The sea polychaete Phyllodoce mucosa lives on shallow bottoms along the Swedish west coast. It is one of the more than 20,000 different species of ringworms in the world.
Photo: Fredrik Pleijel

New masterpiece on versatile worms


They have been difficult to collect and photograph, but after five years, the masterpiece about ringworms is complete. "Annelida" contains everything and a bit more about a diverse group of animals with great ecological significance. A new family tree for ringworms is also presented here, based on DNA technology. Fredrik Pleijel is researcher at the Department of Marine Sciences and one of the authors behind the book.

Fredrik Pleijel
Fredrik Pleijel is researcher at the Department of Marine Sciences and works at Tjärnö Laboratory.
Photo: Susanne Liljenström

Oblong and chubby, with or without bristles, transparent and colorful, free-swimming and deep in the mud. Ringworms are a large and diverse herd, and with just over 20,000 different species, they are among the most common animals on earth.

The vast majority of ringworms belong to the polychaets. According to Fredik Pleijel, it is a group with an almost incalculable significance in the sea

– If you pick up a sample from the seabed, probably half of all animals you find will be seapolychaets, and half of all species as well. They live in every conceivable environment; from muddy bottoms at several thousand meters depth to drained seaweed belts on tidal beaches. They can be predators or sediment eaters, or feed on plankton that they filter out of the water. Thus, they are a large and important part of the marine ecosysstem.

A challenge to photograph

The more than 400-page grand "Annelida" is the result of extensive work, it has taken almost five years to get the book ready. The authors have reviewed numerous scientific articles and also collected and photographed worms from all the earth's oceans. The result is a solid compilation of 77 different worm families and their biology, ecology and kinship.

Polychaete Eulalia clavigera
The polychaete Eulalia clavigera lives in the waters around the British Isles, along the west coast of France and around the Iberian Peninsula. It has a very similar "cousin" on the Swedish west coast, Eulalia viridis.
Photo: Fredrik Pleijel

In addition to writing texts, Fredrik Pleijel has photographed many of the beautiful color images in the book. A challenging task:

– Collecting whole and representative individuals has probably been the most difficult part. To get the worms, we have dived and snorkeled, used various bottom gear and also remotely operated underwater vehicles. My colleague Greg Rouse has even been down to a thousand meters in a manned craft. Then you must not have a predisposition for claustrophobia, says Fredrik Pleijel.

– All animals have been photographed alive or anesthetized. Many of them produce mucus and must be cleaned from sand grains and other things that get stuck in the mucus.

Skedmasken Bonellia viridis
A female of Bonellia viridis. The males are microscopic and live as parasites inside the female. Gender determination takes place when the larva settles: larvae that land near a female develop into males, otherwise they become females.
Photo: Fredrik Pleijel

New family tree thanks to DNA

The authors present a new family tree for the ringworms. Finding out how different species can be grouped into genera, families and so on, is fundamental in biology. In Linnaeus' time, this was done with the help of various characters in appearance, such as how the mouth parts are designed. Today's researchers have access to much sharper tools thanks to DNA technology.

– The tree we present is based on morphological data supplemented with information from molecular analyses of DNA and RNA. You could say that we re-furnished the old tree. For example, earthworms and leeches were previously their own groups, but now we believe that they are part of the group of polychaets. We hope that this tree will stay stable for a long time.

Help for species identification

"Annelida" addresses an international audience. It is written in English and published by Oxford University Press. Fredrik Plejel believes that it will primarily be bought by colleagues who work with ringworms in their research: systematists, ecologists or those who use worms as a model species for various research issues. But also by companies that are engaged in environmental monitoring.

– Here you get, for example, help with species identification and methods for collecting and handling the animals in the best way. A reference list of 70 pages provides tips on additional in-depth literature. It's been a lot of work but it's great to finally see the book printed!

Text: Susanne Liljenström

The cover of "Annelida" shows some of the morphological variation among the ringworms. Annelida is the scientific name for this group of animials.

The authors behind "Annelida" are Greg Rouse, Scripps Institution of Oceanography, Fredrik Pleijel, University of Gothenburg and Ekin Tilic, University of Copenhagen (currently). It contains detailed descriptions of 77 families of ringworms and is richly illustrated with drawings, scanning electron images and hundreds of color photographs. Published by Oxford University Press 2022.