Fish eat practically everything, from microscopic algae and floating organic particles to other fish, coral polyps, scales ripped from neighbors, and even fruit that falls from trees into flooded forests. With more than 35,000 known species spread across nearly every aquatic habitat on the planet, fish have evolved an astonishing range of diets and the physical equipment to match. What a particular species eats depends on where it lives, how its body is built, what stage of life it is in, and sometimes on behavioral strategies that look surprisingly clever.
The Big Categories of Fish Diet
Broadly, fish fall into a few dietary camps. Herbivores graze on algae and aquatic plants. Carnivores pursue other animals, whether invertebrates, smaller fish, or both. Omnivores split the difference. And then there are specialists that defy neat labels: fish that scrape coral, filter plankton from the water column, eat parasites off other fish, or subsist on fallen fruit during seasonal floods. Even within a single species, diet can shift with the seasons, with body size, or with what happens to be available.
The diversity of fish diets matters because it shapes entire ecosystems. Herbivorous reef fish keep algae from smothering corals. Fruit-eating fish disperse seeds across floodplain forests. Predatory fish regulate populations of smaller species. Understanding what fish eat is really understanding how aquatic food webs hold together.
How Jaw and Tooth Design Shapes Diet
A fish’s mouth is one of the best clues to what it eats. Most predatory fish capture prey by suction feeding, rapidly opening the mouth to create a rush of water that pulls the target in. Jaw protrusion plays a major role here. When a fish shoots its jaws forward toward a prey item, it can increase the force exerted on that prey by up to about 35%, even though the strike begins from farther away.
1PubMed Central. Jaw protrusion enhances forces exerted on prey by suction feeding fishesNearly all suction-feeding fish achieve this through upper jaw protrusion, but some species have evolved unusual alternatives. A small freshwater invertebrate-eater called Nannocharax fasciatus has a novel joint in its lower jaw that boosts lower jaw protrusion by about 25%, giving it an uncommon approach to snatching tiny prey off the bottom.
2PubMed. Suction Feeding Turned on Its Head: A Functional Novelty Facilitates Lower Jaw ProtrusionTooth shape is just as telling. Prickleback fishes, a family found in rocky intertidal and subtidal habitats, span a whole range of tooth types depending on diet. Species that eat soft-bodied prey tend to have large jaws lined with many fine, pointed teeth for gripping. Species that crush hard-shelled invertebrates like snails have shorter jaws equipped with large, rounded teeth built for crunching.
3PubMed Central. Functional Morphology of the Oral Jaws and Dentition Across Diverse Diets and Ontogeny in Prickleback Fishes (Stichaeidae)Cichlid fishes take this even further. Their oral jaws handle initial capture, while a second set of jaws in the throat (pharyngeal jaws) processes food. The developmental programs for both sets of jaws and teeth act as mechanical filters, determining what size and hardness of food items a given species can handle.
4Hydrobiologia. Toward integrating a molecular understanding of the herbivore–carnivore trophic adaptations in cichlid fishesThe Gut Tells the Story Too
Jaw shape is the front end of the adaptation, but what happens inside the digestive tract is just as important. Herbivorous fish consistently have longer intestines and stockier bodies than their carnivorous relatives. Plant matter is harder to break down than animal tissue, so a longer gut provides more surface area and more time for digestion. Among carnivores, fish that eat other fish (piscivores) have the shortest intestines, while those that eat coral have comparatively longer ones.
5PubMed Central. Diet and habitat as determinants of intestine length in fishesThe differences go beyond anatomy into biochemistry. When researchers compared herbivorous and carnivorous minnows within the same family, the plant-eaters had longer digestive tracts and higher activity of enzymes that break down carbohydrates, while the meat-eaters had higher levels of an enzyme called chitinase, used to digest the hard exoskeletons of insects and crustaceans.
6PubMed. Evolution of herbivory in a carnivorous clade of minnows (teleostei: cyprinidae): effects on gut size and digestive physiologyThese differences are most striking between closely related species with different diets, which makes sense from an evolutionary perspective: when two species share a recent ancestor but have diverged in what they eat, you can see the anatomical and chemical retooling most clearly.
What Coral Reef Fish Eat and Why It Matters
Coral reefs are among the most diet-diverse environments for fish. Herbivores like surgeonfish and parrotfish constantly graze algae off reef surfaces. On Caribbean reefs, ocean surgeonfish and princess parrotfish keep newly colonized substrates in an early stage dominated by short, filamentous algae and crustose coralline algae, a community structure that does not suppress coral growth.
7PLOS ONE. Impact of Herbivore Identity on Algal Succession and Coral Growth on a Caribbean ReefSurgeonfish are recognized as particularly important for maintaining algal turf biomass at levels that help prevent reefs from tipping into algal dominance after disturbances like bleaching events.
8Journal of Experimental Marine Biology and Ecology. The role of surgeonfish (Acanthuridae) in maintaining algal turf biomass on coral reefsParrotfish take a more aggressive approach. They bite directly into coral rock, scraping off organic material and calcium carbonate substrate. This bioerosion is ecologically significant, and the degree of it varies widely across parrotfish species depending on their body size, jaw mechanics, and feeding mode. A mix of parrotfish species with different sizes and strategies is important for keeping the reef system in balance.
9PubMed Central. Parrotfish grazing ability: interspecific differences in relation to jaw-lever mechanics and relative weight of adductor mandibulae on an Okinawan coral reefButterflyfish represent the specialist end of the reef diet spectrum. On Okinawan reefs, obligate coral-feeding butterflyfish strongly prefer certain live coral types, particularly tabular and corymbose Acropora, encrusting corals, and massive corals, while avoiding dead coral, rubble, and rock. Facultative coral feeders are somewhat more flexible, with some even targeting dead coral surfaces, likely for the microbial films or invertebrates growing on them.
10PubMed Central. Spatial distribution and feeding substrate of butterflyfishes (family Chaetodontidae) on an Okinawan coral reefFruit-Eating Fish in Flooded Forests
One of the more surprising fish diets involves fruit. In the Amazon basin and the Pantanal wetlands of Brazil, large-bodied fish like the pacu migrate into seasonally flooded forests and gorge on fallen fruits and seeds. In the Pantanal, the most important food item for pacu during the wet season was the fruit of the palm Bactris glaucescens, found in the guts of about 73% of all fish collected. Larger fish, with bigger mouths, carried more intact seeds, making the biggest individuals the most effective seed dispersers.
11Biotropica. Big Fish are the Best: Seed Dispersal of Bactris glaucescens by the Pacu Fish (Piaractus mesopotamicus) in the Pantanal, BrazilThis is not a minor ecological sideshow. In Amazonian floodplains, fruit-eating fish in the genera Colossoma and Piaractus disperse large quantities of seeds from up to 35% of the trees and lianas that fruit during the flood season, and these seeds can still germinate after the waters recede.
12PubMed. High-quality seed dispersal by fruit-eating fishes in Amazonian floodplain habitatsAt a basin-wide scale, frugivorous fish species richness across the roughly six million square kilometers of the Amazon correlates with floodplain forest extent, tree species richness, and flood duration. The relationship runs both ways: diverse forests support more fruit-eating fish species, and diverse fish assemblages help maintain and regenerate diverse forests through seed dispersal.
13PubMed. Floodplain forests drive fruit-eating fish diversity at the Amazon Basin-scaleDeforestation or damming that reduces flood extent threatens this mutually dependent system by cutting the link between fish and trees.
Feeding in the Deep Sea and Open Ocean
The further you get from sunlit surface waters, the scarcer food becomes, and deep-sea fish have adapted accordingly. One overlooked food source in the abyss is large animal carcasses that sink to the seafloor. A study off the coast of Angola documented clusters of large elasmobranch (shark and ray) carcasses on the deep seabed. When researchers calculated the carbon delivered by these food-falls, the rate was equivalent to about 4% of the total organic carbon flux reaching the seafloor in that area, an order of magnitude higher than previous estimates for whale carcasses.
14PLoS ONE. Fish Food in the Deep Sea: Revisiting the Role of Large Food-FallsLarge food-falls turn out to be far more significant to deep-sea food webs than scientists once assumed.
Some fish eat things that barely seem like food at all. Japanese eel larvae (leptocephali) were long a mystery because no one could figure out what they ate in the wild. Isotopic analysis showed that their diet is consistent with marine snow, the diffuse rain of organic particles, discarded gelatinous structures from tiny planktonic animals, bacteria, and protozoans that drifts slowly through the water column.
15PubMed Central. A low trophic position of Japanese eel larvae indicates feeding on marine snowAt the other extreme, open-ocean predators like the shortfin mako shark are pursuit hunters with enormous energy demands. Mako sharks are thought to be among the fastest-swimming sharks, and field measurements estimated their routine metabolic rate at about 185 mg of oxygen per kilogram per hour, reflecting the sheer caloric cost of high-speed hunting in the open sea.
16PubMed Central. Direct measurement of cruising and burst swimming speeds of the shortfin mako shark (Isurus oxyrinchus) with estimates of field metabolic rateSuch energy-intensive predators need to consume a lot of prey, which also makes them sensitive to changes in prey availability or ocean oxygen levels.
How Fish Find Their Food
Vision matters for many species, but fish have sensory systems that go well beyond sight. The lateral line, a series of pressure-sensitive organs running along the body, allows fish to detect subtle water movements around them. This system plays an important role in locating and tracking prey. Even when fish can see their prey, removing lateral line input changes the angle at which they approach live, moving targets, suggesting that the sense of water flow contributes to the precision of a strike even when vision is available.
17PubMed Central. The Silverjaw Minnow, Ericymba buccata: An Extraordinary Lateral Line System and its Contribution to Prey Detection18PubMed. Coping with flow: behavior, neurophysiology and modeling of the fish lateral line system
Sharks and rays possess an additional sense that most fish lack: electroreception. Their ampullae of Lorenzini detect the faint bioelectric fields generated by the muscles and nerves of other animals. Classic experiments demonstrated that dogfish sharks and thornback rays performed well-aimed feeding strikes at flatfish buried in sand, even when visual, chemical, and mechanical cues were screened out. When researchers replaced the hidden flatfish with electrodes mimicking the flatfish’s bioelectric field, the sharks and rays attacked the electrodes with exactly the same feeding behavior.
19Journal of Experimental Biology. The Electric Sense of Sharks and RaysFor a shark hunting over sandy or murky bottoms, this electric sense is the equivalent of having X-ray vision for buried prey.
Filter Feeders and the Largest Fish in the Sea
Not all fish hunt individual prey. Some of the largest fish alive, including whale sharks, basking sharks, and manta rays, feed by filtering enormous volumes of water to extract tiny plankton and small fish. Among filter-feeding sharks and rays, the filtering mechanisms vary. Some species have sticky gill-raker surfaces that trap particles, while others rely on different physical principles like direct sieving (catching particles bigger than the gaps between filter elements), inertial impaction (where fast-moving particles cannot change direction quickly enough to avoid the filter), or cross-flow filtration, where water moves across the filter surface rather than straight through it.
20PubMed. Comparison of the structure and composition of the branchial filters in suspension feeding elasmobranchsThese different strategies let filter feeders exploit slightly different size ranges of plankton and different water conditions.
Diet Shifts Across a Fish’s Lifetime
What a fish eats at one month old is rarely what it eats as an adult. Larval fish are tiny, active predators in the plankton, and many species start by feeding on copepod nauplii, the larval stage of tiny crustaceans. As the fish grows, it gradually shifts to targeting copepodites, the older and larger juvenile stage of the same crustaceans.
21PubMed Central. Morphological covariates of the ontogenetic shift from nauplii to copepodite prey in larval fishThis progression tracks the fish’s increasing mouth size and swimming ability. A larval cod barely visible to the naked eye cannot chase down the same prey as a 50-centimeter juvenile, and its jaw simply cannot accommodate large food items.
Some ontogenetic diet shifts are dramatic. Many reef fish begin life eating plankton in the open water, then switch to algae, invertebrates, or other fish after settling onto the reef. Salmon famously feed on insects in freshwater streams as juveniles, then shift to a diet of smaller fish and krill in the ocean as adults. These shifts mean that a single species can occupy very different roles in the food web at different points in its life.
Scale Eaters, Cleaner Fish, and Other Unusual Specialists
Some fish have dietary habits that border on the bizarre. In Lake Tanganyika, seven species in the cichlid genus Perissodus specialize in eating the scales of other fish.
22PubMed Central. Evolution of feeding specialization in Tanganyikan scale-eating cichlids: a molecular phylogenetic approachThese species have evolved distinct tooth shapes matched to their attack strategy. One species has sharp-edged teeth that function like blades, scraping scales off as the fish slides its mouth laterally along the victim’s flank. Another has spine-like tooth projections that hook under scales and wrench them loose during a press-and-twist motion.
23Journal of Fish Biology. Foraging behaviour and functional morphology of two scale‐eating cichlids from Lake TanganyikaPerhaps the strangest twist is that these scale eaters have evolved mouth asymmetry. Some individuals have mouths that open to the left, attacking the prey’s right flank, while others open to the right and attack the left. The ratio of left-handed to right-handed morphs in the population oscillates over time, maintained by frequency-dependent selection: when one morph becomes common, prey fish learn to watch for attacks from that side, giving the rarer morph an advantage.
24PubMed. Frequency-dependent natural selection in the handedness of scale-eating cichlid fishOn the cooperative end of the spectrum, cleaner fish eat parasites off the bodies of larger “client” fish. In the Azores, two wrasse species actively inspect and remove ectoparasites from their clients. Interestingly, these cleaners are selective: both species preferentially ate gnathiid isopod larvae while ignoring caligid copepods, even though caligid copepods were the most abundant ectoparasite found on client fish.
25Marine Ecology Progress Series. Temperate facultative cleaner wrasses selectively remove ectoparasites from their client-fish in the AzoresThe cleaners are not just eating whatever they find; they are choosing specific prey items from the buffet on a client’s skin.
Tool use, though rare in fish, also exists. A review of the behavior found that tool use seems to be concentrated in a small number of fish groups, particularly wrasses. The pattern echoes what researchers see in mammals and birds, where tool use clusters in specific lineages like primates and corvids rather than being spread broadly across the group.
26Wiley Online Library. Tool use in fishesToxins Acquired Through Diet
Not everything fish eat is benign. Pufferfish are famous for carrying tetrodotoxin, one of the most potent natural poisons. But pufferfish do not produce this toxin themselves. It originates in marine bacteria, and pufferfish accumulate it through their food chain. When pufferfish are raised on toxin-free diets in environments where toxin-bearing organisms cannot get in, they become completely non-toxic.
27PubMed Central. Tetrodotoxin–distribution and accumulation in aquatic organisms, and cases of human intoxicationThis is a vivid reminder that a fish’s toxicity can be a direct product of its diet rather than its genetics. Farmed pufferfish served in some Japanese restaurants are marketed as safe precisely because their controlled diet excludes the bacterial toxin source.
Microplastics and Warming Waters
What fish eat increasingly includes things they should not. Microplastics have become a pervasive contaminant in aquatic environments, and fishery products are now recognized as a significant source of microplastic exposure for humans. Once ingested, these particles can reach the gastrointestinal tract and potentially be absorbed, causing oxidative stress and other cellular damage.
28PubMed Central. Microplastics in Fish and Fishery Products and Risks for Human Health: A ReviewFish at higher trophic levels, meaning predators that eat other contaminated fish, tend to accumulate more microplastics. A study of two commercial fish families found a significant positive correlation between a fish’s trophic position and the amount of microplastic in its body, pointing to bioaccumulation up the food chain.
29International Journal of Environmental Research. Impact of Microplastic Ingestion on Commercial Fish: A Trophic-Level AnalysisClimate change is altering feeding relationships too. In the Southern Ocean, rising sea surface temperatures are reshaping the size relationships between predators and prey. The ratio of predator size to prey size declines by about 11% for each degree Celsius of warming, driven largely by predators getting smaller (about 6% per degree) while their prey size stays roughly the same. Under warmer conditions, the available prey shifts toward intermediate body sizes, which forces large predators to eat prey that are smaller than optimal for them, while smaller predators benefit from the abundance of these mid-sized prey items.
30PubMed Central. Temperature alters the predator-prey size relationships and size-selectivity of Southern Ocean fishThe result is a restructuring of who eats whom, with potential consequences for fisheries and marine food webs that are still being worked out.