The copepods are crustaceans, like shrimps or crabs, with the segmented body covered with a chitinous carapace similar to the pieces of an armour. Their size ranges from some tenths of a millimetre to a couple of centimetres, although most commonly they are around one millimetre in length. In general, their body is semitransparent, but in occasions can accumulate pigments or droplets of reserve oil, and then have different colours, from reddish or green, to black or iridescent. They are the most frequent organisms of the zooplankton, can be found at all latitudes and depths, and are considered to be the most abundant metazoans in our planet, even more than insects.

Centropages violaceus            

Copepod in the process of molting. The old exoskeleton can be seen on the right side.


As all planktonic organisms, copepods live suspended in water, although some species live in the bottom, or are parasites of other marine animals. Their name Copepod derives from the Greek, and means "limbs like rows", due to the rhythmic motion of their appendices when swimming (koipe means row).



Female (right) and male (left) of the copepod Acartia grani. This species bears a strong sexual dimorphism. Notice in the male the modified antenna (on the right of the picture), which can bent to grasp the female, and the long fifth pair of legs, used to transfer the spermatophore onto the female.


In their body we can distinguish an anterior, oblong part (the metasome), and a thinner posterior part, the urosome. In the anterior part there are the antennae, the feeding appendages, four or five pairs of swimming legs and, in some species, two simple, united eyes. The posterior part has no appendages and ends in a double feather-like furca. However, there are many variations in this general structure, mainly regarding parasitic forms. Frequently, male and females differ in size (males are usually smaller) and also in some details of the antennae and the fifth pair of legs, which are used by males in the copula.


The fecundation in copepods is internal. Males attach to the female abdomen the spermatophore, a bag-like structure containing spermatozoids. The spermatozoids will eventually enter the female gonoduct, reach the oocites and fertilize them. They reproduce by eggs, either laid free in the water (free spawners) or carried by the females until hatching (broadcasters), depending of the species. The egg gives birth to a small larval form, the nauplius, which grow and develop into copepodites and adults. Once the copepod reaches the adult stage, growth stops. Like in all crustaceans, the development and growth of copepods involve molting.


Female of the genus Clausocalanus with an attached spermatophore (highlighted)


In broadcasting females the eggs are carried externally, either kept together by a veil or mucus (egg mass, this photograph), or surrounded by a membrane (egg sac).


Detail of the egg mass of a female copepod Clausocalanus furcatus.

Eggs and nauplius of Acartia grani, a free-spawning copepod. Eggs are ca. 70 microns in diameter.


In the antennae there are arrays of hairy structures, some of them feather-like, which play the role of sensory organs. With these structures the copepods are able to detect the minute changes in water movement due to the presence of other organisms (mechanoreception), or "smell" dissolved chemical substances (chemoreception), thus obtaining information about the presence of food, predators or mates.

Scanning electron microscope photograph of the antenna of a male copepod. The bristles are chemo- and mecanoreceptors.


A great deal of the success of copepods in marine environments is due to their capacity to use efficiently the available food. The majority of copepods are herbivorous and feed on microscopic algae, the phytoplankton, although some are carnivorous or even feed on detritus. Phytoplankton, however, is a source of food of quite low quality and highly diluted (the proportion is about one part of phytoplankton in one million parts of water by volume).

The phytoplankter Coscinodiscus sp.

In order to obtain their food, many copepods create water currents which transport particles to the mouth, where they are concentrated, sorted and captured with the feeding appendages. Although the morphology and movements of the mouthparts suggest that copepods gather food by mechanically filtering water, in fact they can individually select food items according to its abundance, size and nutritive value. All this information is obtained with the mechanical and chemical sensors distributed in the antennae and mouthparts.

Feeding appendages of the copepod Oithona sp. Picture taken with a scanning electron microscope.
Detail of the mouth of the same Oithona sp. Notice that the size of the mouth is a. 10 microns.
THEIR ROLE                                            

Copepods play a paramount role in the marine food webs, because they are the main link between the autotrophic phytoplankton (the microscopic plants synthesize organic matter using light, carbon dioxide and small amounts of inorganic N and P), and larger heterotrophic organisms like fish.This direct way followed by the organic matter at sea, in which phytoplankton is eaten by copepods, and these at turn by fish, is known as the herbivorous or classical food web.


But at the same time, copepods can enter in the microbial food webs by feeding on its larger components, the ciliates, making them accessible to fish and larger marine organisms. It can be said that the role of copepods is to make good the topic "large fish eat on smaller fish". In fact, some of the most important fishery resources, like codfish and herring, depend on the abundance of a single or a few species of the copepod genus Calanus. Other common genus are Acartia, Centropages, Temora, Eucalanus, Clausocalanus, Oithona, etc.