Nudibranchs are a group of soft-bodied, marine gastropod mollusks that shed their shells after their larval stage. The word "nudibranch" comes from the Latin nudus "naked" and the Ancient Greek (bránkhia) "gills", the name refers to their gills being outside of their bodies.
Nudibranch with its naked gills circled
Before I started scuba diving, I had no idea these little creatures existed. My very first nudi (as commonly referred to) was pointed out to me on my 6th dive, this was my very first dive after completing my Open Water Diver course and it was a Chocolate Chip nudibranch (as it is locally known). It has a light-colored body with dark blotches.
Once I knew what to look for, I started spotting them everywhere and was mesmerized by all their interesting shapes, sizes, and vibrant colors. I got my first nudibranch identification book - Nudibranchs of the Cape Peninsula and False Bay– and flipping through the pages made me realize there were so many I still had to find.
The front of the nudibranch is where its two rhinophores (sensory tentacles) are located and the gills are at the back around the anus.
Black Nudibranch (Tambja capensis) showing the rhinophores and the gills.
Nudibranchs occur in seas worldwide from the Arctic, to temperate as well as tropical regions and can be found at virtually all depths.
Nudibranchs have very few predators thanks to their protection methods, they use a combination of outstanding camouflage and aggressive toxicity. Slugs either secrete irritant chemicals such as strong acids or absorb toxins from their food and accumulate them in their tissues so as to render themselves unpalatable to most predators. (Segment from the book: A Field Guide to the Marine Animals of the Cape Peninsula)
One of my personal favorites is the Blue Gas Flame Nudibranch (as seen below). The gas flame group has highly variable colouration and cerata (external appendages) that sways to and fro in the surge. These are used for breathing as this specific slug lack gills entirely, they use their cerata to absorb oxygen. They grow to be 50 to 80mm and can be found around the Atlantic coast of the Cape Peninsula to Port Elizabeth.
Blue Gas Flame Nudibranch (Bonisa Nakaza)
Some of the most colourful nudibranchs only grow to be between 10 to 15mm.
Candy nudibranch (Cuthona speciosa)
For many divers, these slugs are a source of entertainment and delight. So, hopefully, this article finds its way to new divers as well as non-divers alike and sparks some interest in the weird and amazing underwater world.
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by Madelein Wolfaardt