What Is a Copepod?

Types

• Ten orders are believed to comprise the subclass Copepoda, a part of the class Crustacea. Scientists debate
  whether two of the orders should be included as copepods. Some scientists study copepods alongside the subclass Branchiura which includes sea lice and other external aquatic parasites.

Geography

• Copepods exist in aquatic environments of all temperatures and types. Some have been found in the extreme cold temperatures of Antarctica. Most copepods are freshwater organisms but some can tolerate salt water environments. Four major groups of copepods have been found in the Great Lakes region of North America.

Identification

• Copepods are multi-celled organisms. The word "copepod" comes from two Greek words that describe a predominant feature of this animal. "Kope" means oar and "podos" means foot. Copepods have appendages on opposite sides of the body which "row" simultaneously like oars to propel the copepod through its environment. The copepod body is divided into two sections. The head and thorax compose the fore body. Two pair of antennae, an eye and three pairs of limbs are on the head and an additional pair of appendages called maxillopeds are on the thorax. The thorax has five segments, each with a pair of appendages for mobilization. The abdomen can have up to five segments onto which more pairs of appendages are attached. At the end of the abdomen are two tails.

Size

• The smallest members of this subclass can be viewed best under a microscope. The shortest copepod adult is only 0.2 mm in length. According to the New World Encyclopedia, the largest copepod in the world, Pennella sp., can grow up to 30 cm and is a parasite of fin whales. Most copepods grow to be only 1 to 2 mm, less than the diameter of a pencil lead.

Benefits

• Many copepods are an important part of the food chain in an aquatic environment. These crustaceans eat single-celled algae and are, in turn, eaten by larger fish or whales. Some of these organisms act as aquatic cleaners, consuming dead matter on the bottom of a marine aquarium, the ocean or other body of water. They are added to aquariums as a food source for gobies, seahorses, and pipefish. Copepods can also be important defenses in controlling mosquito populations in places where malaria is a problem. These crustaceous organisms eat the larvae, preventing large numbers of mosquitoes from hatching.

Warning

• Copepods serve as hosts for internal and external parasites that can infect animals and humans with disease. An example of this is the order of freshwater copepods, which transmit the parasite guineaworm, or Dracunculus mediensis, to humans through consumption of untreated water. Some copepods are internal or external parasites on fish or amphibians and can damage or completely destroy those populations in an area. They are found in or on minke and fin whales, whale sharks, elephant seals, and other marine mammals and fish.