Copepods: Copepoda

Copepods are usually very small and measure 0.019 to 0.78 inches (0.5 to 20 millimeters) in length. A few free-living species, those that are not parasites, reach 0.7 inches (18 millimeters). Parasitic copepods spend most or all of their lives on or in the bodies of fish and eat their body fluids. Some whale parasites, such as Pennella balaenopterae, may grow up to 13 inches (330.2 millimeters).

The copepod body is made up of two regions: the front, or fore body, and the abdomen. Their bodies are distinctly narrowed where these two regions meet. The fore body includes the head and thorax. The head has two pairs of antennae. The first pair of antennae, or antennules (an-TEN-yuls), is not branched, or uniramous (YU-neh-RAY-mus). They are usually very long in most species. The second pair of antennae is either uniramous or biramous (branched). Copepods have a single, simple eye, which have only one lens. Three pairs of appendages make up the mouth, including a pair of biramous jaws, or mandibles.

The first segment of the thorax is tightly joined, or fused to the head. It has a pair of uniramous limbs called maxillipeds (mack-SIH-leh-pehds). The maxillipeds work with the mouthparts. A head shield covers the top of the head and the back of first thoracic segment. The remaining five thoracic segments have four or five pairs of biramous limbs that are used for crawling or swimming. Species living in the open ocean have limbs covered with tiny hairs that prevent them from sinking. Each pair of limbs is joined together at the bases so that they move together. Depending on the group of copepods, the abdomen is either four- or five-segmented. It does not have any appendages underneath. The abdomen ends in a pair of short or long taillike structures. Adult females often have a pair of egg sacs attached to their abdomens.