Copepods are tiny crustaceans, typically 1 to 2 millimeters long, with over 4,500 known species. As one of Earth’s most abundant multicellular organisms, they form a fundamental part of aquatic food webs globally, playing a significant role in energy transfer.
Vast Marine Dwellers
The vast majority of copepod species inhabit marine environments worldwide. In the open ocean’s pelagic zone, copepods are the most abundant zooplankton, making up 55-95% of total metazooplankton. They inhabit surface waters (the photic zone), grazing on phytoplankton and converting solar energy into organic matter. This grazing is crucial for the marine food web, as copepods are a primary food source for fish and other marine life.
Copepods also contribute to the “biological pump,” a process that moves carbon from the ocean surface to deeper waters. They do this by consuming phytoplankton and producing fast-sinking fecal pellets, or by migrating to deeper layers and metabolizing carbon. This process helps sequester carbon in the deep ocean, influencing global carbon cycles. In deeper mesopelagic and bathypelagic zones, copepods participate in daily or seasonal vertical migrations, transporting carbon-rich lipids.
Copepods are highly abundant along coastal waters, including estuaries, bays, and nearshore environments. These areas are often productive due to nutrient inflow, supporting large copepod populations that serve as a food source for local marine life. Copepod communities in these dynamic habitats are influenced by factors like temperature, salinity, and food availability.
In polar regions, copepods adapt to cold, ice-covered waters. They are a foundational component of polar food webs, linking primary producers like ice algae and phytoplankton to higher trophic levels, including fish, seabirds, and whales. For example, Calanus hyperboreus and Calanus glacialis store large lipid reserves to survive long winters and fuel reproduction.
Specialized copepods also inhabit deep-sea hydrothermal vents, which are unique chemosynthetic ecosystems. These species are highly abundant in areas with typical vent conditions and feed on bacteria. They show adaptations to these extreme environments, including specialized swimming and crawling legs.
Life in Freshwater Ecosystems
Copepods are found in nearly every freshwater habitat, adapting to diverse conditions. In lakes and ponds, they are common in both the limnetic zone (open, well-lit water) and the littoral zone (shallow, nearshore areas with rooted plants). In these environments, copepods graze on phytoplankton and detritus, forming an important link in the energy flow. Zooplankton communities in lakes often include dominant species of copepods, along with rotifers and cladocerans.
Rivers and streams also support copepod populations, adapting to flowing water. They consume organic matter and microorganisms, contributing to nutrient cycling within these systems. Their ability to thrive in moving water highlights their adaptability across diverse aquatic settings.
Wetlands, including marshes and swamps, provide additional habitats for copepods. Characterized by shallow water and abundant vegetation, these areas offer food and shelter. Copepods break down organic waste, helping maintain the health of these wetland ecosystems. Throughout freshwater environments, copepods serve as a significant food source for various fish and invertebrates, including fish larvae. Their rich nutritional content, including essential fatty acids and proteins, makes them valuable for the growth and health of aquatic organisms.
Unusual and Hidden Niches
Copepods demonstrate remarkable adaptability, thriving in many less common or extreme environments. Some species live in subterranean aquifers, wells, and cave systems, collectively known as groundwater habitats. These “stygobiont” copepods are obligate groundwater dwellers and often exhibit specific adaptations to these dark, stable environments. They can comprise a substantial portion of the underground fauna, contributing to biogeochemical cycles.
Temporary pools and ephemeral puddles also host copepods. These species survive desiccation by producing resting eggs or entering a dormant state (diapause). This allows them to persist through dry spells and re-emerge when water returns.
Specialized terrestrial copepods are found in humid microhabitats like damp moss and wet soil. These environments provide the moisture necessary for their survival, allowing them to inhabit areas outside typical aquatic bodies. They often reside under leaf litter in wet forests, springs, or water-filled recesses of plants like bromeliads.
Copepods also engage in symbiotic or parasitic relationships, living on or within other organisms. Some species parasitize fish, marine mammals, or other invertebrates. Additionally, some copepods are adapted to cryospheric environments, living in ice and snow.