Feature: Nudibranchs


Nudibranchs are a group of sea slugs that have magnificent bright colors and are known around the world for that. There are really few restrictions to what color these creatures can have. Mostly they are natives to more tropical waters but actually, 178 species have been described in Scandinavian waters.

Anatomy & Ecology: Nudibranchs have no gills. They breathe through their skin or through secondary gills (also known as ‘’naked gill’’) that are found on their backs. A group of Nudibranchs (Cladobranchia) also have some brightly coloured organs on their backs called cerata(in Greek cerata= horns). These are used for many purposes such as respiratory and attack/defense. Sometimes the digestive system extends to the cerata. Nudibranchs are highly stenofagous (in Greek meaning steno=narrow and fagous=eating) which means that they will only prey on 1-2 kinds of organisms. The most common of these are sponges, bryozoans and cnidarians. There are also known some nudibranchs that can incorporate zooxanthelle (algae) in their bodies and live in a symbiosis.

Deffence: Nudibranchs don’t have a shell which makes them more exposed to predation. But through many years of evolution they have acquired some defenses such as toxicity (acid secretion) which can either be provided by self-production or by eating something that contains toxins, and camouflage, which makes them resistant to many predators. They still have a few predators including other Opisthobranchia and sea spiders. Some cnidarians have nematocyst cells (toxin producing cells), and since the nudibranchs eat them, and the digestive system sometimes travels up the cerata, the nematocysts can be extracted and used by the nudibranch as a defense mechanism.

Reproduction: Nudibranchs, like other snails, are hermaphrodites, which means that they have both male and female reproductive organs. So in reproduction, sperm is exchanged from both partners where it is stored until the eggs are ready for fertilization. The eggs are usually laid in spiral formations and have a nice visual effect (see photo above).

Communication: Like many other animals they communicate with chemical signals. For example when they are at larval stage, where they float in the water, they don’t metamorphose until they get a signal from a specific hormone, which could for example be an indicator that there is food in the area. They can sense these chemical stimuli with the help of a pair of tentacles which they can’t withdraw, near their mouth. These tentacles are called rhinophores.

Coryphella/Flabellina verucosa

Grows up to 15-25mm in length and can be found in northern Norway and the west coast of Sweden.

Dendronotus frondosus

Grows up to 100mm and can be found in southern Sweden (Baltic Sea)

Acanthodoris pilosa

Usually reaches 40mm in length and can be found at coast of southern Sweden.