Nudibranches are one of my favourite things to find while diving in Koh Tao, the real hidden treasures of the ocean. They can be as small as 0.5cm and can grow up to about 60cm. They are beautifully coloured little creatures. They are shell-less and boneless.
The name nudibranch derives from the Latin words ‘nudus’ meaning naked and ‘brankhia’ meaning gill. They are members of the class Gastropoda in the Phylum Mollusca, which also includes snails, slugs, limpets and sea hairs. All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.
The more I have read up on these stunning sea slugs, the more fascinated I have become. Not only are they a treat to view, but they have evolved some pretty handy survival skills.
They are able to survive in both warm and cool seas, anywhere from shallow depths, around coral reefs (examples of dive sites around Koh Tao include Mango Bay, Tanote Bay, and Japanese Gardens) and underwater pinnacles (e.g. Twins, White Rock, Red Rock), to depths reaching as far as 2 kilometers deep (at our deeper dive sites e.g. Chumphon Pinnacle, South West and Sail Rock)!
They are commonly found at all the dive sites around Koh Tao, so keep your eyes peeled for these splendid slugs, or ask your divemaster or instructor to point them out to you.
Roughly only half (3000 odd) different species of nudibranchs have been identified. There are two main types of nudibranchs, namly the dorid nudibranchs (these breathe through gills on their back) and the Eolid nudibranchs (they have cerata, which are finger-like appendages that cover their back. These cerata are multifunctional and are used for breathing, digestion and defense).
They are brightly coloured and their skin is bumpy and abrasive. They get their beautiful colourations from the food they eat. They are carnivorous and eat sponges, coral, anemones, hyroids, barnacles, fish, eggs and sea slugs. Nudis are very fussy eaters so families may only eat one kind of prey. They are also known to be cannibals, eating nudibranchs from other species. They use either teeth or enzymes to break down their prey.
Their vibrant colours are either used as camouflage or a sign to ward off predators. Their flesh is tough and unpalatable. A comparison has been made that they resemble chewing on an eraser! Nudibranchs are harmless to humans. They also create their own toxins from the food that they eat and secrete these toxins and stinging cells in a defensive manner when they are feeling threatened.
Any fish that has attempted to eat one of these beautiful creatures soon learns that it is not a desirable snack and will form an association so that the same mistake will not be made in the future.
To get around, they secrete a mucous trail to move or crawl by cillary action or movement from their muscular foot. They are in no hurry to move, and will most likely appear to be stagnant, but some species are even able to freely swim about for short distances. Most of their movements occur at night.
Their eye sight is poor and they are only able to see light and dark, but they are able to explore their environment by smell, taste and touch. They have head mounted sensory appendages and oral tentacles. They contain both female and male reproductive organs, so they are hermaphroditic.
Self-fertilization doesn’t occur, but when reproduction does happen they are able to copulate, where both nudis give and receive sperm making fertilization very successful.
Each can lay up to 2 million eggs at a time. They lay spiral- shaped or coiled eggs in masses. As the larvae hatch they are able to swim freely until they come to rest on the ocean floor as adults.
They live anywhere between a few of weeks, up to about a year, depending on the favourability of their living conditions. When they do die, there is no trace of their remains left due to their boneless bodies.
Interestingly, scientists have been doing extensive research into the nervous system of nudibranchs as they seem to have helpful qualities to develop medicines that will benefit humans.
Author: Heather Nicholson (PADI DM #370231)