Invertebrates eat virtually everything the living world has to offer, from single-celled algae scraped off rocks to prey larger than themselves. Because invertebrates make up roughly 97% of all known animal species, their collective diet spans every energy source available on Earth: sunlight captured by symbiotic algae, chemicals seeping from hydrothermal vents, decaying leaf litter on the forest floor, blood drawn from a sleeping host, and fresh kills subdued with venom. Understanding how these animals feed is really understanding how ecosystems move energy around, since invertebrates do most of that work.
Straining Meals From the Water
Some of the most successful invertebrate feeding strategies involve doing almost nothing at all. Suspension feeders and filter feeders pull tiny food particles out of the water column, and they have evolved an impressive variety of tools to do it. Sponges pump water through their bodies and trap bacteria on specialized cells. Bivalves like mussels and oysters draw water across internal gills that act as pleated filters, catching phytoplankton and organic particles. Gorgonians and crinoids take an even more passive approach, holding fan-like structures into the current and letting the water deliver food to them.1PubMed Central. Suspension feeders: diversity, principles of particle separation and biomimetic potential Some species build external filtering devices: larvaceans, tiny relatives of sea squirts, secrete intricate mucous “houses” that serve as particle traps, filtering food from surrounding seawater before the animal eats the entire mucous structure and builds a new one.
In freshwater ecosystems, filter-feeding and particle-collecting invertebrates often dominate the community. A survey of aquatic insects in Moroccan streams found that collector-gatherers and collector-filterers together made up about 40% of all insects present, with predators coming in at around 28%.2PubMed Central. Assessment of functional feeding groups (FFG) structure of aquatic insects in North- western Rif – Morocco That proportion can shift depending on season and habitat, but in most flowing-water systems, animals that gather fine particles or filter them from the current account for the largest share of the invertebrate community.
Grazing on Algae and Eating Plants
Herbivorous invertebrates attack everything from microscopic algae films on stream rocks to tough woody plant tissue. Freshwater snails are among the most visible algae grazers. They use a ribbon-like feeding organ called the radula, which slides over surfaces and scrapes off algal cells. The mechanics are surprisingly refined: in pond snails, the radula and the cartilage beneath it move semi-independently during each feeding stroke, with the relative speed between the two varying depending on how much food is available.3Journal of Experimental Biology. Radular Kinetics During Grazing in Helisoma Trivolvis (Gastropoda: Pulmonata) Sea urchins, limpets, and many marine snails use their own versions of this scraping apparatus to graze algae off coral reefs and rocky shores.
Eating plants presents a bigger biochemical challenge than eating algae. Plant cell walls contain cellulose, and for a long time researchers assumed that animals needed gut bacteria to break it down. That turned out to be partly wrong. Stick insects, for example, produce their own cellulase enzymes capable of handling several types of cell-wall polymers without microbial help. The discovery overturned the long-held assumption that cellulose digestion in insects was exclusively a microbial job. Many caterpillars, grasshoppers, and beetle larvae also feed on live plant tissue, though the degree to which they rely on their own enzymes versus gut microbes varies widely across species.
Growing and Eating Fungi
Plenty of invertebrates eat fungi, but three groups of insects went a step further and invented agriculture. Leafcutter ants, certain bark beetles, and some termites all actively cultivate fungal gardens, tending them with a sophistication that took researchers decades to fully appreciate. Leafcutter ants, for instance, do not actually eat the leaf fragments they carry back to the colony. Instead, they chew the leaves into a pulp that serves as a growth medium for specific fungi, mostly in the tribe Leucocoprini. The ants then feed on the fungal tissue that grows from this substrate.4PubMed Central. Mycophagy: A Global Review of Interactions between Invertebrates and Fungi Ambrosia beetles bore into wood and introduce fungal spores into their tunnels, cultivating the resulting growth as their primary food source. Fungus-farming termites do something similar using plant material gathered from the surrounding soil.
Beyond these specialized farmers, many soil-dwelling arthropods are more casual fungivores. Springtails, mites, and various beetle larvae consume fungal hyphae and spores they encounter in leaf litter and topsoil. These generalist fungus-eaters tend to be polyphagous, meaning they feed across multiple fungal species without being deterred by the chemical differences between them.4PubMed Central. Mycophagy: A Global Review of Interactions between Invertebrates and Fungi
Breaking Down Dead Matter
Detritivores eat what everything else has discarded: fallen leaves, dead wood, animal carcasses, feces, and the general organic sludge that accumulates in soil and sediment. Earthworms are the most familiar example, and their contribution to decomposition is enormous. As organic material passes through an earthworm’s gut, the resident microbial community goes to work. Key bacterial groups break down cellulose and other tough plant polymers, and experiments show that when those gut microbes are removed, earthworms’ ability to degrade cellulose drops dramatically.5PubMed. Gut microbial communities and their potential roles in cellulose digestion and thermal adaptation of earthworms The gut environment selectively enriches microbes that handle carbon breakdown, nitrogen transformation, and phosphorus release, while simultaneously suppressing pathogenic organisms.6Annals of Microbiology. Earthworm gut microbiota and their role in vermicomposting: a review
Aquatic detritivores work in similar ways. Amphipods and isopods shred leaf litter that falls into streams, making it available to smaller organisms. Fly larvae, particularly chironomids, gather fine organic particles from sediment. In the deep ocean, detritivory takes on surreal dimensions: when a whale carcass sinks to the seafloor, it creates an entire ecosystem that can persist for decades. The succession of invertebrate communities on a whale fall passes through distinct stages, and these communities harbor many species found nowhere else, including bone-eating worms and specialized snails that graze on sulfur-oxidizing bacteria growing on the skeleton.7PubMed. Whale-fall ecosystems: recent insights into ecology, paleoecology, and evolution
Active Predators and Their Weapons
Invertebrate predators have evolved some of the most dramatic weaponry in the animal kingdom. Cnidarians like jellyfish and sea anemones use nematocysts, tiny harpoon-like capsules that fire a thread into prey on contact. The discharge mechanism involves calcium ions held within the capsule, and when the thread fires and penetrates tissue, that calcium appears to be injected along with venom, promoting cell death in the target.8PubMed. Ultrastructure of nematocyst discharge in catch tentacles of the sea anemone Haliplanella luciae (Cnidaria: Anthozoa) A single sea anemone tentacle can carry millions of these capsules.
On land, spiders and centipedes are among the most efficient invertebrate predators, and both show remarkably strategic venom use. Giant centipedes preferentially attack the head or thorax of their insect prey rather than the abdomen, and they will reposition a captured insect to bite those regions first. The likely reason is speed: injecting neurotoxins near the brain or the nerve centers that control leg movement incapacitates the prey faster.9PubMed. Prey orientation and the role of venom availability in the predatory behaviour of the centipede Scolopendra subspinipes mutilans (Arthropoda: Chilopoda) Spiders adjust their venom investment based on prey size. The wandering spider Cupiennius salei delivers venom slowly to small prey over several minutes, parceling out its supply through multiple small injections. For large, dangerous prey, the spider dumps nearly its entire venom supply in a single rapid bite lasting under a minute.10Journal of Zoology. Injection of venom into an insect prey by the free hunting spider Cupiennius salei (Araneae, Ctenidae)
Octopuses use a different approach, combining intelligence with a venomous bite to subdue shelled prey like clams and crabs. Some species bore holes through shells using a combination of their radula and secreted enzymes, then inject saliva to paralyze the animal inside. Dragonfly nymphs, mantis shrimp, and predatory diving beetles round out a long list of invertebrate hunters that actively chase, ambush, or trap their meals.
Blood, Sap, and Stolen Fluids
Fluid-feeding invertebrates have found ways to tap into other organisms like living juice boxes. Mosquitoes, ticks, leeches, and bedbugs all feed on blood, and each has evolved specialized mouthparts and biochemistry to make it work. Leeches are a good example of how elaborate these adaptations become: marine leeches in the family Piscicolidae, which make up about 20% of all leech species, feed on hosts ranging from bony fish to sharks and sea turtles. Researchers have identified over 149 distinct anticoagulant and bioactive compounds in leech saliva, targeting multiple steps in the blood-clotting process, including inhibition of clotting factors and platelet aggregation.11Journal of Parasitology. Marine Leech Anticoagulant Diversity and Evolution That biochemical arsenal ensures the host’s blood keeps flowing freely while the leech feeds.
Parasitoid wasps take host exploitation to another level entirely. Rather than simply feeding on a host, many parasitoid species lay eggs inside or on another insect’s body, and the developing larvae reshape the host’s metabolism to serve as both nursery and food supply. The larvae redirect the host’s amino acid, carbohydrate, and fat metabolism toward their own growth, while simultaneously suppressing the host’s development so it stays alive and nutritious for as long as possible.12PubMed Central. Transforming hosts into nurseries and nutrients: strategic manipulation by endoparasitoid wasps Plant-feeding fluid specialists like aphids and scale insects take a gentler approach, piercing plant tissue with needle-like mouthparts and drinking phloem sap. Since sap is rich in sugar but poor in amino acids, aphids must process enormous volumes of it, excreting the excess as honeydew.
Living on Sunlight and Volcanic Chemistry
Some invertebrates skip the trouble of finding food altogether by hosting organisms that produce it internally. Reef-building corals are the most ecologically significant example. Corals harbor photosynthetic algae called zooxanthellae within their tissues, and the sugars these algae produce from sunlight can fully meet the coral’s energy needs.13PubMed Central. Reef-building corals farm and feed on their photosynthetic symbionts The relationship is not passive: corals actively manage their algal populations, farming and feeding on them rather than simply waiting for photosynthetic overflow.
In the deep ocean, far from any sunlight, giant tubeworms at hydrothermal vents have taken symbiosis to its logical extreme. The tubeworm Riftia pachyptila has no mouth, no gut, and no digestive system whatsoever. Instead, it houses chemosynthetic bacteria inside a specialized internal organ called the trophosome. These bacteria use energy from hydrogen sulfide, which the worm’s blood actively concentrates from vent fluid and delivers to the symbiont, to fix carbon dioxide into organic molecules.14PubMed. Chemoautotrophic Potential of the Hydrothermal Vent Tube Worm, Riftia pachyptila Jones (Vestimentifera)15PubMed. Sulfide Binding by the Blood of the Hydrothermal Vent Tube Worm Riftia pachyptila The arrangement works so well that Riftia achieves remarkably high growth rates, fed entirely by its internal bacteria.16Harvard University DASH. Metabolic versatility of the hydrothermal vent worm Riftia pachyptila: Allying transcriptional and metabolic responses to a dynamic environment
Stolen Chloroplasts and Photosynthetic Slugs
Among the strangest feeding adaptations in the invertebrate world, a handful of sea slugs have figured out how to steal chloroplasts from the algae they eat and keep them functioning inside their own cells. These sacoglossan slugs pierce algal cells with a specialized radula, suck out the contents, and somehow retain the chloroplasts in their digestive gland cells rather than digesting them. The stolen chloroplasts continue photosynthesizing for weeks to months, providing the slug with an energy supplement when food is scarce.17PubMed Central. Identification of sequestered chloroplasts in photosynthetic and non-photosynthetic sacoglossan sea slugs (Mollusca, Gastropoda) This ability, called kleptoplasty, is unique among animals to these specific sea slugs.18PubMed Central. Kleptoplasty: Getting away with stolen chloroplasts The bright green color of species like Elysia chlorotica comes directly from the chloroplasts visible through their translucent skin. How the slugs maintain foreign organelles without the algal nucleus to supply new proteins remains an active area of research.
Diets That Change With Age
Many invertebrates eat completely different things at different life stages. Butterflies and moths are the most familiar example: caterpillars chew leaves voraciously, building up protein and fat reserves, while adults sip nectar or rotting fruit juice through a coiled proboscis. This shift is not just a change in food preference; the nutritional needs and strategies of larvae and adults are fundamentally linked, with larval food storage fueling adult reproduction.19Entomologia Experimentalis et Applicata. Effects of larval starvation and adult diet‐derived amino acids on reproduction in a fruit‐feeding butterfly A caterpillar that eats poorly produces an adult with fewer eggs.
Aquatic invertebrates show similar shifts. Many marine crabs start life as tiny planktonic larvae that filter-feed on phytoplankton before settling to the bottom and becoming predators or scavengers. Dragonfly nymphs are fearsome underwater predators that eat other aquatic invertebrates and even small fish, while the adults catch flying insects on the wing. Mosquito larvae filter microorganisms from standing water; adults feed on nectar for energy, and only the females also take blood meals to get the protein needed for egg production. These ontogenetic shifts mean that a single species can occupy multiple feeding roles in its ecosystem over the course of its lifetime.
Sexual Cannibalism as a Feeding Strategy
In praying mantises and some spiders, the female sometimes eats the male during or after mating. This is not random aggression. Research on the mantis Pseudomantis albofimbriata found that cannibalistic females substantially improved their body condition and produced heavier egg cases than females that did not eat their mates.20Behavioral Ecology. Female praying mantids use sexual cannibalism as a foraging strategy to increase fecundity In spider studies, cannibalistic females produced bigger clutches with heavier eggs, and their offspring survived longer than those of non-cannibalistic females, independent of how many males the female had mated with.21Animal Behaviour. Sexual cannibalism benefits offspring survival
The nutritional transfer is direct and measurable. Tracking amino acids from consumed males into the female’s eggs showed that cannibalistic females incorporated significantly more male-derived amino acids into their eggs and reproductive tissues than non-cannibalistic females. The result was a measurable increase in the number of eggs produced.22Proceedings of the Royal Society B: Biological Sciences. Sexual cannibalism increases male material investment in offspring: quantifying terminal reproductive effort in a praying mantis From the female’s perspective, the male is a high-quality meal that shows up voluntarily, and the evidence suggests cannibalism functions as a legitimate foraging strategy rather than a mating malfunction.
Balancing the Menu
Invertebrates are not indiscriminate eaters. Even species with simple nervous systems actively regulate their nutrient intake. Research on honey bee larvae demonstrated that the ratio of protein to carbohydrate in their food significantly affected survival, and the relationship was not simply “more is better.” Carbohydrate content had a nonlinear effect on survival, meaning there was a sweet spot: too little or too much was harmful.23PubMed Central. The geometric framework for nutrition reveals interactions between protein and carbohydrate during larval growth in honey bees Protein and carbohydrate also interacted with each other, so the optimal amount of one depended on how much of the other was available.
This kind of nutritional balancing has been documented across insects, crustaceans, and other invertebrate groups. Locusts that are given a choice between protein-rich and carbohydrate-rich foods will alternate between them to reach a target ratio. Predatory ground beetles adjust their prey selection based on what nutrients they are lacking. The takeaway is that invertebrate feeding behavior is far more nuanced than “see food, eat food.” These animals make dietary decisions that optimize their nutritional intake, even when their brains contain fewer neurons than a single human retina.
When Feeding Strategies Go Wrong in a Polluted World
The same feeding mechanisms that have served invertebrates for hundreds of millions of years now make many of them vulnerable to modern pollutants, particularly microplastics. Filter feeders and deposit feeders cannot easily distinguish a plastic particle from a food particle, and laboratory experiments confirm that invertebrates across multiple feeding types ingest microplastics in proportion to how many are present in the environment. The water flea Daphnia magna, a filter feeder, consumed up to about 6,180 microplastic particles per hour at high concentrations, while the deposit-feeding worm Lumbriculus variegatus took in around 8 particles per hour under the same conditions.24PubMed Central. Feeding type and development drive the ingestion of microplastics by freshwater invertebrates The difference reflects feeding strategy: filter feeders process water rapidly and encounter particles at much higher rates.
Interestingly, the size of the animal matters too. Larger individuals of the same species tended to ingest more and larger microplastics. And when natural food particles like algae were present alongside the plastic, invertebrates generally ate less plastic, suggesting they can partially discriminate between food and non-food when given a choice.24PubMed Central. Feeding type and development drive the ingestion of microplastics by freshwater invertebrates That partial discrimination is cold comfort in heavily polluted waterways, where the sheer volume of plastic fragments can overwhelm whatever selectivity these animals possess. Because invertebrates sit near the base of most food webs, the microplastics they ingest travel upward through the food chain to fish, birds, and eventually to us.