Nudibranchs: The Secret Colourful World of Sea Slugs
Photo StoryPublished : Oct 15, 2024Updated : Oct 16, 2024
Some sea slugs camouflage and hide from predators, others are vibrantly coloured, warning predators they may be toxic (even if they are not), and still others absorb toxins from their food and store it, ready to dish out a nasty sting when required
Text by: Evan Nazareth
Some sea slugs camouflage and hide from predators, others are vibrantly coloured, warning predators they may be toxic (even if they are not), and still others absorb toxins from their food and store it, ready to dish out a nasty sting when required
Sea slugs are the exotic relatives of the snails and slugs in your garden. They can be found from the shallowest of coastal waters down to the darkest depths of all the world’s oceans. They are a diverse and fascinating group of animals, which also encompasses nudibranchs (aka nudies). So, what is it that sets nudies apart from other sea slugs? The first clue lies in their Order name, Nudibranchia, which is a combination of the Latin word for naked and the Greek word for gills. Unlike other sea slugs, nudies’ gills most often present as intricate branching structures wafting about on their backs. In addition to this, young nudies are almost colourless, and their soft bodies are nestled within a coiled shell, much like snails. As they grow and develop their striking colours, they shed their shell—because if you look as pretty as a nudie, then why keep that hidden away?
Why the dazzling colours? There are several reasons. Camouflage may not be something that comes to mind when you think of eye-catching colours, but surprisingly enough, the strong fashion statement of many nudies helps them blend in with the equally colourful sponges and anemones they feed on. While some nudies may remind you of delicious candy, this could not be further from the truth. Their bright colours serve as a warning to would-be predators that they are toxic or distasteful. They get this colouring by concentrating toxins or stinging cells in their bodies, which they get from things they consume. The blue sea dragon (Glaucus atlanticus), for instance, demonstrates this well. Don’t be fooled by how tiny and adorable they may look, like little blue snowflakes drifting on the ocean’s surface. These tiny nudies feed on the stinging tentacles of the Portuguese man o’war (Physalia physalia), a large floating jellyfish-like creature. In doing so, the nudie steals the stinging cells and stores them in its tissue, ready to dish out a nasty sting to anything that messes with it. Interestingly, there are some non-toxic nudies that take advantage of this, impersonating toxic species by copying their colours.
While there are over 3,000 species of nudies, they mostly fall into two groups: Dorids and Aeolids. Nudies are really nature’s flamboyant answer to: “What is the most absurd body plan one can conjure up with no skeleton or shell to conform to?”. Dorid nudies look like a toddler’s attempt at colouring a bunny rabbit in a colouring book using the wrong colours and having no regard for staying within the lines. Aeolid nudies look like the vibrantly coloured spiky hairstyle of a punk rock musician.
Examples of Dorid body plans. (1) Gymnodoris striata has its gills bunched up together as a fluffy tuft on its back and two rhinophores poking out of the top of its head. This nudie eats other nudies. (2) Phyllidia varicose has its gills hidden away under a skirt that surrounds its body, and like other nudies, it has two rhinophores on its head. Photos: Evan Nazareth
Cover video: Neil Shah
Examples of Aeolid body plans. (1) Cerberilla ambonensis and (2) C. annulata both have a dense patch of cerata on their backs, which are extensions of their digestive tract and also function as gills. Photos: Dhritiman Mukherjee (1), Evan Nazareth (2)
Aeolid nudies feed on hydroids, which are small translucent creatures with several tentacles ridden with stinging cells. The nudie, however, is unaffected by these and stores the stinging cells in cerata scattered across its back. When a predator comes in contact with these cerata, it triggers the cells to fire, leaving it with a nasty sting. Photo: Dhritiman Mukherjee
The blue dragon nudie (Pteraeolidia ianthina), not to be confused with the blue sea dragon mentioned earlier, has a royal blue colouration but takes on a brown-green hue after feeding on microscopic plants (zooxanthellae). The flashy blonde streaks in nudies are a result of them storing zooxanthellae in their tissue. These zooxanthellae continue photosynthesising, i.e., producing food in the presence of light, which provides all the food the nudie needs. Photo: Evan Nazareth
The delicate feathery gills on a nudie’s back and the two rhinophores on its head can be retracted into its body when the animal is disturbed. Photo: Dhritiman Mukherjee
Even though nudies come in an array of colours that we can admire, they cannot see this for themselves. Their rudimentary eyes can just about detect light from the dark. Nudies sense the world around them by using a pair of cephalic (head) tentacles and rhinophores. Cephalic tentacles are somewhat analogous to our tongue; they are sensitive to touch and taste. As the nudie moves about, these tentacles feel their way around and check for anything that may make a tasty meal. Rhinophores, on the other hand, protrude out the top of their head, wafting about in the water current. They are like our noses turned inside out, helping to detect scents and odours. In addition to helping them find their way around and locate food, these senses also help them find a mate. Photo: Sahil Lokhandwala
Cerberilla annulate effortlessly glide around the seafloor with rhythmic undulations of their slimy foot. Their cephalic tentacles feel around, helping them navigate based on touch, but they also taste and find food and mates. Their rhinophores help them sniff out other single nudies in the area, and once close enough, their cephalic tentacles lock onto the slime trails left behind. After a chase (at a snail’s pace), the two nudies finally meet—and this is where things get complicated. You see, nudies are hermaphrodites (possessing both male and female reproductive organs) but cannot fertilise themselves. Their reproductive organs are on the right side of their body, so the two nudies swirl around each other in a yin-yang sort of waltz. Lining up with one another. During what looks like a very intimate handshake, each of them fertilises the other’s eggs. Within a few minutes, both nudies part ways with their own clutch of fertilised eggs. Then it’s time to lay these eggs, and the way nudies do this is by creating a gelatinous ribbon containing hundreds of eggs, which is attached to the seabed in a spiral. Video: Evan Nazareth
Nudies indicate to one another when they are ready to mate by using chemical pheromones that are mixed in with the slime trails they leave behind. Other nudies that come across this trail follow it like one would follow the smell of a freshly baked cake to the kitchen where it was made. These trails can sometimes be three to four individuals long. This will eventually lead to what looks like a queue of nudies following each other, head to tail, resulting in them mating with one another. Photo: Evan Nazareth
Nudies egg masses, which often contain hundreds of eggs, are sometimes as colourful as the nudies themselves. This is because the egg masses, much like the nudies, are laced with toxins that deter other creatures from feeding on them. Photo: Evan Nazareth
About the contributor
Evan Nazareth
is a marine biologist at the Nature Conservation Foundation and is currently working on giant guitarfish and other rays in the Andaman Islands.
