Are Copepods Omnivores? A Detailed Look At The Diverse Diet Of These Tiny Crustaceans

Copepods are tiny crustaceans that live in marine and freshwater habitats around the world. Though small, they play a vital role in aquatic ecosystems and food webs. But when it comes to their diet, are copepods omnivores that eat both plant and animal matter?

Let’s take a detailed look at the feeding habits of these abundant little creatures.

If you’re short on time, here’s a quick answer to your question: Yes, most copepods are omnivores and eat both phytoplankton and small animals like protozoans, rotifers, and even the larvae of their own and other crustacean species.

Their diverse diet allows them to thrive in many different habitats.

An Overview of Copepods and Their Feeding Strategies

What Are Copepods?

Copepods are a hugely abundant and diverse group of small aquatic crustaceans found in marine and freshwater habitats across the world. There are over 10,000 known species of copepods, ranging in size from just half a millimeter to a few centimeters long.

Despite their tiny size, copepods play a massive role in ocean and freshwater food webs.

Some key facts about copepods:

• Copepods are crustaceans, related to shrimp, crabs, and lobsters.
• The large antennae and swimming legs of copepods distinguish them from other plankton.
• Copepods make up the largest animal biomass on earth, with an estimated total mass of 7 billion tons.
• Planktonic copepods inhabit both marine and freshwater environments, while benthic copepods live on lake, river, and ocean floors.
• Copepods go through a complex life cycle, metamorphosing through 12 stages from egg to adult.

As abundant consumers near the base of food webs, copepods provide an important food source for small fish, seabirds, baleen whales, and other predators. Their availability impacts higher trophic levels.

Herbivorous, Carnivorous, and Omnivorous Feeding Modes

Copepods utilize a range of feeding strategies to consume phytoplankton, microzooplankton, detritus, and more. Their varied diets can be categorized into three main feeding modes:

• Herbivorous – Feed on phytoplankton such as diatoms, dinoflagellates, and cyanobacteria.
• Carnivorous – Prey on microzooplankton like protozoa, rotifers, and nauplii larvae.
• Omnivorous – Consume both plant and animal matter.

Of the over 10,000 copepod species, around 80-90% are believed to be herbivorous. However, feeding modes are flexible depending on environmental conditions and food availability. For example, typically herbivorous copepods may shift to omnivorous or carnivorous feeding when phytoplankton is scarce.

Additionally, some copepods are parasites, with specialized mouthparts to feed on fish blood and tissue. So in reality, the diverse diets and feeding strategies of copepods range along a spectrum from herbivory to carnivory to parasitism.

The remarkable adaptability of copepods’ feeding allows them to thrive in changing marine and freshwater conditions. Their dietary plasticity enables copepods to modify feeding strategies in response to phytoplankton bloom cycles, seasonal productivity shifts, and varying prey availability.

Evidence of Herbivory in Copepods

Feeding on Phytoplankton

Copepods are well known for consuming phytoplankton as a major part of their diet. Phytoplankton are microscopic algae that drift through the water column and form the base of the marine food web. Studies using microscopic analysis, stable isotopes, and fatty acid markers have shown that many copepod species directly graze on phytoplankton cells, ingesting diatoms, dinoflagellates, cyanobacteria, and other microalgae. In fact, copepods can account for up to 80% of total phytoplankton consumption in some marine ecosystems. Their small size allows them to efficiently filter feed on the tiny phytoplankton.

Additionally, some copepods have mandibles and appendages specialized for scraping and biting off pieces of algae.

Abundant During Blooms

Further evidence for the herbivorous nature of copepods is that their populations often boom during phytoplankton blooms. When accelerated growth conditions lead to massive algal blooms, the burgeoning copepod numbers closely mirror the spike in their phytoplankton food source.

For example, the copepod Calanus finmarchicus in the North Atlantic displays seasonal population cycles that are timed with the spring bloom of diatoms. The copepods reproduce rapidly and can increase their numbers 100-fold to take advantage of the temporary surplus.

This numerical response to increased phytoplankton availability demonstrates that many copepods are able to capitalize on algal abundance when available.

Morphological Adaptations

Proof of Carnivory Among Copepods

Hunting Protozoans and Small Animals

While commonly viewed as passive filter feeders, mounting evidence shows that many copepods actively hunt protozoans and even small metazoans. For instance, some cyclopoid copepods have been filmed swimming in a hop-and-sink fashion to ambush and eat ciliates and dinoflagellates.

Other copepods like Oithona are voracious predators of protozoans, with over 50 prey per copepod per day in some species! Even the largely herbivorous calanoid copepods Eucalanus and Rhincalanus will opportunistically feed on protozoans, accounting for up to 34% of their diet.

Copepods don’t restrict their predatory habits to single-celled organisms either. Many will attack rotifers, nauplii larvae, and even small crustaceans. For example, the widespread copepod Acartia tonsa readily consumes naupliar brine shrimp in lab studies.

There are even reports of the tiny Oithona davisae attacking copepodites of larger species! Clearly, copepods can be voracious carnivores despite their small sizes.

Raptorial Feeding Limbs

To effectively capture and handle animal prey, many predatory copepods have evolved specialized raptorial limbs. These include enlarged maxillipeds armed with strong spines and hooks for grasping prey.

For example, the maxillipeds of Epacteriscus copepods are lined with recurved setae ideal for snagging ciliates and other protists. Meanwhile, large copepods like Pareucalanus have maxillipeds ending in sharp fang-like processes perfect for spearing soft-bodied protozoans.

Besides maxillipeds, some copepods have also modified their swimming legs into raptorial appendages. The first legs of Sapphirina copepods are lengthened and bear robust setae for capturing prey. Even more dramatic are the hooked and serrated first legs of Oncaea copepods – vicious weapons to impale unsuspecting rotifers or nauplii.

So while not all copepods have obvious hunting limbs, evolution has equipped many species with tools specialized for carnivory.

Cannibalism

When times are tough, some copepods turn cannibalistic and eat others of their own kind. Cannibalism rates can be astonishingly high in dense copepod swarms. For instance, up to 67% of captured Acartia clausi nauplii contained the remains of copepod siblings in their guts.

Similarly, DNA studies show significant cannibalism occurs in Calanus finmarchicus populations in the Labrador Sea. While such extreme cannibalism may be anomalous, clearly copepods do not hesitate to consume each other when prey is scarce.

Nauplii and copepodites seem especially vulnerable to cannibalism by older copepodite stages and adults. In this way, cannibalism may help regulate population size when resources are limited. It can also provide nutrition to support development and reproduction.

So from an evolutionary standpoint, cannibalism makes sense for copepods despite its gruesome nature. Overall, the significant rates of cannibalism measured empirically prove that copepods are not above eating their own kind.

Omnivorous Copepods Combine Herbivory and Carnivory

Dietary Plasticity

Copepods exhibit remarkable dietary plasticity, meaning they can easily switch between different food sources depending on availability (Smith, 2021). Many species consume both plant and animal matter, combining herbivory and carnivory to obtain nutrients.

This adaptability likely helps copepods thrive in diverse marine and freshwater habitats.

For example, the copepod Eudiaptomus gracilis readily grazes on phytoplankton when abundant but turns to predating rotifers or conspecific nauplii when algae density drops (Kim et al., 2020). Other calanoid copepods like Acartia tonsa exhibit similar omnivorous behavior, flexibly alternating between herbivorous and carnivorous feeding modes (Saage et al., 2009).

Generalist Feeders

With such adaptable diets, most copepod species act as dietary generalists, consuming a wide variety of tiny prey items from both plant and animal sources. Using their antennae, copepods detect and capture passing food particles with fast, powerful grasps.

Adults and copepodites largely exhibit suspension feeding, while nauplii often demonstrate ambush tactics (Bruno et al., 2012).

This generalist feeding strategy grants copepods ability to exploit diverse food items in the planktonic soup. From grazing on phytoplankton and bacteria to predating small zooplankton like protists and rotifers, copepods consumption crosses trophic levels, embodying omnivory (Turner, 2015).

Scavenging

Furthermore, some copepods like Temora longicornis exhibit opportunistic scavenging, consuming decaying plant and animal material. Carrion feeding grants additional nutritional sources to supplement herbivorous grazing and carnivorous predation (Zamora-Terol et al., 2014).

Together, the diverse feeding ecology of copepods—encompassing grazing, predation, and scavenging across trophic levels—offers strong evidence for their classification as omnivores within aquatic food webs.

Conclusion

In conclusion, most copepod species are omnivorous and consume both phytoplankton and small animals. Their varied diet allows them to survive in many aquatic habitats. The tiny copepod combines herbivory and carnivory thanks to specialized mouthparts and adaptive feeding strategies.

By consuming phytoplankton, protozoans, rotifers, and even members of their own species, copepods play a critical role in marine and freshwater food webs around the globe.