When swimming in a world of microscopic plants, you’re also diving into a zoo of tiny crustaceans called copepods. As the University of Tasmania notes, these miniature beings are the most common animals that you dive with. Included in the zooplankton classification, the majority of these organisms measures between 1 and 5 mm long (<4/100ths – 1/5th of an inch) barely visible to the naked eye. Some are carnivores and some are parasites, which drain their hosts for sustenance. Most, however, happily subsist as herbivores dining on microscopic plants called phytoplankton. These copepods ingest minute, floating plants by sieving vast amounts of water through fine mesh on their limbs.
While the anatomy of copepods varies, the free-living forms often feature a short, cylindrical body with two pairs of antennae and a single eye. Drifting with the currents, these weak-swimming animals rely on their shrimp-like legs and occasionally their antennae to propel themselves. Advancing through an amazing 11 larval stages, these crustaceans shed their little shells through several molts before becoming adults. However, figuring out the individual species of a copepod remains difficult, often requiring dissection and view of the creature’s limbs under a microscope.
Copepods outnumber even insects
Not only are these beings the largest group of crustaceans, Professor Giuseppe Pesce notes that copepods prevail as the most plentiful multicellular group on Earth, outnumbering even insects. Successfully colonizing nearly all of the available water habitats around the globe from freshwater to hypersaline, these tiny animals can live almost anywhere. From open ocean and dark caves to deep-sea hydrothermal vents and polar regions, these miniscule crustaceans thrive. Copepods can even persist in strange habitats, such as ephemeral water bodies, as well as small pools of rain and dew caught between the leaves of tropical plants and the wells of discarded car tires. In dormant egg form, some types can even survive long periods of adverse environmental conditions such as killing droughts and extreme temperatures.
Video of what is believed to be copepod larvae hatching from an egg sac. Video courtesy of Merry Passage
Copepods are widespread but at risk
With these creatures living practically everywhere, it’s not surprising that more than 57,000 published works exist about copepods. The earliest records date back more than 2,000 years, according to the World Association of Copepodologists. However, scientists have recently revealed alarming news about these tiny animals. As published in the journal Archives of Environmental Contamination and Toxicology, researchers recorded one out of every 34 ocean-dwelling copepods having ingested particles of microplastic pollution. Since many animals feed on copepods, scientists believe that this plastic accumulates inside a variety of species, which also end up inside of humans who eat fish.
Another study published in Proceedings of the Academy of Sciences has established that some copepods actively transport and deposit carbon dioxide in deep water where this climate-affecting compound can be safely stored for thousands of years. With copepods themselves threatened by pollution and climate change, the risks to these animals may result in major consequences for our planet and are yet another reason to help the ocean.
Today, copepods remain one of the world’s most abundant animals and the smallest crustaceans encountered underwater. The next time you dive, watch for the semi-invisible mass you see flickering out of the corner of your eye because the single-eyed copepods are swimming all around you.