Copepods are tiny crustaceans that belong to the class Copepoda. They are one of the most diverse and abundant animals, with over 14,000 known species. They are found in almost every freshwater and saltwater habitat on our planet. Most copepods exist as plankton. Some copepods are also parasitic, living on or inside other animals, including humans.
But did you know these little crustaceans are also incredibly important in marine food webs? They serve as a vital food source for many commercially important fish species. Plus, copepods have great adaptations that help them survive in different environments and avoid predators.
And that’s not all! They also play a crucial role in regulating the carbon cycle and nutrient balance of the water. So, let’s learn more about these amazing creatures!
About Copepods – A Quick Biology
Let’s take a quick glance at Copepods.
|Family||About 220 families|
|Species||More than 14,000 species|
|Diet||Plant and animal plankton|
|Geography||Live in almost all marine and freshwater habitats|
|Size||Average size measures between 1-2 mm|
Taxonomy and Classification of Copepods
Copepods belong to the phylum Arthropoda. Within Arthropoda, they are classified as crustaceans, with a hard exoskeleton, two pairs of antennae, and gills. They belong to the class Maxillopoda and subclass Copepoda.
Copepoda is divided into 10 orders, but only 3 are common. Each order is divided into more than 140 families and over 10,000 species. The 3 common orders of Copepoda are
- Calanoida: The largest and most diverse order of copepods, comprising about 2300 species. They are mostly marine and planktonic. But some live in freshwater or benthic habitats. They have a cylindrical body shape and long antennae. Examples include Calanus, Eurytemora, and Acartia.
- Cyclopoida: The second largest order of copepods, comprising about 2450 species. Most are freshwater or brackish water dwellers. But some are marine or parasitic. They have a spherical body shape and short antennae. Examples include Cyclops, Mesocyclops, and Oithona.
- Harpacticoida: The third largest order of copepods, comprising about 3000 species. They are mostly benthic or interstitial (living in spaces between sand grains). But some are planktonic or parasitic. They have long antennules and a tapered body. Examples include Harpacticus, Tigriopus, and Canthocamptus. (Source)
The other orders of copepods include – Siphonostomatoida, Monstrilloida, Mormonilloida, Misophrioida, Platycopioida, Gelyelloida , Poecilostomatoida
Copepods are widely distributed globally. They can be found in all major biogeographic regions. Some copepods are cosmopolitan, meaning they occur in many different regions. Others are endemic, meaning they are restricted to a specific area.
For example, some copepod species are only found in Antarctica or certain hydrothermal vents. Copepods are also very diverse within each region and habitat. For instance, there are more than 2,800 freshwater copepod species and over 4,000 marine copepod species.
Habitat and Range – Where Do Copepods Live?
Copepods are present in nearly every freshwater and saltwater habitat on Earth. They can tolerate various environmental conditions such as temperature, salinity, light, oxygen, and pH.
You can find them in oceans, seas, lakes, rivers, streams, ponds, springs, caves, sinkholes, wetlands, and estuaries. They are also abundant in some terrestrial habitats that are moist or wet, such as swamps, bogs, mosses, leaf litter, bromeliads, and pitcher plants.
Physical Characteristics/Anatomy – What Do Copepods Look Like?
Size and Shape
Copepods are tiny creatures, usually 0.5 to 2 millimeters long. Their bodies are generally teardrop-shaped, with a rounded front and a tapered rear. However, some species have more elongated or cylindrical bodies.
Did you know? The largest copepod is Pennella balaenopterae, a parasite of fin whales that can grow up to 32 cm long. The smallest copepod is Sphaeronellopsis monothrix, a parasite of marine ostracods only 0.11 mm long.
Copepods have three main body segments: the head, the thorax, and the abdomen. The head and thorax are mostly fused together, forming a structure called the cephalothorax. The abdomen is smaller and narrower, ending in a tail-like structure.
- The head bears the mouthparts and the antennae.
- The thorax has six segments (each bearing a pair of legs).
- The abdomen has five segments. The last abdominal segment bears a pair of appendages called caudal rami (tail-like projections) that help with steering and balance.
Copepods have two pairs of antennae on their heads. The first pair is longer and used for swimming. Whereas the second pair is shorter and used for sensing their surroundings. These antennae are important for navigation and finding food.
Important Note: The first pair is conspicuous, while the second is hidden under the head.
Copepods have an external skeleton made of chitin that protects them from predators and water loss.
Most species’ skeletons are thin and transparent, making them almost invisible in the water. Some copepods have pigments that give them coloration ranging from red to green to blue.
Most copepods have a single, simple eye called an ocellus. It is usually bright red and located in the center of their head. This eye can detect light and dark but doesn’t form clear images as our eyes do.
Some copepods have two eyes that form a telescope-like structure that allows them to see far distances. Some copepods living in dark or subterranean habitats have no eyes.
Copepods have several pairs of legs, usually ranging from three to five pairs. These legs are used for swimming, crawling, and grasping food. In some species, the first pair of legs are modified for feeding or mating.
Copepods have specialized mouthparts for feeding. They have a pair of mandibles used for cutting and grinding food. They also have a pair of maxillae, which help to manipulate and move food toward the mouth.
Copepods have various behavioral adaptations that help them survive and thrive in their habitats. These adaptations include:
Diel vertical migration
Many planktonic copepods move up and down the water column according to the day-night cycle. They usually stay deeper during the daytime to avoid predators. They move closer to the surface during the dark to feed on phytoplankton that is more abundant near the surface. This behavior helps them balance their energy intake and expenditure.
Copepods can detect changes in water pressure or vibrations caused by approaching predators. They respond by rapidly jumping, bending, and kicking their legs (caudal rami). This creates a jet of water that propels them away from danger. This response is speedy and can reach speeds of up to 500 body lengths per second.
Many copepods have transparent or translucent bodies that make them hard to see in the water. Some copepods have pigments that match their background or reflect light differently to camouflage themselves.
Some copepods can enter a state of dormancy when the environmental conditions are unfavorable, such as low temperature, salinity, or oxygen. They reduce their metabolic rate and activity and wait for better conditions to resume their normal life cycle. This allows them to survive for long periods without food or movement until conditions improve.
Some copepods can produce light by chemical reactions in their bodies. This bioluminescence can be used for communication, attraction, defense, or counter-illumination (matching the ambient light from above to reduce their silhouette).
Did you know? Copepods can exist as free-living organisms, form symbiotic relationships, or be external or internal parasites.
Feeding Habits and Diet – Are Copepods Carnivores?
Copepods have a diverse diet that depends on their size, species, and habitat. They also have different mouthparts that suit their feeding habits.
Some are herbivorous (plant-eating), feeding mainly on phytoplankton or detritus (dead organic matter). Herbivorous copepods have fine hairs that help them capture algae from the water.
Some copepods are carnivorous (animal-eating). They prey on other zooplankton (microscopic animals), such as rotifers, cladocerans, and even smaller copepods. Carnivorous copepods have sharp mandibles that help them grasp and tear apart their prey.
Some are omnivorous, eating both plant and animal matter. Omnivorous copepods have a combination of both types of mouthparts.
Fun fact: Some copepods are detritivorous, scavenging on organic debris and dead organisms.
Reproduction and Lifecycle of Copepods
Copepods reproduce sexually, with males transferring sperm to females using specialized appendages. Male and female copepods have different reproductive organs. Males have modified antennae that they use to grasp females during mating.
Some copepods mate in pairs, while others form swarms or aggregations for mating. Females carry eggs in sacs attached to their abdomen until they hatch or release into the water. Depending on the species, the eggs may hatch immediately or after a period of dormancy.
Interesting Fact: Some copepods can also reproduce through parthenogenesis, where the female’s eggs undergo development into offspring without being fertilized by a male.
Copepods have a complex life cycle. Young copepods go through several stages of development before reaching adulthood. These stages are called nauplius, copepodite, and adult.
- The eggs hatch into nauplii. These are larval forms with a head and a tail but no thorax or abdomen.
- The nauplii molt six times until they reach the copepodite stage. A copepodid is a juvenile form that resembles the adult but is smaller and less developed
- The copepodite molts five more times until they reach the adult stage. It is the final and reproductive stage of the copepod’s life.
The duration of these stages varies depending on the species and environmental conditions.
The duration and timing of the copepod lifecycle vary depending on the species and environmental factors. Some copepods can complete their lifecycle in a few days or weeks. Whereas others can take months or years. Some copepods can even synchronize their reproduction with seasonal food availability or temperature changes. (Source)
Predators – What Animals Eat Copepods?
Copepods are eaten by a wide range of animals in aquatic ecosystems. Some animals that eat copepods are:
- Fishes: Many fishes rely on copepods as their main or supplementary food source, especially during their larval and juvenile stages. They can consume large numbers of copepods at once by filtering them from the water or actively hunting them. Some examples of fish that eat copepods are herring, anchovies, sardines, salmon, cod, tuna, mackerel, and flounder.
- Whales: Some whales, such as baleen and right whales, feed on large quantities of copepods by filtering them with their baleen plates. Copepods can make up more than 50% of their diet.
- Seabirds: Some seabirds, such as auks, puffins, terns, and gulls, dive into the water to catch copepods and other small prey. Copepods provide them with protein and fat for their reproduction and migration.
- Cnidarians: These animals have stinging cells called nematocysts, such as jellyfish, corals, anemones, and hydroids. Cnidarians can capture copepods using their tentacles or mouthparts to inject venom or entangle them.
- Ctenophores: These animals have rows of cilia called comb plates, such as comb jellies or sea gooseberries. Ctenophores can catch copepods using sticky tentacles or lobes to adhere them to their body surface.
- Chaetognaths: These animals have grasping spines called chaetae, such as arrow worms or glass worms. Chaetognaths can snatch copepods using their jaws to pierce their exoskeleton and inject digestive enzymes.
- Other crustaceans: Some crustaceans feed on copepods as part of their diet. Some examples are amphipods, isopods, mysids, shrimps, crabs, lobsters, and other copepods.
Economic/Ecological Importance – What is the Purpose of Copepods?
According to an article published in the International Journal of Current Microbiology and Applied Sciences, copepods have many ecological functions that benefit the environment and other organisms. Let’s find out:
- Copepods are an important link in the aquatic food web. They transfer energy from primary producers (phytoplankton) to higher trophic levels (fishes).
- They are bioindicators that reflect the health and quality of the water they live in, as they are sensitive to temperature, salinity, oxygen, and pollution changes.
- Copepods contribute to the global carbon cycle by grazing on phytoplankton and producing fecal pellets that sink to the ocean floor, where bacteria can bury or decompose them. (Source)
- They also affect the oceans’ biogeochemistry by producing dimethyl sulfide (DMS). This gas influences cloud formation and climate regulation.
- Copepods are also involved in transporting pollutants and pathogens, as they can accumulate toxins or parasites in their bodies and transfer them to higher trophic levels.
Copepods face several threats from human activities and environmental changes. These include:
- Overfishing: Overfishing can reduce the availability of copepods for their predators, disrupting the balance of marine food webs.
- Pollution: Pollution can affect the quality and quantity of copepod habitats. It can also influence affect their physiology and behavior.
- Climate change: Climate change can alter the temperature, salinity, pH, oxygen levels, and currents of the oceans. This affects copepod distribution, abundance, diversity, and phenology (the timing of biological events).
- Ocean acidification: Ocean acidification can impair copepod calcification (the formation of calcium carbonate structures), which is important for some species to maintain buoyancy and protection. Invasive species can compete with or prey on copepods, reducing their native populations.
Exciting Facts About Copepods
- Copepods found in freshwater could serve as a natural means of regulating malaria by preying on the larvae of mosquitoes. (Source)
- The microscopic freshwater species of Copepods, i.e., water fleas, can transfer the guinea worm to humans.
- Male copepods are typically smaller than females and are generally less abundant.
- Copepods are speedy swimmers. Some can go up to 295 feet in an hour, like a human swimming 81 km/h.
- In 24 hours, a single copepod can eat 11,000 to 373,000 diatoms.
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Are copepods parasites?
Some copepods are parasites that attach to or live inside other animals, such as fish, mollusks, crustaceans, and even whales. Parasitic copepods can cause diseases or injuries to their hosts, such as skin lesions, gill damage, reduced growth, or mortality. However, not all copepods are parasites. Most copepods are free-living and do not harm other animals.
Are copepods harmful to fish?
Copepods can harm fish if they are parasitic or carry pathogens or toxins. Parasitic copepods can cause physical damage or infections to fish by attaching to their skin, gills, eyes, mouth, or internal organs.
Copepods can also transmit diseases or parasites to fish by acting as intermediate hosts or vectors. For example, some copepods can carry the larvae of tapeworms or nematodes that infect fish.
Do copepods need oxygen?
Yes, copepods need oxygen to survive. They take oxygen from the water around them through their gills, which are located on their legs. These gills extract oxygen from the water and release carbon dioxide, which is expelled from the copepod’s body.
Copepods are remarkable animals that have a significant impact on aquatic ecosystems. They are diverse in their forms and functions and have adapted to various habitats and conditions. They are essential for the food web and the ocean’s biogeochemical cycles. They are also indicators of environmental changes and challenges. Copepods deserve our attention and appreciation for their role in sustaining life on Earth.