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Pronounced as ‘nudi-brank’ which literally means ‘naked gill’, nudibranchs are some of the most interesting lifeforms in the ocean. These colourful critters have evolved to protect themselves without the use of a shell and instead make use of chemicals and toxins to shield themselves from harm. In fact, some species of nudibranchs contain such toxic chemicals that, when released as a sign of distress, will kill the marine life within a 50 cm radius. As the literal translation of the word suggests, nudibranchs have external gills.

There are at least 3000 different nudibranch species and these vary wildly in size from tiny 4 mm critters to larger 600 mm species. Nudibranchs are found all over the ocean except in heavy surf conditions, however the widest variety of nudibranchs can be found in Indo Pacific waters.

Nudibranch Introduction Videos

Amazing color, beautiful form, funny appetites, unique defense and one that believes in feminism and masculinity, the nudibranch has it all.

While the sights of larger reef inhabitants amaze other divers, my husband and I dive with magnifying lens strapped on to our BCD. Many a time, I have got amused laughs, curious stares and a flow of questions regarding this curious piece of dive equipment. Why? Playing Sherlock Holmes underwater has many rewards especially when hunting down nudibranchs.

Nudibranchs have fascinated divers and continue to hold their audience in captivity with dazzling color variety and shapes thus earning them the distinction of being trendsetters. Oddly enough, many divers have passed one by without recognition, as these trendsetters are masters of forms. Some have two horned tentacles (rhinophores) on their head, others feathery gills on their back, most are brightly colored though some blend into their environment. While most are small (1-8 cm) there is the Spanish dancer that can grow to 50 cm. With at least 3,000 species displayed in the world oceans, they are no strict rules as to where they may be found. However, the vast majorities inhabit the warm waters of the Indo-Pacific reefs.

What are they?

Nudibranch or more commonly known as the true sea slug is closely related to Gastropods (shell-bearing organisms). The species lacks a shell and are soft-bodied creatures that use their foot-like appendage to crawl over the surface of the reef in search of food. If you have ever seen one move, they do not seem to be in much hurry to find the next meal at all!

Nudibranch literally translates as naked gills, a perfect description of its breathing apparatus. In contrast to their relatives the gastropods who have their gills well hidden from prying eyes, most nudibranchs take great pride in displaying their gills in the form of feathery structures on their back or sides. Despite their variety, all nudibranchs can be grouped as into 4 main types which are indeed a blessing with 3000 of them on the reefs!

Harlequin Nudibranch (Doridaceans): The largest group, these nudibranchs are characterized by the combination of a pair of tentacles on the top of their head and a tuft of feathery gills on the rear part of the back. But beware, just when you thought that’s easy come the phyllids who also belong to this group but lack conspicuous gills and have lumpy ridges on their backs!! Did I say it was easy?

Tubercular Nudibranch (Aeolidaceans): The second largest group are the easiest to recognize as their long tapering bodies which are covered with numerous finger-like appendages (cerata) that are non-retractable.

Side-gilled Nudibranch (Dendrotaceans): have several feathery gills along each side of the back as well as a cup-like sheath around each rhinophore on the head that can be retracted.

Veiled Nudibranch (Arminaceans): a fleshy extended lobe on its head. Some members possess longitudinal ridges on their back and others have well-developed cerata.

Life as A Nudibranch

Nudibranchs lead a nomadic life appearing when and whereas they please from 100m abyssal walls to shallow sandy flats. It is because of this characteristic that they have eluded biologist. How can you study them when they cannot keep an appointment at a certain place and time?

What do they eat? All nudibranchs have a brittle tongue called the radula to feed and use their tentacles to locate food and potential mates. Surprisingly enough, most are carnivores with a craving for hydroids (organisms with sting cells called nematocyst), corals, sponges, fish eggs, crustaceans and tunicates (sea squirts). Others feed on algae and bryozoans (moss-like animal). But the most industrious have taken up farming by growing algae (zooxanthellae) on their cerata which produce carbohydrates for the nudibranch through photosynthesis. They carry breakfast, lunch and dinner with them as they travel. Such talent deserves applause!

Reproduction as with all living organisms also takes place in the life of the nudibranch. The nudibranch is hermaphroditic meaning that both sexes are present in a single nudibranch but self-fertilization does not occur. Eggs are deposited on the reef as colorful ribbon strands often on favorite food like sponges. Once hatched the larva is free swimming and may travel with the oceanic current until a suitable surface is found. How long do they live for? Nudibranchs that feed on slow growing organisms such as sponges and sea pens can live up to a year and others that feed on transitory organisms such as bryozoans have short life spans of a few weeks.

Survival for the nudibranch is dependent on chemical warfare. Their bright colors serve, as a warning to would-be predators of their inedible qualities. Their flesh is frequently toxic or distasteful due to various chemical secretions. The aeolids feed on sea anemone and hydroids with an ulterior motive! They accumulate the nematocysts (stinging cells) from these animals, concentrate it in their finger like appendages and use it as weapons. Eating with a reason comes into a new meaning.

Designers, farmers, warriors and survivors, remember to look out for these colorful reef inhabitants with their amazing abilities on your next dive.

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