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What are nudibranchs?

If you are a diver or you are friends with one, you have probably heard of these charming mollusks. Nudibranchs are an order of sea slugs that earned a special spot in the heart of many divers and underwater photographers, thanks to their elegant bodies and vibrant color patterns. This coloration is an adaptation called aposematism, a warning for predators, alerting them about the deadly toxins carried by these slugs and that they should not attack or eat them. Surprisingly, many species of nudibranchs don’t produce these chemicals by themselves. They obtain their toxins by eating other poisonous animals, such as sponges or corals, accumulating it in their bodies. They spend their days ingesting toxic substances, stealing the defense mechanisms of other animals, and wandering carelessly in the ocean wearing flashy outfits, confident that other predators won’t mess with them. Truly pirates indeed!

Nudibranchs are a diverse order, conformed by more than 3000 species. They are found all over the world, from polar waters to tropical areas, and from tide pools to deep waters. There are two main groups of nudibranchs: dorid and aeolid nudibranchs. Dorid nudibranchs usually have a smoother shape, with a ring of feather-like gills towards the back of their bodies, while aeolid nudibranchs instead breathe with anatomic structures called cerata covering their backs.

Aeolid Nudibranchs

Cerata can have other functions besides breathing, acting as a defense mechanism. These appendices can accumulate nematocysts obtained from predated cnidarians, a phylum that includes corals and jellyfish. Nematocysts are harpoon-like cellular structures that are used to capture prey and defend against possible predators. These stings are discharged upon contact, causing the painful and harmful sting caused by jellyfish and other cnidarians. The mechanisms that make nudibranchs able to obtain these nematocysts are not yet fully understood. It seems that certain chemicals in their mucus protect them from nematocysts. Once ingested, the matured unfired nematocysts are excreted, while immature nematocysts are stored in pouches in the points of the cerata, called cnidosacs, where they mature, becoming a functional protection for the slug.

Glaucus Atlanticus, Photo by: lostandcold

The vagrant blue dragon

Most nudibranchs are benthic species, meaning that they are found at the bottom of the ocean, crawling around just like any terrestrial slug. There are a few exceptions to this. Some pelagic species are able to swim in the column of water, while pleustonic species inhabit the surface of the water. One example of the latter is the aeolid Glaucus atlanticus, a species found floating in tropical and subtropical waters, carried around by oceanic currents. Blue dragons predate on pleustonic cnidarians like the by-the-wind sailor, Velella velella, or the feared Portuguese man-o’-war, Physalia physalis. They can further use uneaten parts of their prey to attach their egg strings. This slug, commonly known as blue dragon, uses the ingested nematocysts of their prey as a defensive strategy, being potentially dangerous for humans depending on the species that has been recently consumed.

Portuguese man o war, Physalia physalis, Azores Islands. Photo by: Juanma García Cambeiro

Solar powered sea slug

Pteraeolidia ianthina is an aeolid nudibranch found in the Indo-Pacific, also known as blue dragon. Do not mistake Pteraeolidia ianthina with Glaucus atlanticus, as these two species only share their common name. This slug predates on different species of cnidarians, taking thievery to a whole new level. Not only do they use the nematocysts from the cnidarians that they eat, but they also store the zooxanthellae algae from predated corals in their cerata. These slugs can keep the hostage algae alive doing photosynthesis inside their bodies. The algae provide them with extra nutrients, becoming some sort of “solar-powered” slugs.

Kleptoplasty is a similar process, by which an organism only incorporates chloroplasts from algae in their own cells, gaining the ability to conduct photosynthesis. Sacoglossa, a group of sea slugs distantly related to nudibranchs, are the only animals with this ability, usually done by microorganisms. Sacoglossans use a specialized radula to puncture the algae and suck up the cell’s sap, taking only the chloroplasts and storing them in their digestive cells. One species of sacoglossan, Elysia marginata, is able to regenerate its whole body, including its heart, after voluntarily detaching their head from their old body. It has been theorized that their use of chloroplasts is what allows them to survive this extreme process, gaining nutrients from sunlight while they grow a new body.

The first photo shows Pteraeolidia ianthina, Philippines, Photo by: Karine Marangon. In the second photo we see Elysia marginata, Philippines, Photo by: Andrea Swagler

The spanish dancer

The Spanish Dancer, Hexabranchus sanguineus, is a large doric nudibranch, common in Indo-Pacific coral reefs. They can reach up to 60cm of length, but usually range between 20 and 30cm. They are a benthic species, normally crawling on the bottom, but when they feel threatened, they can extend their mantle and swim away undulating their body. This graceful swimming, combined with its bright colors, earned them their name, as they resemble Spanish flamenco dancers. They prey on poisonous sponges (Halichondria spp.), obtaining their chemical defenses from them. They accumulate these toxins in their dorsal mantle, an area more vulnerable to attacks, and they pass these defensive compounds to its egg ribbons, protecting future generations of Spanish dancers until they hatch.

Hexabranchus sanguineus, Egypt. Photo by: Karine Marangon

Parasitic swimmer 

Phylliroe bucephalum is one of the only two known species of pelagic nudibranchs. Very little is known about this small transparent slug that is just a few millimeters long. It has a laterally flattened body, adapted to swim, and their diet consists of gelatinous animals like jellyfish, siphonophores and salps. Their development goes through a larval stage which parasitises the medusa Zanclea costata, attaching to their body and slowly eating them. When they become bigger than the jellyfish, they begin to swim actively, consuming the tentacles and the remaining parts of its host.

Lion’s mane nudibranch 

Melibe leonina is a nudibranch found in kelp forest in the Eastern Pacific, from Alaska to Baja California. This nudibranch lacks a radula, the rasping tongue used by most gastropods to break down and chew their food. Instead, it has an oral hood used to capture small planktonic animals. They firmly attach to a kelp blade and then sweep their raised hood. When prey lands on the hood, they start to close the sides of the hood, locking it in with their tentacles. Then they contract the hood, forcing the excess water away while swallowing its prey whole.

This nudibranch can release a noxious secretion that keeps most predators away, except for kelp crabs. According to aquarists, this secretion smells like watermelon. When the secretion doesn’t work, Melibe leonina is able to swim by laterally bending its body. They swim most of the time to escape from predators, but they can also swim spontaneously, in order to disperse and find a new suitable habitat.

Melibe leonina, Photo by: Jill Siegrist

Bioluminescent nudibranch

Plocamopherus maderae is an orange dorid nudibranch that lives in the Macaronesian Region, a collection of archipelagos in the Northeast Atlantic. They are found in the warm waters of Cape Verde, Canary Islands and Madeira. This nudibranch has an unique trait: they are bioluminescent. Bioluminescence is the ability to create light as a result of a chemical reaction within a living organism. The ability to produce light requires a significant expense of energy, having a defined function in the different animals able to do it. In the case of Plocamopherus maderae, it is used as a defense mechanism. When they feel threatened, they start to emit light, trying to distract the attention of potential predators while they swim away. They can only produce light a few times before running out of the chemicals needed to produce this reaction, needing to “recharge their batteries” before they can do it again.

Plocamopherus maderae, Photo by: Miguel Angel García Exposito

Anticancerogenic compounds in nudibranchs 

Many chemicals used in drug discovery are used originally by plants and animals for defensive purposes, trying to avoid being eaten. The defensive chemicals found in nudibranchs, either metabolized by them or incorporated from their prey, can be a source for new chemicals and drug discovery. As an example, the nudibranchs from the genus Chromodoris incorporate lantrunculin A from consumed sponges. This toxin has interesting properties, being cytotoxic against cancer cell lines. The sponges that produce lantrunculin A are not the main food source for these nudibranchs. They eat other sponges with different toxins at a higher amount. They selectively sequestrate lantrunculin A, that has a higher toxicity acting as a better defense mechanism than the other sponges’ toxins, excreting the less effective toxins. Understanding how they are able to select and transport this compound can help to create new cancer treatments, able to accurately target the cancer cells, thus reducing the harmful effects of the drugs on the body.

Chromodoris annae, Philippines. Photo by: Karine Marangon

We want to thank the photographers that helped to create this article, sending us their best nudibranch pictures. And we would like to know your opinion. Did we miss your favorite nudibranch? What are your thoughts about these colorful invertebrates? Let us know below in the comment section!


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