Which came first, the chicken or the egg? Or, in our case, Pokémon or nudibranchs? This time, the answer is simple: nudibranchs. Long before Pikachu and the rest of the Pokémon came into our lives, nature had already created creatures with extraordinary colours, unusual shapes and surprising “abilities”. So surprising, in fact, that it is hard to believe they are real. And the best part is that you do not need to travel to the other side of the world to find them: some live right here in the Mediterranean.
What are nudibranchs?
Despite their extraordinary appearance, these small animals are a group of gastropod molluscs, related to snails and slugs. As adults, they have no shell and, in many species, their gills are exposed on their backs, a characteristic that gives them their name: from the Latin nudus (“naked”) and the Greek branchia (“gills”).
Most nudibranchs are just a few centimetres long, with many measuring less than five centimetres, and they move slowly across the seabed. They are also practically blind. Their tiny eyes, known as ocelli, can only distinguish between light and darkness or detect moving shadows.
Felimare tricolor, a small, brightly coloured nudibranch found in the Mediterranean. Credits: Dani San Roman / SUBMON.
With this description, they seem to have everything working against them: they are small and slow, have no shell and have very limited vision. So how do they manage to survive in an ocean full of predators that are much larger and faster?
The secret to survival
The answer lies in the remarkable strategies they have developed to survive. Although they may not have attacks or evolve using a special stone, some nudibranchs possess “abilities” that many Pokémon would envy.
Their striking colours, for example, are not simply for show. In nature, bright colours often serve as a warning to predators, a phenomenon known as aposematism. It is essentially a way of saying: “better not eat me, I’m dangerous”.
And for many nudibranchs, that warning is very real. Many species incorporate or accumulate defensive chemical compounds from the animals they feed on, such as sponges or cnidarians, and use them as protection against predators. Flabellina affinis and Cratena peregrina, for example, feed on hydrozoans and can take up the nematocysts (stinging cells) of their prey and store them at the tips of their cerata, the elongated projections on their backs.
Flabellina affinis, whose characteristic cerata can store nematocysts obtained from its prey. Credits: Dani San Roman / SUBMON.
Where can nudibranchs be found?
More than 3,000 species of nudibranch have been described worldwide, ranging from tropical waters to polar regions.
Most of them are benthic animals, meaning that they live on or closely associated with the seabed. They can be found on rocks, algae, seagrass meadows, corals or sponges, from very shallow coastal waters to depths of several hundred metres, depending on the species.
Despite their striking colours and almost tropical appearance, there is no need to travel to faraway reefs to find them. The Mediterranean is home to a surprising diversity of nudibranchs, many of them every bit as spectacular as those found at some of the world’s most famous diving destinations.
Peltodoris atromaculata, known as the “Swiss cow” nudibranch, is one of the easiest Mediterranean nudibranchs to recognise thanks to its characteristic dark spots on a white background. It is usually found in dimly lit areas, often on the sponges it feeds on. Credits: Dani San Roman / SUBMON.
Cratena peregrina is one of the most common species in the Mediterranean. Its characteristic orange and violet cerata are not just decorative: at their tips, it can store stinging cells obtained from its prey and use them for defence. The image also shows this species’ delicate ribbon of white eggs, coiled into a spiral. Credits: Dani San Roman / SUBMON.
Did you know they are not the only ones?
Although nudibranchs are probably among the marine invertebrates best known for their striking colours, they are not the only creatures that look as though they have come straight out of a video game. Marine planarians can also be found on the seabed. These flatworms belong to the phylum Platyhelminthes.
At first glance, marine planarians can be mistaken for some nudibranchs because of their vivid colours and undulating shapes. However, they are not closely related. They also have another remarkable characteristic: their extraordinary ability to regenerate, which allows them to regrow parts of their bodies when damaged.
Prostheceraeus roseus and Prostheceraeus moseley, colourful marine planarians that show that nudibranchs are not the only small, eye-catching inhabitants of the Mediterranean. Credits: Dani San Roman / SUBMON.
Nature always outdoes fiction
Nudibranchs and marine planarians are just a small glimpse of the extraordinary biodiversity found in the Mediterranean. All it takes is a closer look to discover that some of the most surprising creatures do not live in a fictional world, but much closer to home. So, next time you explore the seabed, keep an eye on the little details, you might just come across one of the Mediterranean’s very own “Pokémon”.
