According to Nudibranchs of the Cape Peninsula and False Bay, the mosaic pleurobranch is “uncommon”. We’ve only seen one, and this shortly after discovering that such a creature existed. It’s possible that I’ve swum over countless mosaic pleurobranchs without noticing them. That said, they are very striking creatures and reach a size of at least 12 centimetres. The one we saw, pictured, was closer to 20 centimetres long. He was cruising along the pipeline at Long Beach in Simon’s Town.
The mosaic pleurobranch is of the order Notaspidea, side-gill slugs. Like fellow family member the warty pleurobranch, these slugs have a single gill on the right hand side of their bodies, which is usually obscured by their mantles. In surgy conditions you might see the gill exposed (picture here). Side-gill slugs are carnivores, feeding on sponges, sea squirts and some species even consume fishes.
