Why are nudibranchs so colorful?
As you certainly know, nudibranchs are beautiful sea slugs that display most of the time a full range of vivid and elegant colors. From flashy pink to intense purple, bright yellow to deep green, blue, red, etc… We can frequently observe them in many dive sites in Lembeh like Nudi Fall, Police Pier or Kareko Batu. They display these flamboyant colors for many reasons.
The first scientists and naturalists who studied them thought that nudibranchs of the same species recognized each other by color, but it has since been shown that the sea slugs are not able to see the colors. Moreover, in some cases we can observe a high variability of coloration between individuals of the same species.
So, why are nudibranchs so colorful if it isn´t for that reason? They actually look like defenseless small fragile creatures, and lacking a protective shell like the sea snails, some very close relatives, they should go unnoticed in an environment full of potential predators like the rich waters of Lembeh.
In the animal kingdom, a bright coloration is generally a warning: “I am dangerous!” Blue-ring octopus or flamboyant cuttlefish, often seen in Lembeh, are poisonous or venomous like some colorful snakes or frogs. Biologists call it aposematism; it is the ability to prevent attacks by warning potential predators that the animal has defenses such as being unpalatable or poisonous, displaying flamboyant colors or contrasted stripes.
Many nudibranchs feed on sponges or tunicates, which contain toxic or distasteful chemicals that will be concentrated in their body, making them toxic or inedible for the potential predators. Other nudibranchs feed on hydroids, jellyfish, and anemones, ingesting their stinging cells (nematocysts) that will be recycled in their cerata (outgrowths on the body of some nudibranchs) for defensive purpose, providing them some stinging capacities. Those flashy colors will advise the wannabe predators of their toxicity or inedibility, and if this predator ignores this signal at his first encounter, he will remember it next time.
As some nudibranchs spend a long time feeding on colorful anemones or soft corals, they will adopt the bright color from their prey in order to camouflage themselves. It would be extremely difficult to spot some species of Phylladesmium on a colony of Xenia soft coral of which they perfectly imitate its appearance. The nudibranch order being very broad, some other species won´t display those flamboyant colors in order to match with a duller environment.
Finally, some harmless and palatable nudibranchs will mimic toxic nudibranchs by exhibiting their flashing colors patterns, sending potential predators a warning signal to be less likely to be eaten.
Anyway, for us as divers, it is always delightful to look at and take pictures of those colorful and elegant creatures in the many dive spots and environments of Lembeh, and our experienced dive guides at Two Fish Divers Lembeh have an eagle eye to show us the most camouflaged and tiny nudibranchs.
Dive manager, Two Fish Divers Lembeh