Everything about nudibranchs, or rather sea slugs!

Nudibranch definition: 
Among our beautiful and colorful underwater world, one of the most surprising and amazing critter is no doubt the nudibranch!

We use the word nudibranch in keeping with popular usage, but technically we refer to a broader group of sea slugs known as heterobranchs. True nudibranchs are just one subgroup within this larger classification — specifically the ones with external, visible “naked gills”.

Habitat:
Nudibranchs can be found almost everywhere in the underwaterworld. From shallow water to deep waters, by night and by day. They are usually found in locations that match their preferred food source.

Diet:
Heterobranchs have a varied diet that includes algae, sponges, bryozoans, ascidians, hydroids, sea anemones, corals — and in some cases, even other nudibranchs. It’s good to know that they can also use their food for defense purposes. Some species can absorb toxic compounds from sponges and store them in specialized glands on their mantle for defense. Others are capable of ingesting nematocysts — the stinging cells of their prey — and relocating them to their cerata for protection.

Identification:
To determine which subgroup a sea slug belongs to, we need to examine its anatomy. In some cases, external features are enough, but often a look at the internal anatomy is necessary for accurate identification. FYI: In certain cases, DNA analysis is required — especially to confirm whether it’s a distinct species or a new one altogether.

Reproduction:
All nudibranchs are hermaphrodites from birth, meaning they possess both male and female reproductive organs. However, they can't fertilize themselves — reproduction still requires a partner. After mating, they lay their eggs in colourful, clockwise-spiraled ribbons. Some egg masses can contain up to a million eggs. Depending on the species, the eggs either hatch into tiny, free-swimming larvae that drift with the plankton — helping them disperse — or emerge directly as miniature adults.

Classification / Taxonomy:

Kingdom: Animalia

They are animals — multicellular, mobile, and heterotrophic (they eat other organisms).

Phylum: Mollusca

This is the mollusk group. It includes creatures with soft bodies, many of which have a shell (though nudibranchs have lost theirs).

Class: Gastropoda

Gastropods are snails and slugs — animals with a single, usually spiraled shell (again, nudibranchs don’t have it anymore) and a muscular foot for movement.

Subclass: Heterobranchia

This is a large and diverse group that includes all sea slugs. Nudibranchs are just one part of this group. Heterobranchs include species with very different morphologies and lifestyles, both marine and terrestrial.

Orders:

Main Orders of Sea Slugs (within Heterobranchia)

Family, Genus, Species:

This is where things get specific:

• Family groups nudibranchs with similar body plans or behaviors (e.g. Chromodorididae — known for bright colors).
• Genus is a more specific group within a family (e.g. Goniobranchus or Hypselodoris).
• Species is the individual type of nudibranch (e.g. Goniobranchus kuniei).

To keep in mind:

• All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.
• The term sea slug includes members of many different orders within Heterobranchia.
• These animals can look similar, but they belong to different evolutionary groups with unique features and behaviors.
• On this website, we’ll use families and genera to organize and classify the species we observe.
  
  

Size:
Nudibranchs range in size from just 1 mm to an impressive 750 mm for the largest recorded species.

Lifespan:
Their lifespan depends largely on food availability, ranging from a few weeks to about a year for the larger species.