Scarlet sea spider.
Photo by Johan Swanepoel
Another scarlet sea spider with the same pattern.
Photo by Guido Zsilavecz
Moult of a toothed decorated crab.
Photo by Johan Swanepoel
Live toothed decorated crab.
Photo by Guido Zsilavecz
Johan Swanepoel from Port Elizabeth asked two questions:
1. Is this the scarlet sea spider(Nyphon signatum) with a colour variation?
The answer is yes, it is the scarlet sea spider, and although the specimen differs somewhat in colour pattern to the one shown in "Two Oceans", this variation is common.
2. Is this perhaps the Compact seaspider (Tanystylum brevipes)
No, this is not - it is a young example of a toothed decorated crab (Dehaanius dentatus). Why it looks odd is because I think this is the moult of it, rather than the actual animal, and it is upside-down - which is why you can see its legs at the bottom, making it look like a sea-spider. The front of the crab would be at the pointy bit at the bottom. If you look carefully you can count 10 "legs" - 5 per side, which is 2 too many for a sea spider! I've seen such moults before as well, and it seems to me that, just as dead insects, they land up on their backs!
A field guide to the marine animals of the Cape Peninsula, G Jones, 2008
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994.