Copepoda
There are more copepods than any other kind of zooplankton.
The copepods are small, usually a few mm. They are to be studied with a binocular, but the small ones are best studied with the microscope.
One or two segments of the body have become fused with the head. There are two pairs of appendix on the head followed by a pair of appendix on each body segment. The first appendixes, antenna, are often longer than the others.
The copepods are extremely abundant in sea and they also play a dominant role in fresh water. Ecologically, the free-living copepods are of extreme importance, often dominating the primary consumer level in aquatic communities.
In many marine localities the copepod Calanus is the most numerous organisms in the zooplankton and have the greatest proportion of the total biomass. Calanus forms a major portion of the diet of such economically important fish as herring, and the larvae of larger fish and is an important food item for whales.
Copepods are all filter feeders. Filter feeding involves setae on certain appendages for the collection of food particles. The necessary water current is produced by the beating of the filtering appendages or, more commonly, by special appendages modified for this purpose.
The females lay eggs in clusters of about 50, every 10 to 14 days. From the egg develops larvae called nauplius, quite unlike the adult. Subsequent development may involve a gradual change to the adult body form through at least five shell changes. After that the animal is sexually mature. The male and the female are different. It is quite difficult to identify the different stages in the development from the larvae, nauplius, to adults and to differ between the different sexes.
The copepods are so numerous in species, and each with a series of different developing stages, make it difficult to identify the species. Here only a few examples are presented.
Copepods are divided into three sub-groups
1 calanoid copepods may easily be recognized by the length of their first pair of antennae. These are equal to at least half the length of the body and are often longer. The female carries a single egg sac. They feed mainly on plankton algae. They are common in the sea and in all types of fresh water Length 1 1/2-21/2 mm when adults.
2 cyclopoida copepods with much shorter antennae than the calanoid copepods. Length 1/2 -3 mm. The females are carrying two egg sacs. The genus cyclops is most distributed in fresh water.
3 harpacticoida copepods are bottom dwellers. Length less than 1mm. The first pair of antennae is quite short and not usable for swimming. Some benthic harpacticoids swim at nights, contributing to a wider dispersion of the species. The female has only one egg sac.
The image show an animal found in a rock-pool, the genus Tigriopus.
Video 1 of copepoda .together with the green algae Volvox.
Video 2 of cyclopoid copepoda with two egg sacks and short antennas.
Video 3 of calanoide copepoda with long antennas.
Video 4 of copepoda with nutrients