Agulhas spider crab at Castle Rocks. From video clip by Niklas von Maltzahn
Individual as found at Long Beach - half burried, looking dead.
Photo Guido Zsilavecz
Long Beach individual alive and well.
Photo Guido Zsilavecz
The Agulhas spider-crab is listed in Barnard's Descriptive Catalogue as Mamaia capensis. The genus has since changed to Maja. Barnard acknowledges that this might actually be Maja squinado, but this species is found only in the Mediterranean, and it is most likely that the species is the related (but larger) M. brachydactyla.
The body is about hand-sized, so this is a large crab when fully extended. When I first saw one at a night dive at Froggy Pond, in 1998 (but also in July - maybe they come in only during deep winter?), it first remained calm before galloping away. I found it next to a boulder on sand. The individual Niklas saw at Castle was found on reef and tried to hide under it. The one I found at Long Beach was also near a rock, as shown in the second photo. It seemed to me to be dead, covered by sand and grown over. I took a few photos and swam on, then returned realizing I did not take a photo from directly above, allowing me to see the shape of the carapace, only for the crab to "awaken" and run away at a fair speed.
M. brachydactyla prefers sand and rocky areas, inshore down to 50 m depths. Barnard lists the range as being False Bay to Algoa Bay.
Thanks to Cedric d`Udekem d`Acoz for information.
Descriptive Catalogue of South African Decapod Crustacea (Crabs and Shrimps), KH Barnard, 1972
Crustacea Guide to the World, H Debelius, 2001