• HOME
• COURSES
• DIVING
• DIVE SITES
• EQUIPMENT
• INJURIES
• LESSONS
• SEA LIFE

Home › Sea Life › Marine Animals › Invertebrates › Mollusks › Sea Slugs

Nudibranchia: Facts about Nudibranchs

[Phylum: Mollusca] [Class: Gastropoda] [Subclass: Heterobranchia] [Order: Nudibranchia]

Nudibranch - scientific name "Nudibranchia" - are one of more than 3,000 known species of soft-bodied, shell-less, marine gastropods.

This section contains fun facts and information about nudibranch species, including where they live, what they eat, and how these aquatic sea slugs reproduce.

Nudibranch Habitat and Geographical Range

Sea slugs are found almost anywhere there is water. You can find these feisty little creatures in your local tide pools, in tropical coral reefs, and even in the coldest depths of the Antarctic Ocean floor.

But, the most popular dive sites for scuba divers to see these colourful creatures, include:

• Bali and North Sulawesi
• Philippines (e.g. Anilao Batangas)
• Phuket, Thailand

The squishy bottom-dwelling marine mollusks inhabit most of the world's shallow sea beds in warm water and cold water oceans.

Almost all nudibranch species are benthic animals that live in various depths of the intertidal zones. Hence, most of these marine invertebrates spend their lives crawling around the sea bed and rocky outcrops.

Even so, neustonic Glaucus nudibranchs float upside down close to the surface. There is also an elongated pelagic nudibranch (Cephalopyge trematoides) that swims freely in the water column.

Most of the nudibranch phylum thrive better in salt water environments. Even so, a few of the sea slug species can survive in brackish water (e.g. Bohuslania matsmichaeli).

Nudibranch Characteristics

Nudibranchia belong with the 40,000 plus species of gastropod molluscs, with the adults varying size from four (4) millimetres to sixty (60) centimetres (0.16 to 23.62 inches). The family also includes sea snails, sea hares, and limpets.

Fascinating divers and scientists alike, "Nudibranchia" are noted for their extraordinary vivid colouring and striking characteristics.

Put another way:

The beautiful striking colours with flamboyant striped patterns and polka-dot spots all over these ocean organisms means they are very popular subjects for underwater photography.

The majority of the sea slug phylum displays a remarkable array of sumptuous hues. Typical body colours include blues, reds, yellows, and pinks.

Other standout features of nudibranch mollusks are the anomalous shapes and wickedly funny appendages (called "cerata") - some of which are venomous.

Nudibranch Behavior

If you've ever watched to see how a sea slug moves, you will realise that it has a flat, broad, foot-like muscle. The muscular foot helps them to move, and leaves behind a slimy trail as they travel.

In fact, chemicals in the trail of slime alerts other sea slugs of danger and the strong scent also helps them to find a mate. For this reason, most nudibranchs are found on the seabed. Interestingly though, there are some species that can flex these muscles and swim for a short distance.

Moving on...

Nudibranchia have limited and lacklustre vision - distinguishing only light and dark. So, their eyesight is not strong enough to see their own brilliant coloration. Rhinophores, situated on top of their head, receive enough information about the world that they live in so they can identify their prey.

Even though they're not usually poisonous to humans, the exception being "Glaucus atlanticus (blue sea dragon)", some species of sea slug are toxic to prey when they use their cerata to defend themselves.

Eolid nudibranchs use their cerata to store foul-tasting poisonous stinging cells called nematocysts from their food - such as the Portuguese man-of-war bluebottle - and the toxins can be used to sting their predators.

Dorid varieties have an ability to produce their own toxins and they are able to absorb them from their food. Some species are able to photosynthesize coral and algae combined with solar energy. Thus, they can harness the power of sunlight to produce a rich source of nutrients.

This food source from sunlight and algae chloroplasts are absorbed into the cerata creating a nutritious food supply helping to sustain them for several months.

Pro Tip: Even though there are 3,000+ species already identified, biologists are still discovering new types of sea slug classification.

Sea Slug Species and Identification

Even though many gastropods have shells, adult nudibranchs shed their exoskeleton at the end of the larval stage.

During this time young nudibranchs also undergo a procedure known as 'torsion'.

Torsion means they completely twist the oblong shaped body 180 degrees on the foot and move the gills and the anus to a position on top of their head.

This form of placement means the adults have now become asymmetrical.

Sea slugs have a somewhat innocent existence - perhaps in part because they generally do not live long lives.

Studies suggest that some species will survive for only a few weeks (even in the wild). Nevertheless, the average lifespan is around one year for most of the sea slug genus.

Nudibranch Scientific Name

The name nudibranch is a Latin word meaning "nudus" (naked) and "branch" (gills) which generally describes and refers to naked feathery gills and horns protruding from their back. The two main types of nudibranch are:

• Dorid nudibranchs
• Eolid nudibranchs

Identification is achieved by the mechanism used to breathe. Dorid species have gills at the back end for breathing (posteria). Whereas, eolids have finger-like branched or thread-like tentacles (cerata) covering their back.

Cerata are generally used for breathing, helping with digestion, and for defensive protection. The oral tentacles are located at the front, often with one horn-like filament rhinophore on top, with cerata appendages flowing along the back.

Here's the thing:

Some species of Nudibranchia have colourful tentacles that flow upwards from their head. They also have a pair of tentacles with scent receptors called rhinophores - used for smelling and tasting their food.

These highly sensitive organs function similar to the human nose. Hence, they enable the animal to smell or sense food and provide them with sufficient information to identify their surroundings as well as other sea slugs.

Because the sea slug's rhinophores are targeted by hungry fish, nudibranch can retract the tentacles "at will" back into a hidden skin pocket if they feel threatened or sense danger.

What Do Sea Slugs Eat?

Nudibranchs are carnivorous and they get their impressive bright body colouring from the food they eat. They eat using a radula with tiny teeth. They use it to scrape substrate and graze mostly on algae.

Nonetheless, their diet may also include a combination of:

• Anemones
• Bryozoa (filter feeding invertebrates)
• Coral polyps
• Barnacles
• Fish eggs
• Hydroids
• Other sea slugs
• Sea sponges
• Tunicates (sea squirts)

Fun Fact: Some species of nudibranch are picky eaters, often only feeding on specific food types or even a single source of prey.

Nudibranch Reproduction

The fact that nudibranchs are simultaneous hermaphrodites means that they possess the reproductive sex organs of both male and female.

Being hermaphroditic means they can mate with any mature individual of the species. This is vital to maximise the mating activity, especially because they do not move far on the seabed.

Put another way, finding a mate is difficult and, in general, they are marine creatures that live a solitary lifestyle.

When mating, they can normally mate with any adult passing nearby. By laying a mass of spiral-shaped or coiled eggs, the young hatch as larvae swimming freely in the ocean. They will eventually come to rest on the ocean floor and live their adult life.

Interesting Fact: Sea slugs have become extremely important to the development of some medications. In fact, scientists are actively studying how the nudibranch nervous system works as part of further research.

Different Types of Nudibranch Species

Divers also enjoyed reading about...