Omnivores are far more common in the ocean than most people expect. From the largest fish on Earth to microscopic plankton barely visible under a lens, marine animals that eat both plant and animal material show up at every depth and in nearly every habitat. Research has found that omnivory actually increases as you move from freshwater to marine environments among filter-feeding and predatory fish, suggesting something about ocean life encourages dietary flexibility.
Whale Sharks and Bonnethead Sharks
Two of the most surprising ocean omnivores belong to a group most people associate strictly with meat-eating. Whale sharks, the largest fish alive, were long assumed to be planktivores that passively filtered tiny animals from the water. A multi-technique study using amino acid and fatty acid analysis showed that whale sharks actually have a trophic level consistent with omnivory, and that floating Sargassum seaweed and its associated organisms appear to be a meaningful part of their diet.1PubMed. The world’s largest omnivore is a fish That makes the whale shark, at roughly 12 meters long and several tons, the world’s largest omnivore by a wide margin.
Bonnethead sharks, a small hammerhead species common along the Atlantic and Gulf coasts, offer an even more striking case. Researchers fed bonnetheads a diet of seagrass in controlled conditions and found the sharks digested about half the organic matter from the grass, roughly comparable to the digestive efficiency of a young sea turtle eating the same food.2PubMed Central. Seagrass digestion by a notorious ‘carnivore’ The sharks were not just swallowing seagrass accidentally while chasing prey. They were breaking it down and assimilating its carbon. For a group of animals defined in the public imagination by teeth and speed, that finding challenged a lot of assumptions.
Reef and Coastal Fish
Coral reefs and seagrass meadows host some of the most obvious fish omnivores. Pinfish are a textbook example. These small, abundant fish in seagrass beds shift their diets with the seasons: more carnivorous in spring when animal prey is plentiful, then largely consuming epiphytic algae during summer months.3Journal of Experimental Marine Biology and Ecology. Trophic relationships among concentrated populations of small fishes in seagrass meadows Because pinfish feed at more than one trophic level, they scramble the simple food-chain picture where big things eat medium things that eat small things. In seagrass ecosystems, pinfish produce changes in both the tiny organisms growing on seagrass blades and the seagrasses themselves, but they do not trigger the clean predator-prey cascades ecologists sometimes predict.4Limnology and Oceanography. Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment
Mesocosm experiments testing how predator diversity affects algae showed that adding pinfish, an omnivore, to groups of carnivorous predators actually reversed the expected trophic cascade. Instead of more predators leading to less algae-eating and more algal growth, the omnivorous pinfish complicated the picture and changed the algal community’s composition.5Ecology Letters. Cascading effects of predator diversity and omnivory in a marine food web This is a recurring theme with marine omnivores: they make food webs messier and harder to predict, which, as we will see later, can actually be a stabilizing force.
Damselfish in the genus Stegastes, commonly seen on southwestern Atlantic reefs, offer another example. These territorial fish aggressively guard patches of algae they cultivate on the reef, and more than 70% of their diet consists of algal material. But they also eat invertebrates and detritus as accessory items, making up roughly 15% of the diet.6Journal of Sea Research. Food resource use by two territorial damselfish (Pomacentridae: Stegastes) on South-Western Atlantic algal-dominated reefs The “algae farming” behavior gets most of the attention, but these fish are not strict herbivores by any measure.
Blue Crabs, Sea Urchins, and Other Bottom-Dwellers
Many of the ocean’s most familiar invertebrates are omnivores. Blue crabs are a well-known case. In the wild, they eat clams, fish, worms, shrimp, plant material, and decaying organic matter with little apparent preference. Laboratory experiments have shown just how costly it is for them to go without animal food: crabs restricted to an all-seaweed diet had a threefold drop in the fat stored in their digestive gland and became twice as aggressive compared to crabs eating animal tissue.7PubMed Central. The Influence of Diet Composition on Fitness of the Blue Crab, Callinectes sapidus Crabs on seaweed-only diets also died at higher rates. So while blue crabs can eat plant material and do so routinely, the animal component of their diet appears to be the more nutritionally critical half.
Sea urchins are often thought of as grazers that mow down kelp and algae, but many species are genuinely omnivorous. A study of a widespread urchin species across multiple sites found estimated trophic levels ranging from 2.4 to 4.6, a huge span. A trophic level of 2 is a pure herbivore and 4 is a secondary predator, so these urchins were occupying positions all the way from near-herbivore to near-top-predator depending on what food was available locally.8PubMed. Local flexibility in feeding behaviour and contrasting microhabitat use of an omnivore across latitudes Their diet reflected whatever prey happened to be most accessible in their immediate microhabitat, whether that was epiphytes, understorey algae, or small animals. This kind of opportunistic flexibility is a hallmark of marine omnivory.
Krill and Copepods
Even at the base of the marine food web, omnivory is widespread. Antarctic krill, the tiny crustaceans that fuel everything from penguins to blue whales, shift their feeding as they mature. Juveniles feed mainly on phytoplankton, while adults switch to eating small copepods. The shift is supported by differences in their body fat composition, with adults carrying elevated levels of fatty acids associated with carnivorous feeding.9Limnology and Oceanography. Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—II. Juveniles and adults This pattern of becoming more carnivorous with age appears to be a strategy for conserving energy during the lean Antarctic winter, when phytoplankton blooms fade and animal prey becomes a more reliable calorie source.
Copepods themselves, the small crustaceans that krill adults eat, are often omnivores too. The marine copepod Acartia clausi, common in coastal waters worldwide, switches between eating algae and eating ciliates depending on which prey is more abundant. Lab experiments using radioactively labeled prey showed that this copepod responds to changing food availability in a sophisticated way, ramping up consumption of whichever prey type becomes more plentiful rather than sticking to a fixed diet.10Marine Ecology Progress Series. Prey switching by Acartia clausi: experimental evidence and implications of intraguild predation assessed by a model For an animal smaller than a grain of rice, that is a surprisingly dynamic feeding strategy.
Deep-Sea Scavengers
In the deep ocean, where food is scarce and arrives unpredictably, omnivory becomes less of an option and more of a survival requirement. Amphipods, the small crustaceans that dominate scavenging communities on the deep seafloor, show what researchers call “remarkable trophic plasticity.” Each species uses multiple foraging modes, supplementing its scavenging on dead animals with detritus-eating or active predation on other small organisms.11Limnology and Oceanography. Extreme food webs: Foraging strategies and diets of scavenging amphipods from the ocean’s deepest 5 kilometers When a whale carcass or dead fish lands on the abyssal plain, amphipods swarm it. Between those feasts, they eat whatever else they can find.
A study of the deep-sea amphipod Eurythenes gryllus in the Arctic used DNA analysis of gut contents to figure out exactly what these animals had been eating. Out of 101 amphipods analyzed, the ones with identifiable gut contents had primarily consumed larger animals including bony fish, cephalopods, and even marine mammals, most likely as carcasses that sank from above.12Environmental DNA. The Sinking Dead—Arctic Deep‐Sea Scavengers’ Diet Suggests Nekton as Vector in Benthopelagic Coupling These amphipods serve as a critical link between the productive surface ocean and the food-poor deep seafloor, recycling the nutrients locked in sinking bodies.
Green Turtles and Other Marine Reptiles
Green sea turtles are often described as herbivores, and adults in warm tropical waters do eat mostly seagrass and algae. But the full picture is more complicated. A global review of green turtle diets found that water temperature plays a significant role in how much animal matter they eat. At warm sites where sea surface temperatures exceeded 25°C for at least six months per year, their diet averaged about 93% plant material. At cooler sites where temperatures stayed below 20°C for extended periods, animal matter jumped to roughly half the diet.13Marine Biology. A global review of green turtle diet: sea surface temperature as a potential driver of omnivory levels The temperature effect was statistically significant, making this the first quantitative evidence at a global scale that temperature may drive omnivory in these animals.
This finding has practical implications for how green turtle populations might respond to climate change. If warming oceans push more turtle habitats above the 25°C threshold, diets in those areas could become more herbivorous. Conversely, turtles at the edges of their range in cooler waters are already more omnivorous than their tropical relatives, and shifts in prey availability due to changing ocean conditions could alter that balance further.
Marine Mammals and Seabirds
Sea otters are among the most visible marine mammal omnivores, though their diet leans heavily toward the animal side. They eat sea urchins, crabs, clams, octopuses, and fish, consuming thousands of calories a day to maintain their body temperature in cold Pacific waters. Some populations also eat kelp and other plant material, placing them in the omnivore category even if their reputation as voracious predators of urchins dominates the conversation.
Seabirds that feed at sea offer a different angle on marine omnivory. Herring gulls, studied over a decade of breeding and dietary data, showed that most pairs concentrated on bivalves during the breeding season. Pairs that supplemented their chick diet with energy-rich items like domestic refuse and fishery discards had higher reproductive success.14PubMed Central. The most common diet results in low reproduction in a generalist seabird The gulls are omnivores not just in the sense that they can eat anything, but in the sense that diet composition directly shapes their breeding outcomes.
Glaucous-winged gulls along the Pacific coast have undergone a documented shift in their diet over the last 150 years, moving from a mid-level marine diet toward one that includes lower-trophic marine prey and more terrestrial or freshwater foods. Isotopic analysis of feathers spanning that time period confirmed the trend.15Environmental Science and Technology. Human-Induced Long-Term Shifts in Gull Diet from Marine to Terrestrial Sources in North America’s Coastal Pacific: More Evidence from More Isotopes (δ2H, δ34S) Human activities, particularly changes to fisheries and the expansion of landfills, appear to have reshaped the diets of these omnivorous birds over time.
How Gut Bacteria Support Marine Omnivory
One reason omnivory works in the ocean has to do with what lives inside these animals’ digestive tracts. Research on marine fish with different feeding habits, including carnivores, phytoplanktivores, and omnivores, found cellulolytic bacteria (bacteria capable of breaking down plant cell walls) in the guts of all three types.16Biocatalysis and Agricultural Biotechnology. Four novel strains of cellulolytic symbiotic bacteria isolated and characterized from GI tract of marine fishes of various feeding habits Even fish classified as carnivores carried gut microbes that could digest plant material. This suggests a degree of latent digestive flexibility across marine fish, which could help explain why dietary shifts, like the seasonal ones seen in pinfish, are physiologically possible in the first place.
The bonnethead shark’s ability to digest seagrass almost certainly depends on similar microbial partnerships. No vertebrate produces cellulase on its own; all plant digestion in vertebrates requires symbiotic microorganisms. The presence of these bacteria across fish with very different feeding habits hints that the microbial toolkit for omnivory is more widely distributed in marine animals than their feeding labels suggest.
Why Marine Omnivory Matters for Food Web Stability
Omnivores do not just feed differently from specialists. They change the way entire ecosystems behave. In a high-Arctic fjord exposed to freshwater inputs, researchers found that the benthic food web remained remarkably stable despite big seasonal shifts in what food was available. The explanation was the high degree of trophic plasticity and omnivory among the bottom-dwelling consumers, which allowed several different carbon pathways through the food web to stay active even when some food sources disappeared.17Limnology and Oceanography. Food source diversity, trophic plasticity, and omnivory enhance the stability of a shallow benthic food web from a high‐Arctic fjord exposed to freshwater inputs
The relationship between omnivory and stability is not always straightforward, though. A study of a marine subtidal food web found that increasing the prevalence of omnivory among top predators actually decreased the stability of herbivore populations and microalgae.18PubMed. Food chain length and omnivory determine the stability of a marine subtidal food web Some algal groups benefited while others did not. The picture that emerges is that omnivory can stabilize food webs at a broad level by maintaining energy flow through multiple channels, but it can also destabilize particular populations within that web by introducing unpredictable predation pressure. Whether the net effect is stabilizing or destabilizing depends on the specific community and which species are doing the omnivorous feeding.
This complexity explains why marine omnivory tends to be more common than freshwater omnivory. In marine ecosystems, filter-feeding and predatory fish species show higher levels of omnivory than their freshwater counterparts.19Fish and Fisheries. Ecosystem type shapes trophic position and omnivory in fishes Ocean food webs tend to be longer and more complex, with more potential food sources available. That richness of options may make it evolutionarily advantageous for marine animals to keep their dietary options open rather than specializing on a single food type.
When Omnivores Look Like Something Else
One of the persistent challenges in marine biology is that omnivory often hides behind misleading labels. Whale sharks were classified as planktivores for decades. Bonnetheads were considered pure carnivores. Green turtles were treated as herbivores. In each case, improved analytical tools, especially stable isotope analysis and fatty acid profiling, revealed a more mixed diet than traditional gut-content studies had shown. Gut contents capture a snapshot of the last meal; isotope analysis captures what an animal has been assimilating over weeks or months, which often tells a different story.
Salps, the gelatinous barrel-shaped tunicates that drift in open ocean waters, were traditionally seen as indiscriminate filter feeders that ate whatever the current delivered. More precise sampling has shown that their feeding is not purely passive. In situ measurements of undisturbed salps revealed that they preferentially remove certain prey types, with small picoeukaryotes cleared faster than either larger cells or smaller bacteria.20Limnology and Oceanography. Prey taxonomy rather than size determines salp diets Their prey-to-predator size ratios turned out to be an order of magnitude smaller than previously reported, meaning salps were catching far tinier particles than expected. Whether salps count as omnivores depends partly on how broadly you define the term, but their diet spans multiple trophic levels, including both photosynthetic organisms and the animals that eat them.
The broader lesson is that the ocean’s real dietary categories are blurrier than the neat herbivore-carnivore-omnivore labels suggest. Many animals that spend most of their time eating one type of food will opportunistically switch when conditions change, whether that means a seasonal shift in prey availability, an ontogenetic transition from juvenile to adult, or a geographic difference in water temperature. Omnivory in the ocean is less a fixed identity and more a spectrum that animals slide along throughout their lives.