The class copepoda are the most abundant crustaceans (invertebrates with exoskeletons belonging to the phylum arthropoda) and plankton in general. Copepods are often the primary herbivores that eat photosynthesizing plankton (such as diatoms). They have a single compound eye and most have two joined antennae. The size and shape of the antennae depend on the species of copepod. Copepods generally have no abdominal appendages and their body is an elliptical shape. The three most common free living orders of copepods are the Calanoida, the Harpacticoida and the Cyclopoida.
Calanoid Copepods have antennae that tend to be long and the abdomen has four segments. Calanoid copepods are the most common and diverse pelagic copepod order and tend to be the most abundant members of the zooplankton. They are usually the most abundant type of copepod living far away from shore.
Harpacticoid Copepods have shorter antennae that result in more streamlined navigation of tight spaces but allow less surface area for floatation. Generally, order Harpacticoida are more abundant in samples that occur near shore or close to the sea bed.
Cyclopoid Copepods are usually smaller than Calanoids and their antennae are shorter than their bodies. Cyclopoid Copepods are only represented by only a few different genuses such as the genus Oithona which includes the species Oithona similis that could be the most abundant animal on the planet.