When visibility is low you learn to really appreciate the little things. Sometimes, the REALLY little things. When a nudibranch (pronounced nood-i-brank) was first shown to me I had no idea what I was looking at. Now, I often see that same vacant look I probably had on my face on the faces of divers I show them to. So what are these magnificent blobs of color? Well, let me tell you a little bit about my friends the nudibranchs…
For people who still have that vacant look on their face we’ll start with the basics, nudibranchs are a member of the sea slug family, they have a shell only in the larval stage and then lose it when they reach adulthood. There are over 3,000 known species of nudibranchs, come in a rainbow of colors and can range from about 6mm to 31cm (.25-12 inches). They are found in all of the oceans, in all temperatures of water. Here in Manuel Antonio we find them in about 6-12 meters of water in our volcanic rock reef systems, we see them in red and white, green and pink, just to name a few!
Nudibranchs are carnivores and eat things like sponges, coral, anemones and hydroids. A perfect example of you are what you eat, they eat colorful food and that’s where they get their vibrant colors!
Nudibranchs have terrible eye sight and rely on receptors called rhinophores on their heads for smelling. This is how they smell out food or other nudibranchs, although the two are not always mutually exclusive, nudibranchs will sometimes eat each other. If they’ve already eaten and have their priorities in order and decide to mate with another nudibranch they happen to meet, they’re always in luck! Nudibranchs are all hermaphrodites so they don’t have to worry about finding a mate of the right gender. This is a great situation for something that moves slow and can have a lifespan as short as a few weeks.
You are now prepared to identify our friends the nudibranchs and even possibly spit out a few fun facts to your fellow divers! Just a friendly reminder to always watch your fins and equipment while diving you don’t want to crush your new friends!

source: http://marinelife.about.com/od/invertebrates/tp/Facts-About-Nudibranchs.htm