50 Fun Facts About Invertebrates

Which soft-bodied invertebrate has three hearts, bluish copper-based blood and two-thirds of its neurons in its arms?

Octopus blood is blue because it carries oxygen with hemocyanin rather than iron-rich hemoglobin. One main heart serves the body while two smaller ones push blood through the gills.

Which Antarctic deep-sea animal, weighing about 1,000 pounds, is the heaviest living invertebrate known to science?

The colossal squid also has the largest eyes documented in the animal kingdom, 27 to 30 cm across. The heaviest confirmed specimen weighed 495 kg.

Which microscopic eight-legged invertebrates can dry into a dormant state and survive the vacuum of outer space?

Tardigrades, nicknamed water bears, pull in their legs and shrivel into a lifeless-looking form called a tun. In that state they have endured vacuum, radiation and cold close to absolute zero.

Which armored sea creature has bright blue blood that drug makers use to test injectable medicines for bacterial toxins?

The horseshoe crab is no true crab but a closer relative of spiders. Cells in its copper-based blood clot around bacterial toxins, which makes it a sensitive safety test.

Which spiny-skinned sea animal can regrow lost arms and feeds by pushing its stomach outside its body onto prey?

Starfish pry a clam's shell open a crack, then slip their stomach inside to digest the soft tissue. In some species a single severed arm can regrow a whole new animal.

Which drifting sea animals have no true brain, yet one species can revert to a juvenile stage after maturing?

Jellyfish coordinate their swimming with a ring of nerves instead of a brain. The so-called immortal species, Turritopsis dohrnii, can turn back into a polyp after it has matured.

Which insects form Coleoptera, the largest order of animals, making up about a quarter of all known animal species?

Beetles number about 400,000 described species, more than any other order. Their front wings are hardened into cases called elytra that shield the flight wings underneath.

Which tiny colonial animals, each a polyp secreting limestone, built Australia's Great Barrier Reef?

Corals built the world's biggest single structure made by living organisms, one visible from space. Each polyp is typically only a few millimeters across.

Which small sea creature is a vertebrate rather than an invertebrate, with males that carry the developing young?

The seahorse is a bony fish with a backbone, a relative of the pipefishes. In place of scales it has thin skin stretched over rings of bony plates.

Which animals of the phylum Porifera have no nervous or digestive system and feed by filtering water through pores?

Sponges may be the sister group to all other animals, though the evidence is not settled. Most spend adult life fixed to the seabed, pumping water through their bodies for food and oxygen.

Which phylum of insects, spiders and crustaceans accounts for over 80 percent of all known living animal species?

Arthropoda takes its name from the Greek for jointed leg. Every member has a segmented body, an external skeleton and paired limbs that bend at joints.

Snails, clams and cuttlefish all belong to which soft-bodied phylum, the animal kingdom's second-largest by species?

Mollusca accounts for about 23% of all named marine organisms, more than any other phylum in the sea. Most members have a muscular foot and a mantle whose edge secretes the shell.

Sea urchins, sand dollars and brittle stars belong to which phylum, the largest found only in the sea?

Echinodermata means hedgehog skin in Greek. Its members start life as two-sided larvae but grow into adults built on a five-part plan, and they live on the sea bed at every depth.

Sea squirts and lancelets have no backbone, yet they belong to which phylum alongside fish, birds and humans?

Chordata membership rests on the notochord, a stiffening rod, rather than on a backbone. Sea squirt larvae have one, along with a hollow nerve cord, while they swim freely.

Spiders, mites and harvestmen belong to which class of eight-legged invertebrates that have no antennae or wings?

Arachnids take their name from arachne, the Greek word for spider, shared by the boastful weaver of myth who was turned into one. Adults walk on eight legs, where adult insects have six.

Snails, slugs and whelks belong to which class of animals, at home in the sea, fresh water and on land?

Gastropods are named from the Greek for stomach and foot, because the crawling foot sits beneath the gut. They have spread through the sea, fresh water and dry land.

An adult insect's body, which carries six legs and usually wings, is divided into which sections?

Head, thorax and abdomen each have a job: the first carries the antennae and jaws, the second bears every leg and wing, and the third holds most of the gut.

Which tough material, also found in the cell walls of fungi, forms the exoskeletons of insects and crustaceans?

Chitin is the second most abundant polysaccharide in nature, behind only cellulose. Many crustaceans stiffen theirs further with calcium carbonate.

Because a rigid exoskeleton cannot stretch, which process must insects, spiders and lobsters repeat in order to grow?

Molting means shedding the old outer covering to reveal a new one grown beneath it. Until that fresh covering hardens, the animal is pale, soft and easy prey.

Which soil-dwelling invertebrate breathes through its skin and was the subject of Charles Darwin's last scientific book?

The earthworm takes in oxygen straight through its moist skin. Darwin argued that few other animals have played so important a part in the history of the world.

Sea anemones and hydras capture prey with which specialized stinging cells that fire a toxin-filled capsule on contact?

Cnidocytes are single-use cells: once the hair-like trigger is touched and the capsule fires, the whole cell has to be replaced.

Sea anemones and adult sea urchins usually have parts repeating around a central axis, showing which type of symmetry?

Radial symmetry suits anchored, floating and slow-moving animals, which meet the world from every side. A two-sided body instead favors streamlined, directed movement.

Sea anemones and many soft-bodied worms hold their shape with fluid under pressure, forming which kind of skeleton?

Hydrostatic skeletons work because the trapped fluid cannot be squeezed smaller. When muscles tighten around one part of the body, the pressure forces another part to lengthen.

Which parasitic flatworms have no gut, absorbing nutrients through their body surface inside a host's intestine?

Tapeworms anchor themselves to the intestine wall with a scolex, a knob armed with hooks, suckers or both. Behind it, the neck keeps budding off new body segments.

Caenorhabditis elegans, the first animal to have its whole genome sequenced, is which kind of transparent invertebrate?

The roundworm C. elegans, a nematode about 1 mm long, is so thoroughly studied that biologists have mapped the developmental fate of every single cell in its body.

Which flatworms, favorites of regeneration research, can split into fragments that each regrow a complete animal?

Planarians owe the feat to abundant adult stem cells called neoblasts, which renew themselves and can produce every cell type the body needs.

Which blood-feeding segmented worm returned to hospitals in the 1980s to relieve blood congestion after microsurgery?

The leech secretes hirudin, a peptide that stops blood from clotting while it feeds. Doctors now also use hirudin itself as an anticoagulant drug.

Snails and slugs scrape up food with which minutely toothed ribbon, often compared to a tongue?

The radula carries many rows of tiny teeth, and fresh rows move up from the rear as the front ones wear out. Some predatory slugs use theirs to seize and devour live prey.

Pearls are built from layers of which iridescent material, which also lines the inside of an oyster's shell?

Nacre, also known as mother-of-pearl, is made of microscopic tablets of aragonite, a form of calcium carbonate, stacked in sheets that make it strong and resilient.

Which shelled cephalopod, often called a living fossil, grows by adding ever-larger chambers to its coiled shell?

The nautilus moves into each new, larger chamber and seals the old one behind a wall. Its line has survived relatively unchanged for hundreds of millions of years.

Which shelled animals that cement themselves to rocks and ship hulls did Charles Darwin spend years classifying?

Barnacles are crustaceans, kin to lobsters, and each adult is in effect fixed upside down by its forehead. Deep into his eight-year study, Darwin wrote that he hated them as no man ever had.

Which common garden invertebrates are land-living crustaceans whose gill-like breathing organs must always stay damp?

Pill bugs, or roly-polies, are woodlice that can roll into a ball for defense or to save moisture. Their breathing organs sit on limbs that their water-living relatives use for swimming.

Which many-legged invertebrates have two pairs of legs on most body segments, unlike the one-pair, venomous centipedes?

Millipedes have a name meaning thousand feet, yet no species was known to reach 1,000 legs until one with over 1,300 was discovered in Australia in 2020.

Which eight-legged predators glow under ultraviolet light and carry their newborn young on the mother's back?

Scorpions have fluorescent chemicals in their outer covering, so they shine under a UV lamp. Every known species gives birth to live young rather than laying eggs.

Spiders release their silk through which organs, usually found on the underside of the rear body section?

Spinnerets are not single nozzles: each is covered in many microscopic spigots, and every spigot produces one filament of the finished thread.

Which small swarming crustaceans, a staple food of whales, seals and penguins, anchor the Antarctic food web?

Krill of one Antarctic species add up to an estimated 379 million tonnes, among the largest total biomasses of any species. Predators eat more than half of it every year.

Which tropical marine crustacean punches prey with club-like limbs so fast that vapor bubbles form in the water?

The mantis shrimp hits its prey twice with each strike: first with the club, then with the force of the collapsing bubbles. Its eyes hold 12 to 16 kinds of photoreceptor, against four in humans.

Lobsters, crayfish, prawns and true crabs all belong to which order of crustaceans?

Decapoda comes from the Greek for ten feet. In many members one pair of those legs carries enlarged pincers, and the order holds nearly 15,000 living species.

Which blood-feeding parasites pass on the bacterium behind Lyme disease when they bite people, deer and mice?

Ticks are not insects: adults have eight legs, like their relatives the spiders. The illness is named after Lyme, Connecticut, where it was first recognized as a separate condition in 1975.

Insects breathe without lungs, taking air into branching internal tubes through which paired openings along their sides?

Spiracles feed a network of ever-finer tubes that deliver oxygen directly to the tissues needing it, so the gas never has to travel in the blood.

Butterflies and moths form which insect order, whose name refers to the tiny scales covering their wings?

Lepidoptera combines the Greek words for scale and wing. Its members also share a proboscis, a feeding tube used for drinking nectar.

Butterflies, flies and wasps pass through which nearly immobile life stage between the larva and the adult?

The pupa may look lifeless, but inside it the larval structures are broken down while the adult ones take shape. A butterfly's is called a chrysalis.

Which extinct armored invertebrates, among the commonest fossils, died out in the mass extinction ending the Permian?

Trilobites lived in the oceans for almost 270 million years, and more than 22,000 species have been described. Only a handful were left when the last ones vanished.

Nearly every animal phylum first appears as fossils in which burst of evolution about 539 million years ago?

The Cambrian explosion lasted between 13 and 25 million years, a geological instant in which most modern animal body plans diverged.

Which French naturalist, known for the idea that acquired traits are inherited, coined the term invertebrates?

Jean-Baptiste Lamarck was put in charge of insects and worms at the Paris natural history museum in 1793. He then divided those two old groups into ten.

Which ancient Greek split animals into those with blood and those without, roughly vertebrates and invertebrates?

Aristotle made many of his observations during about two years on the island of Lesbos, where he studied the animals of a sea lagoon in close detail.

Protostomes such as insects and snails are named for the idea that an embryo's first opening becomes which body part?

The mouth was once believed always to form from that first dent, hence a name meaning first mouth. In deuterostomes, the group that includes humans, the dent becomes the anus instead.

Which fluid, the insect counterpart of blood, bathes the organs directly instead of staying inside vessels?

Hemolymph is pushed from the rear of the body cavity toward the front by a single vessel running along the insect's back, then flows freely among the organs.

An insect's compound eye is built from thousands of repeating light-sensing units known by which name?

Ommatidia number around 30,000 in the eye of a large dragonfly but only a handful in some primitive wingless insects. Each one is roughly ten times longer than it is wide.

Which nearly transparent sea drifters swim with eight rows of beating cilia, the largest animals to move this way?

Comb jellies are named for those rows, in which the cilia are stacked like the teeth of a comb. Soft as they look, they make up a phylum entirely of their own.