Nudibranch (pronounced noodabrank) means “naked gill”. This refers to the gill-like appendages sticking out of the backs of most nudibranchs and is the organ that allows them to breath. Most nudibranchs also have appendages called rhinophores, usually located at the head. These have scent receptors and are used to taste, smell, and navigate. Most nudibranchs can retract these tentacle-like organs into a pocket in their skin so they are protected from hungry predators.
Nudibranchs are Gastropods in the phylum Mollusca. They are related to snails, slugs, limpets, and sea hares. Unlike other mollusks nudibranchs do not have shells as adults. The shell is only found in the larvae stage and disappears when it becomes an adult.
There are over 3,000 species of nudibranchs and new species are being added to the list all the time. Nudibranchs come in all sizes from just a few millimeters to over 12 inches and weighing up to 3 pounds like the beautiful Spanish dancer. They can be a myriad of colors, spotted, striped or solid with trim. They are found in cold temperatures and warm temperatures.
There are two main types of nudibranchs dorids and eolids. The dorids breathe through gills that are located on their backs in a cluster. Eolids have appendages called cerata that can cover their entire backs and can be threadlike, club-like, branched or clustered in several groups. The cerata have multiple functions including breathing, digestion, and defense.
Nudibranchs see only light and dark. Information about their environment is gathered by the rhinophores or sense of smell and some by sense of taste through tentacles surrounding the mouth. Nudibranchs and all other mollusks, have a radula, an organ with many tiny teeth, which they use to scrape up food. All nudibranchs are carnivorous. They feast on a variety of foods including sponges, hydroids, anemones, fish eggs, barnacles and even other nudibranchs. Some species dine exclusively on one type of food and one species of that food.
The colors of nudibranchs come from the colors of the foods they ingest. Color can camouflage them or, when bright and flamboyant, signal to predators that they are poisonous. Some nudibranchs eat corals that contain algae. They absorb the algae’s chloroplasts and store them in their cerata where the chloroplasts will continue to photosynthesize and supply the nudibranch with nutrients.
What they eat can also supply a means of defense. Eolid nudibranchs that eat organisms that have nematocysts, or stinging cells, can store them in their cerata and use them to sting predators if they are attacked. Some dorid nudibranchs, like the fried egg nudibranch, have a unique defense system. They secrete toxic slime that stinks and mucks up the water, warding off would-be predators. True to their classification as slugs they do very little work to prepare these defenses. They simply absorb toxins manufactured by the foods they eat. Not to stereotype the character of all nudibranchs, there are some industrious species of nudibranchs that manufacture their own toxins.
The majority of nudibranchs cannot harm humans. Two exceptions are Glaucus atlanticus and a close relative Glaucus marginata. They eat Portuguese man-o-war, absorb the nematocysts, and can use them to cause a pretty nasty sting even to humans.
All nudibranchs are hermaphrodites which means one individual is both male and female. The lucky little guys can mate with any individual that happens by. This is a plus because during the majority of a nudibranchs life they live alone and searching for a mate is inhibited by the fact that they never move very far and they can’t move very fast. When two adults meet they each extend a tubular organ that they connect together and through this they fertilize each other’s eggs. The eggs are deposited in a spiral pattern, all rings evenly spaced from one another. They are suspended in a gelatinous substance that holds them in place and gives the egg mass a ribbon-like appearance. The larger “ribbons” of the Spanish dancer eggs can even look like an underwater rose.
Sadly nudibranchs don’t live very long. Some live up to a year but most live only a matter of weeks.
Facts about some common Hawaiian nudibranchs:
Phyllidiidae varcosa, more commonly known as the Scrambled Egg Nudibranch or the Fried Egg Nudibranch. The latter name seems more fitting to me because the white ridges on the body with the yellow spots resemble an egg white and yolk. These are very commonly seen in the waters off Kona. An individual can reach up to 3.5 inches (14cm) in length. They are common throughout the Philippines and the Indo-Pacific too. The fried egg nudibranch, like all other Phyllidiidae species, will secrete a toxic mucous when disturbed. This excretion is deadly to other marine animals. The toxin comes from sponges they eat. They eat the toxic sponges than store the toxin in their bodies to use as a defense mechanism.
Glossodoris rufomarginata also known as the White Margin Nudibranch is another species seen in Kona. It is also found in other areas of the Indo-Pacific and the Red Sea. These are small nudibranchs ranging from 1-2 inches (2.5 to 5cm). They will be found in depths of 15 to 100ft (5 to 30m) but I usually see them in the 25 to 35 foot (8-12m) range hanging on the ceilings of arch formations.
Halgerda terramtuentis, the gold lace nudibranch, is endemic to Hawaii which means it is only found in Hawaiian waters. They are abundant in Hawaii and can be found in shallow waters usually no deeper than 30ft (10m). They grow to a maximum of about 2in (5cm).
Pteraeolididae ianthina, or blue dragon nudibranch is an example of a nudibranch that gets its color from the food that it eats. They eat hydroids that have zooxanthellae living in them. The zooxanthellae photosynthesize to produce nutrients and this also produces the color that is transferred to the nudibranch when it eats the hydroid. Curiously, just like plants, the zooxanthellae can produce different colors so the “blue” dragon nudibranch can be colors other than blue. In Hawaii I have only seen blue colored blue dragon nudibranchs but they are found throughout the Indo-Pacific in various colors. No matter the color of the body, they all have purple band markings on their oral tentacles. These nudibranchs also ingest the nematocysts of the hydroids, store them in their cerata and use them for self-defense.
Hiatodoris fellowsi is another endemic nudibranch. Its common name, Fellow’s Nudibranch. They are named after biologist David Fellows who was the first to collect a specimen. These are strikingly beautiful nudibranchs with their snowy white bodies and contrasting jet black gills and rhinophores.