. The first pair of thoracic appendages are modified to form maxillipeds, which assist in feeding. The abdomen is typically narrower than the thorax, and contains five segments without any appendages, except for some tail-like "rami" at the tip.

Because of their small size, copepods have no need of any heart or circulatory system (the members of the order Calanoida have a heart but no blood vessels), and most also lack gills, being able to absorb oxygen directly into their bodies. Their excretory system consists of maxillary glands.

Physiology
Copepods have a variety of sensory capabilities. The most noteworthy are bristle-like setae that act as mechanoreceptors responding to flow that causes bending. An array of such sensors allows detection of patterns of water flow around the body caused by approaching prey or predator, and the copepod can distinguish between the two. The sensors are highly specialized for sensitivity and the nerves are even myelinated for fast conduction.

Behavior
The second pair of antennae are the main source of propulsion, beating like oars to pull the animal through the water.

Many of the smaller copepods feed directly on phytoplankton, catching cells singly. Some of the larger species are predators of their smaller relatives. Many benthic copepods eat organic detritus or the bacteria that grow in it, and their mouth parts are adapted for scraping and biting. Herbivorous copepods, particularly those in rich cold seas, store up energy from their food as oil droplets while they feed in the spring and summer plankton blooms. These droplets may take up over half of the volume of the body in polar species.

This scene was scanned with the ecoSCOPE, an underwater high-speed microscope. Very little is known about the details of these kinds of predator/prey interactions, in spite of their 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 from normal camera systems or scuba divers.

Some copepods have extremely fast escape responses when a predator is sensed and can jump with high speed over a few millimeters (see animated image). Many species have neurons surrounded by myelin (for increased conduction speed), which is very rare among invertebrates (other examples are some annelids and malacostracan crustaceans like palaemonid shrimp and penaeids). Even rarer, the myelin is highly organized, resembling the well-organized wrapping found in vertebrates (Gnathostomata).

Finding a mate in the three-dimensional space of open water is challenging. In some copepods the problem is solved by pheromone chemicals emitted by the swimming female, which leaves a trail in the water that the male can follow to locate the female.

Life cycle
During mating, the male copepod grips the female with his first pair of antennae, which is sometimes modified for this purpose. The male then produces an adhesive package of sperm and transfers it to the female's genital opening with his thoracic limbs. Eggs are sometimes laid directly into the water, but many species enclose them within a sac attached to female's body until they hatch. In some pond-dwelling species, the eggs have a tough shell and can lie dormant for extended periods if the pond dries up.

The eggs hatch into nauplius larvae, which consists of a head with a small tail, but no thorax or true abdomen. The nauplius moults five or six times, before emerging as a "copepodid larva". This stage resembles the adult, but has a simple, unsegmented abdomen and only three pairs of thoracic limbs. After a further five moults, the copepod finally takes on the adult form. The entire process from hatching to adulthood can take anything from a week to a year, depending on the species.

Classification
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 who studies copepods is a copepodologist.

Copepods in marine aquariums
Live copepods are used in the saltwater aquarium hobby as a food source and are generally considered beneficial in most reef tanks. They are scavengers and also may feed on algae, including coralline algae.

Live copepods are popular among hobbyists who are attempting to keep particularly difficult species such as the mandarin dragonet, or to breed other marine species in captivity. In a saltwater aquarium, copepods are typically stocked in the refugium.

Water supplies
Copepods are sometimes found in the public mains water supply, especially systems where the water is not filtered, such as New York City, Boston, Massachusetts, and San Francisco. 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 larvae of the guinea worm must develop within a copepod's digestive tract before being transmitted to humans. The risk of infection with these diseases can be reduced by filtering out the copepods (and other matter), for example with a cloth filter.

Copepods have been used successfully in Vietnam to control disease-bearing mosquitoes such as Aedes aegypti that transmit dengue fever and other human parasitic diseases.

The copepods can be added to water-storage containers where the mosquitoes breed. Copepods, primarily of the genera Mesocyclops and Macrocyclops (such as Macrocyclops albidus), can survive for periods of months in the containers, if the containers are not completely drained by their users. They will attack, kill, and eat the younger 1st and 2nd instar larvae of the mosquitoes. This biological control method is complemented by community trash removal and recycling to eliminate other possible mosquito-breeding sites. Because the water in these containers is drawn from uncontaminated sources such as rainfall, there is little risk of contamination by cholera bacteria, and in fact no cases of cholera have been linked to copepods introduced into water-storage containers. Trials using copepods to control container-breeding mosquitoes are underway in several other countries, including Thailand and the southern United States.

The matter of copepods in the water supply, however, has raised a problem for Jewish people who observe Kashrut in that copepods, being crustaceans, are not kosher, and are not small enough to be ignored as non-food microscopic organisms (since some specimens can be seen with the naked eye). The discovery of copepods in the New York water supply in the summer of 2004 in particular caused significant debate in rabbinical circles and caused many observant Jews to buy filters for their water.