Do you remember the character Rogue from Marvel’s X-Men? Her superpower is the ability to use the powers of the other heroes in the story. It’s actually kind of common in the comic book world (1); but there’s reality behind this fiction, as stealing another species’ power and using it yourself is not that far from what the humble sea slug does every day. Nudibranchs (pronounced new-dee-brank) are not just any sea slugs.
What is a nudibranch and what makes them special?
The first thing you will notice about nudibranchs is that they are beautiful, flamboyantly colored, and incredibly diverse. Even among the striking color patterns that are found in many coral reef species, nudibranchs stand out if you look closely enough. There are thousands of species and they live in tropical and temperate waters throughout the world’s oceans. In general, they are rather small. Most species average around 3-4 inches (<10cm). The smallest species, however, are barely visible without a microscope, and the largest can be more than a foot long (40 cm)! (2)
The basic anatomy is shown here:
The rhinophores (from Greek words rhino- meaning nose and -phore meaning carrier) are on the head and act as chemical sensors (like taste and smell for us). The cerata (from the Greek word keratos meaning horn) are projections from the back of the slug that operate for gas exchange. They don’t have shells as adults, and unlike typical sea slugs, they have to get oxygen from the outside of their body through their skin. These cerata increase the surface area of the skin and therefore act sort of like the gills for the animal. The name nudibranch, in fact, translates to “naked gills” in Greek.
Most are benthic (live on the sea floor), but a few are pelagic (swim freely in the open ocean). One amazing species (Glaucus atlanticus), I commonly called the blue dragon sea slug, is pelagic and beautiful, but also has one of the nudibranch’s special abilities that make them even more amazing. It eats the venomous, jellyfish-like Portuguese Man-of-War, and steals its venomous abilities. So if you thought nudibranchs were weird already, brace yourself…
Kleptocnidae in Aeolid Nudibranchs
First, what is a cnidarian?
Cnidaria is the phylum (see our taxonomy lesson here) that includes jellyfish, corals, anemones, and a few of the less charismatic aquatic critters. They all employ specialized cells called cnidocytes that hold nematocyst organelles. Nematocysts are like microscopic harpoons that are spring loaded to eject venomous barbs.
It might sound a bit violent, especially when you consider that each jellyfish tentacle is loaded with them, and can have millions of them that fire at once. Despite this, there are few cnidarians that pose a threat to humans. These barbs are intended to be used for both hunting and defense. When you are a simple, brainless sack of protein and water that floats along with the current, it helps to have something potent to subdue the prey when it comes along.
A group of nudibranchs called the aeolids have figured out a way to eat cnidarians without being harmed by them. Nudibranchs' mobility (not to mention that they have an actual brain) makes them more complex than their cnidarian prey, and they have developed a way to separate immature nematocysts from their food, which are incapable of firing, and keep them from being digested with the rest of the food. Then, they move them to the cerata for storage, and can fire them at predators who try to attack them. They now have the same ready-to-go stinging mechanism that jellyfish, anemones, and corals have, without having had to go through the trouble of making it. In fact, they got defense and a meal all in one (3)!
Kleptoplasty in Sacoglossan Nudibranchs
This adorable little creature is like a tiny solar cell. In this other group of nudibranchs called the sacoglassans, the cerata are packed with chloroplasts that they stole from the algae that they ate, and they can use them to photosynthesize just like the algae itself! Sacoglassans are vegetarians among the otherwise carnivorous nudibranchs (remember that corals and anemones are animals and not plants).
These "solar-powered sea slugs" are similar to their aeolid cousins, because both have figured out how to take one organelle from the cells of their food and keep it functional for themselves. Just like aeolids keeping nematocysts, sacoglassans keep chloroplasts. Chloroplasts are the organelles in which photosynthesis takes place. In a complicated series of reactions, light energy is used to complete a reaction in which water and carbon dioxide are converted to sugar and oxygen in the chloroplast. They get sugars from the algae they ate, and then continue to get sugar just from basking in the sunlight.
It varies by the species of both nudibranch and algae eaten, but research has suggested that even though the rest of the algae has been digested, the chloroplast can keep photosynthesizing and providing energy to the nudibranch for hours to months after they have been taken (4). With such a brilliant scheme, you can imagine that kleptoplasty is not just used by a single brilliant species. There are actually hundreds of recognized sacoglassan nudibranchs (5).
What do you think of the super-powered sea slugs?
Is one cooler than the other?