COPEPODS: Copepoda

PHYSICAL CHARACTERISTICS

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.

GEOGRAPHIC RANGE

Copepods are found on every continent, including Antarctica. They also live in all of the world's oceans and seas.

HABITAT

Free-living copepods live in nearly all bodies of water on the planet, including temporary pools and rain puddles. Depending on the species, they are active in open water or live on or in sandy or muddy bottoms. They are often found among plants growing along the seashore. Many species swim near the surface of the ocean, while others are found to depths of 32,814 feet (10,000 meters). A few species are found only in undersea caves, deep-sea coldwater springs, or underwater geysers called hydrothermal vents. Others live in wet sand, mud, or mosses. They are also are found in hot springs and other bodies of freshwater underground and in caves. Some copepods live in extremely salty lakes on land. They have even been found in freshwater lakes on the slopes of the Himalayas at an elevation of 18,175 feet (5,540 meters). About half of all known copepods, many of which are parasites, live on or inside the bodies of sponges, worms, echinoderms, snails, tunicates, fishes, and marine mammals.

DIET

As a group, copepods eat a wide variety of foods. Some species grab tiny organisms, or bits of their tissues, floating in the water. Others are specialists and will eat only tiny algae (AL-jee). Species living on the bottom usually scavenge dead organisms, but some prey on tiny animals. Parasites living on or inside their hosts feed on body tissues and fluids.

BEHAVIOR AND REPRODUCTION

Many free-living copepods gather food from the water by using their antennae and maxillipeds. The swimming movements of these appendages help to create a flow of food-carrying water toward their mouths.

Both male and female copepods are required for reproduction. Males locate females by following their pheromone trails in the water. Pheromones (FEH-re-moans) are chemicals that attract members of the same species. Males place a single sperm packet on the female's abdomen. The sperm escapes the packet, enters the female's body through the opening of her reproductive system, and is stored in special sacs. The fertilized eggs are usually carried in a pair of sacs attached to the abdomen.

The newly hatched nauplius (NAH-plee-us) larva does not resemble the adult. Nauplius larvae do not feed and have only antennae and mouthparts for appendages. They use these appendages for swimming. The nauplius molts, or sheds its external skeleton (exoskeleton), six times before reaching a larval stage that resembles the adult. This stage must molt five times before reaching adulthood. In parasitic species, the development of the nauplius larva is much shorter, if it exists at all.

COPEPODS AND PEOPLE

Copepods are extremely abundant. It is estimated that there are more individual copepods on Earth (1.37 × 10²¹) than there are insects. They are a vital link in ocean food chains. They eat floating plants, which are then eaten by all kinds of fish. Many of these fish are harvested from the sea and used as food for people. A few freshwater species that are found in drinking water can spread human parasites known as the guineaworm, Dracunculus mediensis. Sealice and other parasitic species are major pests in fish farms. A few species attack marine algae grown as food for people in parts of Asia. But not all of contacts with copepods are bad. Some species are important predators of mosquito larvae. These predatory copepods have been introduced to various parts of the world to stop the spread of deadly malaria, which is carried by the mosquito.

PUTTING THE BITE ON SHARKS

Approximately one-fifth of all copepods are parasites. Many species live in large numbers on the bodies of sharks. They attach themselves to the bodies of blues, hammerheads, threshers, and others. Some even live between the teeth of the great white shark. One species latches on to the eyes of the Greenland shark. Individual sharks can carry up to 100 of these crustaceans on their fins, 400 in their nose, and 4,000 in their gills.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists 108 species of freshwater copepods, all of which are found only in caves or underground springs. Three species are listed as Extinct, or no longer in existence; six are Critically Endangered, or facing extremely high risk of extinction in the wild; 73 are Vulnerable, or facing a high risk of extinction in the wild; six are listed as Lower Risk, or at risk of becoming threatened with extinction in the future, and 20 are listed as Data Deficient, which means there is not enough information to make a judgment about the threat of extinction.

NO COMMON NAME (Oithona plumifera): SPECIES ACCOUNTS

Physical characteristics: Adult females measure 0.039 to 0.059 inches (1 to 1.5 millimeters) in length, while males are 0.029 to 0.039 inches (0.75 to 1 millimeters). The fore body is egg-shaped and has a beaklike projection on the front. The female's antennules have very long hairlike structures, while those of the male are bent. The max-illipeds are well developed. The first four pairs of thoracic limbs are also well developed, but the last pair is small and feathery. Each has a bristle at the base. The abdomen is slender and is nearly equal in length to the fore body.

Geographic range: They are found in all oceans.

Habitat: Oithona plumifera live along coastlines and out in the open sea, from the surface down to 328 feet (100 meters).

Diet: They eat bits of tiny plants and animals floating in the water.

Behavior and reproduction: Their life cycle consists of six stages of nauplius larvae, followed by five larval stages resembling the adult.

Little is known about their mating behavior. Males and females probably meet in swarms in the water. The male probably grasps the female's fourth pair of thoracic limbs before depositing his sperm packet. The fertilized eggs are stored in a pair of sacs.

Oithona plumifera and people: Oithona plumifera forms an important middle link in the oceanic food web between tiny algae and commercially harvested fishes breeding in estuaries and in coastal waters.

Conservation status: This species is not considered threatened or endangered. ∎

NO COMMON NAME (Monstrilla grandis): SPECIES ACCOUNTS

Physical characteristics: Adult females measure 0.147 inches (3.75 millimeters) in length, while males are 0.074 inches (1.9 millimeters). The fore body is long and the mouthparts are absent. The antennules are short in both males and females; those of the males are bent. The first four pairs of thoracic limbs are well developed, but the last pair is small. They are in the shape of two rounded structures, or lobes. The short abdomen has a pair of egg-bearing spines and a pair of large structures on the tip.

Geographic range: This species is found in the Northeastern Atlantic Ocean.

Habitat: The free-living adults live along the coast and are found in the upper layers of water. Parasitic nauplius larvae develop inside the bodies of snails or worms.

Diet: Adults lack mouthparts and do not eat. The nauplius larvae absorb nutrients from their hosts.

Behavior and reproduction: Females carry their eggs on long spines. The eggs hatch into free-living nauplius larvae. The nauplius finds a host and attaches itself with special claws on the antennae and jaws, or mandibles. They eventually burrow through the skin. Once inside, the nauplius becomes sacklike in shape and its appendages rootlike. They use their appendages to soak up nutrients from the host's body. When the last larval stage is reached, the larva leaves the host and molts to become an adult.

Little is known about mating behavior. Males presumably grab females with their antennules and use their swimming limbs to transfer the sperm packet.

Monstrilla grandis and people: This species does not impact people or their activities.

Conservation status: This species is not considered threatened or endangered. ∎

SALMON LOUSE (Lepeophtheirus salmonis): SPECIES ACCOUNTS

Physical characteristics: Adult females measure 0.27 to 0.49 inches (7 to 12.5 millimeters) in length, while males are 0.17 to 0.26 inches (4.5 to 6.7 millimeters). Their bodies are flat from top to bottom. The antennules of both males and females are short. The clawed antennae and mouthparts are used to grab the skin of their hosts. The mouthparts form a cone-shaped structure. The third pair of thoracic limbs makes an apronlike structure that forms part of a sucker on the fore body. The fourth pair of limbs is uniramous. The abdomen is long and slender.

Geographic range: They are found in the North Atlantic and North Pacific oceans, including Arctic waters.

Habitat: Salmon louse live as external parasites on the bodies of salmon and their relatives living in the sea. They abandon their hosts when they enter freshwaters to reproduce.

Diet: Salmon louse eat the tissue and blood of their host.

Behavior and reproduction: They eat by scraping the salmon skin with the needlelike tips of their mandibles. After a while the wound begins to bleed and they feed on the blood.

The life cycle begins with two free-living nauplius larval stages that do not eat. Each nauplius stage lasts about a day. These are followed by a larval stage that resembles the adult. It is this stage that attacks the salmon host. It can survive up to 10 days without a host. However, its chances of survival are much better if it can find a host in the first 48 hours. Six additional larval stages take place on the host before adulthood is reached. The first four larval stages after the nauplius have a special threadlike structure that helps to keep them firmly attached to the host. The last two larval stages are able to crawl around on the host, just like the adults.

Males locate females while they are in their last larval stage and guard them. Mating only takes place after the female molts and reaches adulthood. Males deposit a pair of sperm packets on the female. Females store the sperm in special sacs inside their bodies until the eggs are laid. They produce several pairs of eggs in strings. Each string has as many as 700 disc-shaped eggs all stacked together.

Salmon louse and people: The feeding activities of salmon lice weakens fishes through the loss of blood and by leaving open wounds that become infected. They are a serious pest in salmon farms in northern Europe and North America, causing losses of up to $30 million per year in Europe alone.

Conservation status: Salmon louse are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Huys, R., and G.A. Boxshall. Copepod Evolution. London: The Ray Society, 1991.

Mauchline, J. The Biology of Calanoid Copepods. Advances in Marine Biology. New York and London: Academic Press, 1998.

Periodicals:

Reebs, S. "Samples: Fold Three Times and Drink to Prevent Cholera in Rural Bangladesh." Natural History 112, no. 4 (May 2003): 16.

Wheeler, M. "Light Element: In the Nose of Jaws. Some Parasitic Copepods Have Seized on a Unique Piece of Real Estate." Discover 19, no. 3 (March 1998).

Caloyianis, N. "Greenland Sharks." National Geographic 194, no. 3 (September 1998): 60-71.

Web sites:

Calanoida (Copepoda, Maxillipoda).http://www.crustacea.net/crustace/www/calanoid.htm (accessed on March 22, 2005).