All species of earthworms, including the worm farmer’s favorite, the red wiggler (Eisenia fetidae) and the common earthworm (Lumbricus terrestris), reproduce in much the same way. Earthworms, according to Charles Darwin are “lowly organized creatures”, but it must be said that to most of us, their reproductive system seems highly complex and rather weird.
The first thing to understand is that all earthworms are hermaphrodite. This means that each individual has both male and female genitalia and therefore cannot be considered exclusively male nor female. When two worms couple for mating, both worms produce sperm, which is exchanged directly with their partner during the sexual encounter and the exchanged sperm is subsequently used to fertilize the eggs of each individual.
The various worm species take different times to reach sexual maturity. For Eisenia fetidae and other composting worms it is about eight to ten weeks. The sign of approaching maturity is the development and increasing prominence of the clitellum band. This is a swollen ring or belt of lighter coloured flesh that develops about a centimeter or so behind the mouth at tip of the worm. This bulge is usually a light pink color, but in some species the clitellum has a distinctive yellowish tinge. The clitellum band has a leading role to play in the reproductive cycle of the worms.
MATURE RED WIGGLER, EXHIBITING PALE CLITELLUM BAND
When the mature worms begin ovulating they seek out sexual partners and begin to twist around each other either in pairs or more often in small tangled groups or clumps. As the sexually aroused worms entwine with each other, their bodies are stimulated by the mating process and large quantity of sticky sperm is produced, which attaches itself to the upper bodies of their partners. Shortly after the coupling has been successfully completed, the clitellum band begins to excrete a mucous substance that soon stiffens into a jelly-like ring. This donut shaped ring then gradually loosens up and begins to slip off the worm. As it passes over the worm’s body the jelly ring collects some of the attached sperm and at the same time also gathers up a number of eggs that are simultaneously released in its path. As it slips off the “parent” worm, the jelly ring curls itself into a ball , enclosing some of the eggs and sperm within itself in a protective cocoon. The outer portion of the cocoon quickly hardens into a shell or casing enclosing the inner gel mass, which provides a nutritious environment for the fertilization and gestation of the eggs.
The cocoons are about the size of grape pips and have the shape of birds eggs. They start off at a kind of yellowish grey color but in time become more of a mahogany brown. These cocoons or egg casings, as they are often called, provide a safe environment to protect the fertilized embryos. The cocoon is the equivalent of an external womb, keeping the the embryos safe until they are fully developed. The baby worms emerge from the cocoon around seven weeks after fertilization of the eggs and each egg casing would typically give birth to between seven and fifteen pale pink baby worms.