The copepods are minute crustaceans that have an enormous contribution in the aquatic ecosystem. They are found in oceans, lakes, and rivers all over the USA and constitute an important connection between microscopic plankton and some larger predators such as fish, seabirds, and whales. Although they are very small, they have an impressive evolutionary record over the last 450 million years, adjusting to live in various habitats, such as freshwater ponds and the depths of the Atlantic.
These microcrustaceans measure 0.2 to 5 millimeters, and they belong to marine, freshwater, and parasitic microcrustaceans. Oceans are inhabited by the marine copepods, including Calanus finmarchicus, which have been beneficial to the commercial fisheries by acting as food to the young fish. In the lakes, ponds, and rivers, freshwater copepods such as the Cyclops spp. can be found, giving a clue on the water quality and acting as a source of natural food to the aquarium fish. The parasites are parasitic copepods such as Lernaea spp., which can have effects on the health of fish and should be scientifically observed in fish aquaculture.
The ecological significance of copepods is hardly overestimated. They are also the main consumers in aquatic food webs, consuming phytoplankton and detritus and passing the energy to upper trophic levels. They also play a role in the oceanic carbon cycle in that they also help in the sequestration of carbon as they eat phytoplankton and excrete fecal pellets that contain carbon and are solidified in the deep waters. Changes in seasonal abundance are an indicator of fish populations and provide an indication of the health of freshwater and marine systems, which are important as environmental monitoring systems.
Copepods are relatively easy to observe and study by enthusiasts and students. With the help of a plankton net, a microscope, and digital imaging, a hobbyist is able to observe the populations of ponding or aquarium. Sampling in the morning, preservation of specimens in ethanol to study them later, and differentiating between free-living and parasitic species are some of the best practices to avoid confusion. In the case of aquariums, live copepods are approximately 10 to 25 dollars per culture pack in the USA and offer natural and healthy food to small fish and invertebrates.