Bild på en nakensnäcka. — There are 95 species of nudibranchs in Sweden - and new species are discovered every year. As the name suggests, nudibranchs have no shell, and many have a colourful exterior with strange skin projections and folds. The slug in the picture is a Polycera quadrilineata.

Photo: Fredrik Pleijel

The beauty of nudibranchs displayed in book project

Published

18 March 2021

at

Red eyeglass nudi, blue-spotted sunbather, and large slug pinecone. These are the Swedish common names of three of the nudibranchs we meet in the new volume of Nationalnyckeln, the book project that will describe all of Sweden's plants, animals, and fungi. Fredrik Pleijel, a researcher at the Department of Marine Sciences, has photographed many of the beautiful photographs.

Fredrik Pleijel with microscope — Fredrik Pleijel is researching marine invertebrates. Photography has become an important complement in unravelling scientific classification.

Photo: Susanne Liljenström

Fredrik Pleijel is a researcher on polychaete worms, but has extensive knowledge of most of the sea's invertebrate inhabitants. In recent years, he has devoted more and more time to photography, and has won several awards for his work. In the new Nationalnyckeln, he has photographed and written texts about the often colourful and vibrant nudibranchs.

“Nudibranchs are interesting in many ways. Many have very species-specific colours that can act as warning colours for predators. They can ‘burn’ their enemies with the help of cnidocytes that they have acquired by eating hydroids, a kind of bottom-dwelling relative of jellyfish,” explains Fredrik Pleijel.

High-resolution SLR camera

“Hydroids, like jellyfish, have cnidocytes that are used for defence and to capture prey. However, some nudibranchs can attack and eat the hydroids without activating the cnidocytes. Instead, they are transported to the top of the snail's dorsal protrusion where they act as a shield for the slug”, says Fredrik Pleijel.

Most of the nudibranchs were retrieved from the ocean with the help of divers. The photography was done indoors at the Tjärnö Marine Laboratory. Fredrik Pleijel uses a high-resolution SLR camera with a macro or super macro lens depending on the size of the animal. To correctly reproduce the fantastic colours of the nudibranchs, they are photographed alive.

The blue-spotted sunbather, Elysia viridis, is a nudibranch that can produce its own energy using sunlight, like plants. The nudibranch eats algae and absorbs the algae's chloroplasts, which continue to photosynthesise inside the slug.

Photo: Fredrik Pleijel

“I usually have the camera mounted on a microscope stand and I have four flashes for this. I control the camera from a computer and can see the image on the computer screen in high resolution as soon as I take it.”

Patience is necessary

Capturing the character of a nudibranch on camera is a true test of patience. Most species are 1 to 2 centimetres long, but some are only a few millimetres.

“The small size of many of the animals makes photography more difficult. Another problem is that the animals move around, and it can be difficult to keep up with camera settings. Sometimes everything goes easily and smoothly, and I only take a few shots, other times it can take up to 50 pictures before I feel satisfied. And sometimes never,” says Fredrik Pleijel.

Do you have any favourite in the book?

“There are many to choose from! Perhaps Elysia viridis, the one we call the blue-spotted sunbather in Swedish. It’s a peaceful vegetarian that eats green algae. The slug takes over the chloroplasts of the algae with chlorophyll, i.e. the organelles where photosynthesis takes place, in much the same way as other nudibranchs take over cnidocytes. The chloroplasts are placed in the slug's body wall where they continue to photosynthesise and produce sugars that the shell can absorb. So it is a photosynthesising animal!"