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Weird & Wonderful Sea Slugs

Nudibranchs for beginners

Nudibranchs are a group of soft-bodied, marine gastropod molluscs which 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 naked gills
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-coloured body with dark blotches.

Once I knew what to look for, I started spotting them everywhere and was quite mesmerized by all their interesting shapes, sizes and vibrant colours. 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.

Very early on in my diving career, I acquired an underwater housing and a little compact Canon camera and that was it, I instantly became a macro (close up) photography enthusiast hunting for nudibranchs to shoot on every dive. After each dive, I would go through my pictures and identify which slugs I found and made notes of it in my dive log. I still do this when I find new specimens that I haven’t seen before.

In the beginning, I got super excited about my photos, well actually I still do, come to think of it, anyway, I was overly excited and showed the DM my pictures on the camera. She looked at the photos and asked: “why are you photographing their bums?” We burst out laughing and I was educated on the basic anatomy of the sea slug. The front of the nudibranch is where it’s two rhinophores (sensory tentacles) are located and the gills are at the back around the anus.

Nudibranch rhinophores and gills
Black Nudibranch (Tambja capensis) showing the rhinophores and the gills.

Nudibranchs occur in seas worldwide from 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 own tissues so as to render themselves unpalatable to most predators. (Segment taken from the book: A Field Guide to the Marine Animals of the Cape Peninsula)

One of my personal favourites is the Blue Gas Flame Nudibranch. The gas flame group have 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
Blue Gas Flame Nudibranch (Bonisa Nakaza)

Some of the most colourful nudibranchs only grow to be between 10 to 15mm.

Candy nudibranch 2  Purple Lady nudibranchs 1
Candy nudibranch (Cuthona speciosa) 

For many divers, these slugs are a source of entertainment and delight but they often just get blank stares from non-divers. So, hopefully this article finds its way to new divers as well as non-divers alike and spark some interest into the weird and amazing underwater world.

Get your copy of Nudibranchs of the Cape Peninsula and False Bay or A Guide to the Sea Slugs of the Maputuland Coast in our online shop.

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Madelein Wolfaardt Author

by Madelein Wolfaardt