Copepods, named for their ‘oar-feet,’ are small crustaceans recognized by tear-shaped bodies and large pairs of antennae. In their larval stage, these metazoans lack a true thorax or abdomen, emerging as nauplius with only a head and tail. Copepods are members of the arthropod class Copepoda within the superclass Multicrustacea.
They inhabit different habitats, from the ocean’s depths to freshwater and even in moist terrestrial environments. These versatile creatures drift in the water column as plankton or reside on sediments as benthic organisms.
Adult copepods typically range between 1 and 2 mm in length, though some parasitic species can grow significantly larger. Their transparent or semi-transparent bodies often make them difficult to spot despite their abundance in aquatic ecosystems.
Copepods are vital to the aquatic food web as they are the primary food source for many marine animals.
Although most copepods measure between 1 and 2 mm (0.039 to 0.078 in) in length, some polar species can grow as large as 10 mm (0.5 in).
In contrast, some adult copepods are exceptionally tiny, measuring only about 0.2 mm (0.007 in).
Most copepods have a teardrop-shaped body with a single compound eye in the middle of the head, although members of the genera Copilia and Corycaeus have two eyes. Like other crustaceans, they have two pairs of sensory antennae, with the first pair being larger and a thin, transparent exoskeleton covering their body.
Members of the orders Calanoida, Cyclopoida, and Harpacticoida have short, cylindrical bodies with rounded or beak-like heads. The first one or two thoracic segments are fused with the head, followed by a thorax comprising three to five segments, each with limbs. The abdomen, a broader section, consists of five segments without appendages but often ends with a tail-like structure called a ‘rami’.
Copepods can absorb oxygen directly through their exoskeleton; thus, they do not need circulatory or respiratory systems. However, they possess maxillar glands that excrete metabolic waste.
Currently, around 14,000 species of copepods are classified into ten orders.
Due to the tiny size of these invertebrates, fossil records of copepods are scanty. The oldest, most probable fossils of copepods were unearthed from a fragment of bitumen in the glacial rocks of Oman. These fossils date back to the late Carboniferous Period (Pennsylvanian Subperiod), around 303 million years ago. Though most of these fossils remain unidentified, some are often considered members of the family Canthocamptidae (order Harpacticoida). A few microfossils from the Cambrian Period have been discovered in North America.
Studies suggest that copepods evolved independently at least 14 times before they acquired parasitic capabilities. Evidence of damage to echinoid fossils suggests that parasitic forms were prevalent in France around the Middle Jurassic Period (168 million years ago).
Copepods are adapted to live in diverse habitats, from high mountain streams and polar regions to the deepest ocean trenches and underground caves. They are found worldwide, thriving in oceans and freshwater environments such as rivers, estuaries, lakes, and in hot hydrothermal vents.
In addition to their free-living forms, some copepods are ectoparasites, attaching themselves to the bodies of various hosts. These hosts include bony fish, sharks, marine mammals, and invertebrates like corals, mollusks, tunicates, and other crustaceans.
Most copepods are herbivores, primarily consuming phytoplankton (each copepod can ingest up to 373,000 phytoplankton cells daily). While some larger copepod species occasionally prey on smaller ones, many benthic copepods feed on organic detritus and the bacteria growing on them.
In contrast, parasitic copepods such as fish lice sustain themselves by feeding on the flesh and fluids of their hosts.
Free-swimming copepods use their second pair of cephalic appendages like oars to propel themselves through the water. While some copepods stay stationary for extended periods, they move in short bursts or make sudden, rapid movements to evade predators.
Copepods can leap several millimeters at high speed when they detect a predator. They also use bioluminescence to produce light, which helps to disorient their predators, allowing them to escape quickly.
Copepods are usually gonochoric (separate male and female sexes) and engage in an elaborate mating strategy.
The complete metamorphosis from hatching to adulthood could take a week to a year, depending on the species, resource availability, and environmental conditions. For instance, the calanoid Parvocalanus crassirostris reaches adulthood in a week at 25°C, while at 15°C, it takes about 19 days.
Sea horses, jellyfish, whales, and other larger copepod species are their common predators.
Copepods are a major component of zooplankton and an important food source for many aquatic animals, including dragonets, banded killifish, Alaska pollock, and other crustaceans like krill. They are often considered to form the largest animal biomass on Earth and play a vital role in the carbon cycle and secondary productivity of the world’s oceans.
The ocean’s surface is the world’s largest carbon sink, absorbing almost 2 billion tons of carbon annually. Copepods contribute to this process by feeding at the surface at night and sinking to deeper waters during the day. Through respiration, excretion, and the molting of their exoskeletons, they add to the carbon content of the deep sea.
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