Introduction to the Copepods

Copepods are small crustaceans that comprise their own taxonomic class, the Copepoda. There are over 7,500 described species of copepods; although the majority of these are found in marine habitats, freshwater and even terrestrial (i.e., moist soil-associated) forms occur as well. Free living species are perhaps most familiar, but there are also many highly specialized parasitic copepods. Most free living copepod species range from less than 1 mm to 5 mm in size.

Free living marine copepods are among the dominant species in pelagic zooplankton communities. Most species are suspension-feeding grazers on phytoplankton and microzooplankton. They represent an important linkage between primary production and higher trophic levels in pelagic food webs. Calanus finmarchicus can consume nearly 375,000 diatoms in a 24 hour period.

Many free living copepod species, including Calanus, exhibit daily vertical migrations allowing them to escape detection by most visual predators by concentrating in deeper, dim water strata by day; at night, vertically migrating copepods swim up into the euphotic upper water layers where food is most abundant. Most vertical migrators apparently synchronize their daily movements by visually tracking isolumes - specific threshold light levels - up and down in the water column.

Anatomy of a Copepod

In the accompanying illustration a generalized calanoid copepod is shown in ventral view. The copepod trunk is composed of a (six-segmented) thorax and (5-segmented) abdomen. The head is fused with the first one or two thoracic somites (segments). The body is more or less cylindrical, usually tapering at the posterior.

Among the more conspicuous body parts are the elongated first antennae which are typically held out at right angles to the body. These antennae are not used for swimming; rather they are used to slow sinking in the water column, allowing the animals to maintain position with a minimal expenditure of energy. The tiny hairs that cover the antennae and other appendages, called setae, further reduce the rate of sinking.

Such body plan elaborations serve to maximize the amount of surface area exposed to contact with the water. Water itself actually behaves more like molasses at a scale of microns to millimeters because the attractive forces among individual water molecules (cohesion) and between water molecules and suspended bodies (adhesion) are especially pronounced at this scale. Thus, a small (i.e., copepod-sized) animal with a body plan that deviates substantially from a simple sphere can exhibit dramatically reduce sinking rates.

The second antennae, along with the thoracic swimming feet, are used in locomotion. The rotary-beating biramous (two-branched) second antennae are the main locomotory appendages in the calanoids, the order to which the ecologically important Calanus finmarchicus belongs.

The first pair of thoracic appendages has become modified to form maxilipeds used in feeding. The second maxillae are also modified for use in food capture.

Reproduction and Development

Copepods reproduce by means of sexual reproduction requiring copulation. In most species, male copepods are smaller than females. Females are usually more abundant than males within the population.

During copulation, males grasp females using their antennae and (depending on the species) with a modified pair of thoracic appendages. During copulation, the male transfers a sperm-containing package called a spermatophore to the female using its thoracic appendages; the spermatophore attaches to the female at the genital opening where it will be used to fertilize the eggs.

In the calanoids, fertilized eggs are usually shed singly into the environment to develop. In contrast, many free-living copepod species belonging to other orders retain their eggs within attached ovisacs until hatching. Eggs hatch into nauplii; the naupliar stage lasts through 5 or 6 instars (molts), at which time the animals become copepodids. After molting through 5 copepodid stages, copepods attain adulthood and cease molting.