Why fuss over a little slug?
To non-divers, it might be difficult to explain a love for nudibranchs; small, squishy bottom-creeping, slug-like creatures. To the converted these little splashes of colour enhance the substrate like nuggets of gold.
There are more than 3,000 known species with new ones being identified all the time. There are enormous variations in terms of colour, pattern, size and body shape so it is difficult to generalise about these crazy little critters. However a soft oblong body and ornate coloration seem to be the norm.
As you will discover there is little else normal about the humble but fascinating nudibranch (pronounced "noody-brank").
Nudibranch Fact Sheet
Family name: Nudibranchia
Order name: Mollusca
Sub-order name: Opisthobranchia
Common name: Nudibranch
Scientific name: Each species has its own unique scientific name
Nudibranchs are gastropod molluscs, belonging to the phylum Mollusca. This contains soft bodied creatures usually with a shell and a distinctive gill shape. The main groups include Cephalopoda (octopus and squid), Bivalvia (clams) and Gastropoda (snails and slugs). Gastropods have 3 sub-classes: Pulmonata (land snails and slugs), Prosobranchia (sea snails with shells) and Opisthobranchia (sea slugs).
Nudibranchs normally grow to between 4 and 10 cm. However, certain forms can grow to up to 60 cm and weigh up to 1.5 kg. They have rhinophores which are sensory tentacles on the head for taste, touch and smell. Sight seems to be limited to distinguishing between light and dark and managed by 2 eyes set into their bodies. Secondary gills are exposed in the form of a branchus (nudibranch means 'naked gills') which arises from the dorsal region.
There are a large number of different families of nudibranchs all of which can be placed into 1 of 2 main types, according to their gill structures:
- Dorid nudibranchs
- Eolid nudibranchs
Eolid nudibranchs display finger-like appendages called 'cerata' on their upper surfaces, while dorid nudibranchs breathe through gills on their back. Spanish Dancers are one of the best known dorid nudibranchs, named after it's flowing red frills, reminiscent of the ornate flapping skirt of an Andalusian senorita. The unmistakable Blue Dragon is a classic eolid.
Most of their time is spent eating, with only 3 to 5 hours of rest. Normally you will see nudibranchs of the sea floor or on a wall where they move around, about 10m per day, using their 'foot' which leaves a slimy trail.
The carnivorous nudibranch diet consists of barnacles, anemones, sponges and sea slugs and occasionally each other. They often assume the colour of their prey which can also serve as camouflage. Some run on solar with the photosynthesising algae that live on their outer tissues producing sugars which the nudibranch uses.
Other nudibranchs, turtles, some crabs and humans are their greatest threats. Colours serve to camouflage as well as suggest toxic content. Some can emit chemical odours as a defence. Eolid nudibranchs can ingest the stinging cells of anemones and store them in their cerata tips to sting would-be predators or catch food.
Having both male and female organs, nudibranchs are hermaphroditic but still need "two to tango". There are sometimes disputes over who is to play the male role and who the female. After fertilisation the nudibranchs can produce up to 1 million eggs in a spiral structure than resembles a ribbon and is toxic to any predators.
The eggs are laid near to a food source to allow the larvae easy access to food when they appear. They soon move to deeper water to develop into adults. Life expectancy for most averages at about 1 year.
Nudibranchs aren't fished commercially and are not very successful creatures in aquariums which means they are not specifically targeted for extraction. However destructive fishing methods, pollution and coral bleaching all negatively affect the nudibranch's habitat. As slow-moving creatures they are unable to flee to places with more favourable conditions. Being very sensitive to environmental change, habitat loss or degradation is a serious threat to any resident nudibranchs.
Although found in all the world's oceans, nudibranchs are more common in the tropics and warm temperate zones. Incredibly one species, the Sea Angel, has been recorded near the North Pole.