Copepod

Copepods are tiny animals that belong to a group of small crustaceans. You can find them almost everywhere in water, whether it is freshwater like lakes and rivers or saltwater like oceans. Some copepods float freely in the water, while others live on the bottom near the sediments.

These little creatures come in many types. Some spend part of their lives attached to other animals, while others live in very special places such as swamps, under leaves in wet forests, or even inside plants that collect water, like bromeliads and pitcher plants. You might even find them deep underground in caves, sinkholes, or at the bottom of streams.

Copepods are important for scientists because they can help show how healthy an environment is. By studying these tiny animals, we can learn a lot about the biodiversity and health of different water habitats.

Classification and diversity

Copepods belong to a group called Copepoda, which is part of a larger group known as Multicrustacea in the subphylum Crustacea. They can also be considered part of the class Hexanauplia. Scientists have identified about 13,000 different kinds of copepods, and around 2,800 of these live in freshwater.

Characteristics

Copepods are tiny animals, usually about 1 to 2 mm long, with a teardrop shape and big antennae. They have a hard outer shell, but because they are so small, their bodies are almost see-through. Some polar copepods can grow up to 1 cm long. Most have one big middle compound eye, often red, right in the center of their heads. Some underground species have no eyes, while others have special eyes that act like telescopes.

Free-living copepods usually have short, tube-like bodies with rounded heads. Their heads join with one or two body parts, and the rest of their bodies have three to five segments with legs. They use special mouth parts called maxillipeds to help them eat. The back part of their bodies, called the abdomen, is narrower and has five segments but usually no legs, except for some small tail parts at the end. Parasitic copepods look very different and it’s hard to describe them all the same way.

Because copepods are so tiny, they don’t need hearts or blood to move things around. Most don’t even have gills; instead, they take in oxygen straight from the water. They get rid of waste using special glands.

Behavior

Copepods move in different ways. Some swim slowly, some move in bursts, and others move all the time. When they sense danger, they can jump quickly to escape. Some copepods can glow in the dark, which helps protect them from being eaten.

Finding a partner in the open water is tricky. Some females leave a special scent trail in the water that males can follow.

Diet

Most copepods eat tiny plants called phytoplankton one at a time. Some bigger copepods eat smaller ones. Many that live on the ocean floor eat bits of dead material or bacteria. Some copepods live on other animals and eat bits of their host’s food.

Life cycle

Most copepods stay in the water their whole lives. When it’s time to have babies, the male holds onto the female and puts a special package of sperm near her body. She may lay eggs right in the water or carry them in a sac until they hatch.

The eggs hatch into tiny larvae called nauplius, which have a head and a small tail but no body segments yet. After several molts, they change into copepodid larvae, which look more like adults but still have simple bodies. After a few more molts, they become full-grown copepods. This whole process can take from a week to a year, depending on the species and the temperature.

Ecology

Planktonic copepods play an important role in the world's oceans and ecology. They are a major part of zooplankton and are a key food source for small fish like the dragonet and banded killifish, as well as for other creatures such as krill. Some scientists think copepods might have the largest number of these tiny animals on Earth.

Copepods are very important for the oceans because they help create new life and carry carbon to deep water. They eat near the surface at night and then move deeper during the day to stay safe. Their old shells and waste also help move carbon to the deep sea.

About half of all copepod species live by attaching to other animals like fish, sharks, and corals. These copepods change their bodies to fit this lifestyle. They can also carry other tiny organisms that make them sick, which can affect the whole ocean environment.

Evolution

Copepods are very common today, but because they are tiny and fragile, we rarely find them as fossils. The oldest fossils of copepods come from about 303 million years ago, found in Oman in a piece of bitumen. These ancient copepods probably lived in a lake under a glacier. We also have possible signs of copepods from even older times, about 500 million years ago, in North America. Some copepods have changed to live as parasites many times throughout history, with the oldest evidence of this found in France from about 168 million years ago.

Practical aspects

Live copepods are often used in saltwater aquariums as food. They help keep the water clean by eating tiny plants and other small bits of material. Hobbyists who keep special fish, like the mandarin dragonet or scooter blenny, find copepods very helpful. In aquariums, copepods are usually placed in a special area called a refugium.

Copepods can sometimes be found in drinking water, especially in places where the water isn’t filtered, like in New York City, Boston, and San Francisco. This isn’t usually a problem if the water is treated. However, in some tropical countries, copepods can carry harmful bacteria that cause sickness. To reduce this risk, people can use simple filters to remove copepods and other particles from the water.

Copepods have also been used to help control mosquitoes that spread diseases. When added to water where mosquitoes breed, certain types of copepods can eat mosquito larvae, helping to stop the spread of diseases like dengue fever. This natural method is used in places like Vietnam and is being tested in other countries too.