“Worm” is a catch-all name for several unrelated animal groups, from earthworms and leeches to flatworms, roundworms and tapeworms, so they don’t share one way of reproducing. Between them, worms use almost every method known in animals, from mating with a partner to splitting in two or making eggs with no father at all. Many are both male and female at once, yet still often seek a partner, and some switch between going it alone and mating depending on how good conditions are.
How Earthworms Mate and Lay Cocoons
Each earthworm is a hermaphrodite, meaning one body carries both male and female sex organs. You might expect that to make mating unnecessary, but an earthworm still pairs up with another worm rather than fertilizing itself. The two line up side by side with their heads pointing in opposite directions.1University of Illinois Extension. How I Reproduce | The Adventures of Herman the Worm Each then passes sperm to the other, and the received sperm is stored in small sacs.2University of Illinois Extension. How I Reproduce | The Adventures of Herman the Worm
Mating mostly happens at night or in damp conditions on or just beneath the soil surface, so most people never see it. Mucus holds the pair together, which is why they are hard to pull apart without hurting them. They often stay joined for an hour or more, and if disturbed they usually separate and retreat underground.
After mating, a cocoon forms on each worm around the thick, lighter-coloured band near the front of the body.3University of Illinois Extension. How I Reproduce | The Adventures of Herman the Worm As the worm backs out of the narrowing cocoon, eggs and the stored sperm are deposited inside it.4University of Illinois Extension. How I Reproduce | The Adventures of Herman the Worm So fertilization happens inside the cocoon, outside the worm’s body. Because each worm both gives and receives sperm, both go on to produce cocoons. Stored sperm lets one mating fertilize several cocoons over time. Only adults have a visible band; young worms lack it and can’t breed yet. Leeches, close relatives of earthworms, make cocoons the same way, slipping the sheath over the head and sealing both ends with a glue from the band.5PubMed. Operculum ultrastructure in leech cocoons
In soil or compost, the cocoons are tiny lemon-shaped capsules, roughly the size of a grain of rice. They start pale and darken toward brown as the young develop. Finding them in a compost bin is a good sign the worms are breeding, so leave them in place; the young hatch on their own.
Hatching takes weeks to a few months, depending on temperature and moisture, so a cocoon in cool soil may sit for a long time with nothing visibly happening. A cocoon often holds more than one egg, but not all develop, so only a few babies usually emerge. They are thin, pale, thread-like miniatures of the adults and easy to overlook in compost.
Worms That Are Both Male and Female
Growing and carrying fertilized eggs takes a lot of energy, so in some flatworms each partner would rather be the male.6Shape of Life. The Crazy World of Penis Fencing In some species a worm stabs sperm straight through its partner’s skin.7Shape of Life. The Crazy World of Penis Fencing The first to stab becomes the father, and the other is left to carry the eggs.8Shape of Life. The Crazy World of Penis Fencing
This contest is often called penis fencing. The worms rear up and jab at each other with a sharp, needle-like organ, each trying to inject sperm first. It isn’t a fight to the death, and the worms usually survive it.
Being both sexes doesn’t mean reproducing alone: only some, like tapeworms and the roundworm below, regularly fertilize themselves. The soil roundworm C. elegans is mostly a self-fertilizing hermaphrodite, with true males kept at low numbers in the wild.9PubMed Central. The evolutionary role of males in C. elegans Its body is essentially female, but it makes a limited batch of sperm early on, about 150 per side of its reproductive organ, then switches to making only eggs.10Wikipedia. Caenorhabditis elegans Eggs are fertilized inside it by its own sperm and then laid.11Wikipedia. Caenorhabditis elegans It can also mate with a male, and when it does it uses the male’s sperm in preference to its own.12Wikipedia. Caenorhabditis elegans
Self-fertilizing has limits. The sperm supply runs out once used, which caps how many eggs get fertilized, and the offspring are near copies of the parent, so there is less variety to cope with change. Mating with a male adds both more sperm and new genes. Mated hermaphrodites can lay far more fertilized eggs than lone ones.
Each segment of a tapeworm has its own complete set of male and female organs.13Invertebrate Anatomy OnLine. Taenia pisiformis Tapeworms commonly fertilize themselves, which suits an animal that may be the only one of its kind inside an animal’s gut.14Invertebrate Anatomy OnLine. Taenia pisiformis If more than one tapeworm shares the same animal, they can cross-fertilize.15Invertebrate Anatomy OnLine. Taenia pisiformis
A tapeworm is a chain of segments, and the ones nearest the end are the oldest, holding the most developed eggs. Those segments eventually break off and pass out with the host’s droppings, carrying fertilized eggs to the outside world. New segments keep forming near the head, so a single tapeworm can go on producing eggs for a long time.
Worms That Reproduce by Splitting in Two
Some freshwater flatworms called planarians reproduce by tearing their body into two pieces.16PubMed Central. Mechanics dictate where and how freshwater planarians fission The worm first narrows into a waist, pulses to stretch it, then ruptures across.17PubMed Central. Mechanics dictate where and how freshwater planarians fission The head piece and tail piece each regrow their missing half in about a week, making two worms.18PubMed Central. Mechanics dictate where and how freshwater planarians fission
Planarians often turn up in aquariums and ponds, and a sudden boom of small flatworms in a tank often comes from splitting. It works without a partner, so one worm can start a population of genetic copies.
Some segmented water worms build first and split later. The new worm’s organs form before the split finishes, so the two separate cleanly without leaving a wound.19Biology Bulletin. Regeneration and asexual reproduction in annelids: Cells, genes, and evolution
Cutting an earthworm in half does not make two worms. At best the head end survives and regrows a short tail, and the tail end can’t regrow a head and dies. A planarian can regrow because its body holds many stem-like cells able to rebuild any missing part, while an earthworm’s regrowth is far more limited. Planarian splitting is a deliberate process, not an injury, so cutting worms in a garden only harms them.
Eggs Without Sperm
Some roundworms reproduce by parthenogenesis, where an egg develops into a new worm without being fertilized. Some, such as root-knot roundworms that damage crop roots, can reproduce only this way or switch between this and mating.20Nemaplex. How Do Worms Reproduce? Sexual & Asexual Methods Different species solve the problem of starting an embryo without sperm in different ways.21PubMed. Egg development in parthenogenetic nematodes: variations in meiosis and axis formation
Normally sperm triggers the egg to begin dividing, and these species have found other ways to start that process. In some, the egg keeps a full set of chromosomes by skipping or reversing a step in egg formation, so the young are near-copies of the mother. A population can then be all female, with every individual able to found a new one.
For gardeners, this means one root-knot roundworm can start an infestation without a mate, and populations build quickly in a single season. That is one reason crop rotation is advised against them.
How Parasitic Flukes Multiply
Flukes reproduce sexually in a vertebrate host, the animal a parasite lives in as it matures, such as a dog or cat.22Wikipedia. Trematoda Their eggs leave in the host’s droppings and hatch in water into a swimming larva.23dvm360. Trematode parasites 101 That larva has only a short time to find a snail or die. It burrows into the snail.24dvm360. Trematode parasites 101 Inside the snail it multiplies asexually into the next larval stage, which goes on to infect another animal.25dvm360. Trematode parasites 101
Along the way the parasite passes through further larval stages, each multiplying, before a form appears that can leave the snail. That stage then needs to reach the final animal, sometimes by being eaten along with a second animal such as a fish, or by entering through skin or plants.
The parasite takes over the snail’s reproductive organs and uses all its resources.26Colorado Arts and Sciences Magazine. Not just a fluke: learning more about trematode infection An infected snail often becomes sterile and can’t breed itself, but it lives on as a production site. A single infected snail can release a couple of thousand larvae every day.27Colorado Arts and Sciences Magazine. Not just a fluke: learning more about trematode infection
Mating in the main host mixes genes, and copying in the snail multiplies numbers fast. Most larvae never reach a snail or a final host, so only a tiny share of the output needs to succeed.
For pets and people, infected droppings near water can restart the cycle. Keeping pets from eating raw fish or snails lowers the risk, and a vet can check droppings for fluke eggs.
The Strangest Worm Reproduction
Osedax worms live on the bones of dead whales, with females rooted in the bone.28PubMed. Osedax: bone-eating marine worms with dwarf males Their males are microscopic, essentially arrested larvae that make sperm from leftover yolk.29PubMed. A dwarf male reversal in bone-eating worms Many dwarf males live together inside the tube around one female.30PubMed. A dwarf male reversal in bone-eating worms A dwarf male never grows up and lives off its yolk reserves, and its job is to supply sperm, so the female is rarely without a mate. She dissolves the bone with acid-producing roots to reach nutrients, so the bone is both food and home.
In some roundworms, the male leaves a plug that seals the female’s genital opening after mating.31PubMed Central. Strategic adjustment of copulatory plug size in a nematode The substance hardens over the opening and acts like a seal, helping stop rival males from mating with her. It gives his sperm time to fertilize the eggs, making it more likely that his sperm, rather than a rival’s, gets used.
Roundworm sperm have no tail and instead crawl like tiny amoebas.32PubMed. Nematode sperm: amoeboid movement without actin They move by extending and pulling in a flexible edge, and they must crawl through the female’s reproductive tract to reach the eggs. Some are guided by chemical signals from the eggs.
Most worms abandon their eggs once laid, but some freshwater leeches carry their hatchlings on their underside, protecting them until they leave.33PubMed Central. Juvenile detachment, an early sign of departure from parental care, in the leech Orientobdelloides siamensis (Oka, 1917)