During dives you might had the chance to spot lots of these interesting animals. I myself had quite a go sometimes. :) Thank you Joaquin for your support and helping hand setting up the right light for each shot, when we went out the Great Barrier Reef.
There are over 3,000 species of nudibranchs, from very shallow to depths of 2,500m. Nudibranchs are a group of soft-bodied, marine gastropod molluscs. They can be found in all the world's oceans but they are more common in shallow & warm temperate and tropical seas. Nudibranchs can be found at nearly all depths but generally grow to larger sizes in shallow water. One form of protection is camouflage to blend in with the background and mimicked surroundings. Another is to exhibit bright or striking colours as a warning to predators that they are toxic. Some nudibranchs are poisonous while others pretend to be poisonous, which is evident by their vibrant colors. They feed on stinging cells of hydrozoids and store them in the rear of their body for protection. They can also ingest toxins from sponges and become toxic and inedible. Some nudibranchs even produce their own chemicals.
Some nudibranchs are solar-powered. These species harvest algae from plants or coral and house this algae inside of their bodies. This algae derives its energy from light via photosynthesis, which is then used by the nudibranch. Nudibranchs are hermaphrodites! That’s right, each one is simultaneously a male and a female – they possess reproductive organs of both sexes. While mating they fertilize each other, although a nudibranch cannot fertilize itself. Because of their bright and attractive colors, many nudibranch species would appear to make popular aquarium subjects. Indeed, from time to time, you might be able to find nudibranchs for sale in shops or online. However, there are certain aspects of nudibranch biology which make the group as a whole too challenging even for expert aquarists! Nudibranchs are so difficult to keep alive in captivity, not to speak about breeding them and so most nudibranch found for sale would have been collected from the wild.
The two ‘horns’ of a nudibranch, on top of the head, are actually called rhinophores. These are basically chemical receptors that allow the nudibranch to find food and mates. Having these two horns sticking out of their bodies makes nudibranchs more visible to predators so they have the ability to withdraw them into their bodies when in danger. If you look closely, they are almost feather shaped, which increases the surface area of the rhinophore, giving them a greater ability to detect chemicals.
Some humans actually eat nudibranchs. Chileans and some islanders in Russia and Alaska roast or boil sea slugs and sometimes eat them raw. It has been described as “chewing an eraser.”
Never dive alone and let the stress at the surface! =)