Welcome to the vibrant world of nudibranchs
For me nudi-hunting is like collecting stamps...
You always want to find new ones for your private collection.
Now, I am the type of diver who can usually be found hovering in one place for minutes at a time treasure hunting with my mask a short distance from the reef or seafloor. Many people don’t know that the biodiversity in a square meter of coral reef is unmatched by any other ecosystem on the planet, a rule to which the Andaman Sea in Thailand and Mergui Archipelago in Myanmar (Burma) are no exception. The regions are filled with a cornucopia of colours, shapes and textures that if you take your time and look closely you can find some bizarre little rarities that can bring a tear to your eye.
What exactly is a nudibranch?
These “sea slugs” are hermaphroditic which means that each individual has both female and male reproductive organs on the right side of their body. They can lay up to 25,000 eggs at a time in a mucous ribbon-like formation that will take roughly a week to hatch.
Nudibranchs are carnivorous and love to eat sponge, hydroids, algae, coral, anemones and even other nudibranchs. They can be found mostly in shallow reefs in warm salt water, but there are some species that live in deeper or cold waters and some can even survive the lower salinity of brackish water. They are found in great abundance at any of the dive sites of the Mergui Archipelago in Burma.
What is so special about nudibranchs?
Although they are small, they have developed some very industrious techniques to protect themselves. Many nudibranchs display bright colours for a multitude of reasons. Some practice what’s called aposematism which is colouration to suggest to predators that they aren’t tasty or can be toxic. This is a technique used by the Phylidiidae family that we often see in the Indo-Pacific.
Some nudibranchs that feed on the stinging cells of jellyfish or hydroids can actually recycle them and use them as weapons. Glaucus atlanticus or the Blue dragon nudi can be found floating near the surface and feeds on the nematocysts of the Portuguese man of war Jellyfish and similar species. It will eat the stinging cells and pass them harmlessly through their body where they can then be displayed on their skin or in appendages called cerata making them harmful to predators.
Some can also make their own chemicals (regardless of what they eat or where they live) to serve similar purposes or can even secrete an acidic mucous when disturbed.
Some nudibranchs will feed on plant cells and use them to create their own food sources. A particular group of sacoglossan sea slugs will eat certain algae found in soft corals and recycle their chloroplasts so they can create their own nutrients through photosynthesis. This means that once it has eaten enough it doesn't need to eat any longer. The algae in its body will use the sunlight to create enough nutrient for its host to survive.
How do you identify a nudibranch?
Using their scientific names is the best way to accurately identify nudibranchs even if it can be a bit of a mouthful. Taxonomy is the way we classify living things in biology and it is an intricate science. Basically speaking, plants and animals are divided into family trees that get more specific as you travel downwards eventually allowing you to name individuals.
Instead of branchial plumes, Aeolid
nudibranchs are covered in appendages called cerata which increase surface area for gas exchange and also can be used to display ingested chemicals or stinging cells for defence.
There are 4 main Nudibranch families:
Doridoida, Aeolida, Dendronotida and Euarminida. The most common families are "Aeolids" and "Dorids" which each have specific identifiable features.
Dorids all have a distinguishable branchial plume on their back end which they use to breathe. Their mantles are often smoother and more regular shaped than Aeolids which are typically much spikier.
Some of my favourites are ID Please (Marine Creature Identification) and Nudibranch Central on Facebook.
What kind of nudibranchs can we find in Thailand and the Mergui Archipelago?
A selection of the prettiest Nudis we found in Thailand and Myanmar
Thanks to the authors and contributors!
This blog post was written by Katie O. with the help of Camille L. and several facebook groups who could identify some less common nudis.