Invertebrates eat practically everything. As a group that includes over 95% of all animal species, their diets span from microscopic algae and decaying leaves to live fish and even marine birds. What any particular invertebrate eats depends largely on its body plan, its habitat, and the specialized feeding structures it evolved to exploit a specific food source.
Filter Feeders and Microscopic Food
Many marine invertebrates feed by straining enormous volumes of water to capture tiny particles of food. Sponges, clams, oysters, and gelatinous organisms like salps and pyrosomes all use some version of this strategy. Their primary food consists of phytoplankton (single-celled algae), bacteria, and other organic particles suspended in the water column. The phytoplankton they consume are incredibly small, often around 10 micrometers in diameter, roughly one-fifth the width of a human hair. Some gelatinous filter feeders capture prey at size ratios exceeding 10,000 to 1, meaning the animal doing the eating can be ten thousand times larger than what it consumes.
Corals take a dual approach. Reef-building corals host microscopic algae called zooxanthellae inside their tissues. These algae photosynthesize and transfer as much as 90% of the organic material they produce directly to the coral. That covers most of the coral’s energy needs. But coral polyps also extend tiny tentacles at night to snag passing plankton, supplementing their diet with animal protein.
How Insects Match Mouthparts to Food
Insects are the most species-rich group of invertebrates, and their diets are remarkably diverse. The key to understanding what any insect eats is its mouthparts, which fall into two broad categories: chewing and sucking.
Insects with biting and chewing mouthparts have mandibles built for solid food. Herbivorous species like locusts have mandibles with well-defined grinding surfaces for tough plant material, and a single swarm can strip crops across wide areas. Longhorn beetles bore into heartwood, branches, and stems using their mandibles to consume woody tissue from the inside out. Predatory insects like ground beetles have sharp-tipped mandibles designed for piercing and crushing other invertebrates.
Fluid-feeding insects have a proboscis, a tube-like organ that works like a straw. Butterflies and moths use smooth, rigid proboscises to reach deep into flowers and draw out nectar, which is primarily composed of sugars and free amino acids. Female mosquitoes use a needle-like proboscis to puncture skin and feed on blood, which provides the protein needed to produce eggs. There are even vampire moths equipped with tiny hooks and barbs on their proboscises that let them pierce animal tissue for blood meals.
Diet Shifts Across a Lifetime
Many insects don’t eat the same thing their entire lives. In species that undergo complete metamorphosis, like butterflies and beetles, the larval and adult stages typically have very different nutritional requirements. Caterpillars are voracious leaf-eaters, consuming massive quantities of plant tissue to fuel rapid growth. Once they transform into adult butterflies or moths, most switch entirely to nectar. This liquid diet provides the sugars and amino acids needed for flight, mating, and egg production, but it’s a fundamentally different food source than what sustained the larva. Insufficient nectar intake in adulthood reduces lifespan, mating success, and the number of eggs a female can produce.
Predators That Digest Outside Their Bodies
Spiders are among the most widespread invertebrate predators on land, and they eat in a way most people don’t expect. After immobilizing prey with venom, a spider injects digestive fluids from its gut directly into the prey’s body. These secretions contain a full toolkit of enzymes that break down proteins, fats, and even the chitin in insect exoskeletons. The spider then sucks the liquefied contents back into its gut, pumping fluids in and out repeatedly until the prey is reduced to an empty husk. This process, called extra-oral digestion, means spiders essentially turn their prey into a smoothie before consuming it.
Scorpions use a similar approach, making extra-oral digestion a widespread strategy among arachnids.
Cephalopods: Flexible Hunters
Octopuses, squid, and cuttlefish are active predators that eat crustaceans, fish, and other mollusks. Their hunting methods are surprisingly varied. Octopuses ambush, stalk, and even use a “speculative pounce,” spreading their webbed arms over an area of seafloor to feel for hidden prey underneath. When eating clams or snails, octopuses drill through the shell using a rasping tongue-like organ called a radula, then inject venom to relax the prey’s muscles and extract the meat.
Squid tend to be more pursuit-oriented hunters. The jumbo squid feeds on prey as large as tuna, while pygmy squid use ink as a smokescreen during attacks, releasing it between themselves and prey before striking. Some deep-sea squid use bioluminescence to spot non-glowing prey in the dark. Not all cephalopods are active hunters, though. The vampire squid, despite its dramatic name, drifts through the deep ocean feeding on detritus, bits of dead organic material that sink from above.
Jellyfish and Stinging Prey Capture
Jellyfish, sea anemones, and their relatives in the cnidarian group capture food using specialized stinging cells called nematocysts. Each nematocyst is a pressurized capsule containing a coiled, harpoon-like thread. When triggered, water rushes into the capsule, roughly doubling its volume in an instant. The resulting pressure launches the coiled thread outward at extraordinary speed, puncturing the target and delivering a cocktail of neurotoxins. The thread turns inside-out as it extends, a process that completes in milliseconds. This stuns or kills small fish, shrimp, and plankton, which are then guided toward the mouth by the animal’s tentacles.
Scavengers and Bottom Feeders
Crabs, lobsters, and other decapod crustaceans are some of nature’s great opportunists. Many are bottom feeders and scavengers that consume dead animals resting on the seafloor. But “scavenger” undersells how varied their diets really are. Mud crabs are opportunistic omnivores that eat slow-moving mollusks, carrion, plant material, and even other crabs. Spiny lobsters show strong preferences depending on their habitat: those living in surf zones favor mussels, while species in deeper rocky habitats prey on sea urchins, other crustaceans, and various mollusks. Their nutritional needs include not just protein and fat, but minerals like calcium, sodium, and iron, which they obtain from the shells and tissues of their prey.
Detritivores and Decomposing Matter
A huge number of invertebrates eat dead organic material, and their role in ecosystems is hard to overstate. Earthworms, millipedes, woodlice, springtails, and mites all feed on decaying plant litter and the microorganisms that colonize it. Together, these soil-dwelling detritivores are responsible for roughly 30 to 40% of the decomposition of soil organic matter. They physically shred leaf litter into smaller pieces, increasing the surface area available for bacteria and fungi, which speeds up nutrient cycling.
This work is quiet but essential. Without invertebrate detritivores, fallen leaves and dead wood would accumulate far faster than microbes alone could break them down, and the nutrients locked inside that material would take much longer to return to the soil where plants can use them.
Snails and the Radula
Snails and slugs use a radula, a ribbon-like tongue studded with rows of tiny chitinous teeth, to process food. The radula moves back and forth against a jaw plate, scraping and tearing food into small particles. Most land snails and slugs are herbivores or detritivores, grazing on algae, fungi, and decaying vegetation. Aquatic snails often scrape algae from rocks and other surfaces. But the radula is versatile enough to support predatory lifestyles too. Cone snails are venomous marine hunters, and moon snails use their radula to drill neat holes through the shells of clams before consuming the soft tissue inside.