Oligochaeta: Characteristics & Reproduction

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Despite their small size and bland appearance, oligochaetes are important components of their environments. In this lesson we'll look at what oligochaetes are, how you can identify them, and how they reproduce.

Right in Your Backyard

Even though they look like simple creatures, earthworms and their relatives are important components of the ecosystems they inhabit. Collectively they form the subclass Oligochaeta, which is in the class Clitellata and phylum Annelida. The name of the class refers to the clitellum on the worm, which is an area that secretes cocoons used for holding eggs.

Unlike the polychaetes (bottom) or bristle worms, oligochaetes (top) have only a few bristles (chaetae) on their bodies
oligochaeta vs polychaeta

The name of the subclass can be broken down into 'oligo' which means 'few' and 'chaeta' which are bristles on their bodies. 'Few' is relative to the bristle worms, or polychaetes, which are covered in chaetae, and are found in the same phylum.

There are about 3,500 species of oligochaetes and they are found all over the world. They live in freshwater, saltwater, and of course, in soil like you find in your backyard. Oligochaetes are important recyclers in their environment. Take the earthworm for example, which makes soil. You can see this in action if you have a worm compost bin, where you put food scraps in and eventually end up with highly nutritional soil that you can use in your garden.

The clitellum, for which the class is named, secretes cocoons that are used for holding eggs and sperm
earthworm cocoons

Characteristics of Oligochaeta

Oligochaete worms have long, segmented, tubular bodies, giving them the appearance of a piece of thick spaghetti. They can range in length from just a few millimeters to over 10 feet! You won't find a head or any limbs on these guys either, but they can still get around pretty well by peristalsis. This is when the body moves by contracting and relaxing its segments so that a wave-like motion pushes it along.

Oligochaetes are also simultaneous hermaphrodites, which means that they are both male and female at the same time. This doesn't mean that they can self-fertilize, but it does mean that they can mate with just about any other member of their species they come across. That's pretty efficient!

Despite the lack of a head, oligochaetes do have a brain and nervous system, with a central nerve cord running the entire length of the worm's body. They don't need a respiratory system because gas exchange occurs through the body itself. Despite lacking eyes and a nose, they can still perceive light and can also sense taste and touch. And they of course have a few of those bristles, which help with movement, and the clitellum which looks like a band around the body of the worm.

Worms are used in compost bins because they turn food scraps into nutrient-rich soil
worm compost bin

Oligochaetes feed on dead and decaying material, which helps take it out of the environment. They replace it with 'new' material that is nutrient rich and ready for use again by other organisms like plants.

Oligochaeta Reproduction

Let's revisit the fact that oligochaete worms are simultaneous hermaphrodites. Again, a single worm can't fertilize itself, but each worm has both male and female sex organs to use when mating with another worm. In fact, each worm can both give sperm to the other worm and lay its own eggs. This means that for two worms mating you can end up with two sets of offspring.

Eggs are laid in the same environment that the worms inhabit but are protected by a cocoon. Depending on the species of worm, some can lay eggs all year round and as often as every few weeks.

These two earthworms are mating by lining up head to tail and passing sperm to one another
earthworms mating

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