Once you’ve got the nudibranch bug, it is impossible to stop searching for more varieties of these amazing little creatures. Just what are they, though, and what is their role in the marine environment? Let’s take a peek.
What are Nudibranchs?
Nudibranchs, which are often affectionately referred to as Nudis, are sometimes called Sea Slugs. While it is true that they are distant relatives of the slimy slugs and snails that dwell on land, and while it is also true that they are related to sea slugs, they are actually extremely specialized creatures with more than 3,000 described species included in their taxonomic clade, Nudibranchia.
With simple eyes that can only detect shades of light and dark, a mouth that scrapes up food rather than biting at it, and a symmetrical bilateral form, they share some features with other gastropods, while other features such as the venomous appendages that some species have are unique to the clade.
Nudibranchs hatch with tiny shells attached, but shed their shells once they reach adulthood. They can vary in size from as small as 20 millimeters or just over ¾ inch, to 600 millimeters, or about two feet long. They breathe through a plume or rosette-shaped “naked gill” which is situated on their backs, which can be pulled into a small pocket when they are frightened. All nudibranchs have cephalic tentacles on their heads, which are sensitive to smell, taste, and touch.
While nudibranchs are hermaphroditic, producing both egg and sperm, they are not capable of reproducing on their own. They are carnivorous, consuming everything from sponges to tiny bryozoans, to other nudibranchs and nudibranch eggs. Some are even cannibalistic, consuming members of their own species.
Nudibranch Defense Mechanisms
Like other small, brightly colored creatures, nudibranchs have unique methods for keeping themselves safe from predators. Besides the venomous, tentacle-like appendages that adorn some species, they contain a variety of toxins. Some even feed on jellyfish, absorbing everything but the stinging cells, which are eventually passed to the surface of their skin. Others secrete acidic mucous from their skins when touched, and others are capable of completely incapacitating predators via neurotoxins.
The Role of Nudibranchs in the Underwater Environment
Despite their small size, nudibranchs perform a few important functions. First, they often feed on small particles of food that other, larger animals leave behind. This helps to keep ocean water clean and clear. Second, they sometimes act as food, themselves. As with most other creatures, there are some animals which have specially adapted to consume them. While most nudibranchs are not considered to be prey, there are some larger fish and other animals that will eat them. Third, they help to keep algae bloom in check. Nudibranchs do graze on excess algae along reefs and near shore, preventing algal growth from spreading out of control.
If you hope to encounter nudibranchs while you are diving, you are in luck! They can be found in every one of the world’s oceans, including the Arctic. The most colorful species are generally found in Indo-Pacific waters, often in shallow areas near shore, but some nudibranchs do live in very deep water, with some species having been discovered at depths more than a mile beneath the surface. Keep your eyes open and your camera ready – you never know what you might see!