It turned out to be a pair of Moon crabs attempting to mate. Or rather, I think it was the male doing the attempting while the female didn't seem to be in the mood.The female has a broad V-shaped section on her underside.
This is actually the 'tail' which is folded under the hard body and holds the eggs. The male has a narrow version of this structure which is used to fertilise the female. I couldn't get a photo of him upside down and I didn't want to disturb the proceedings.
Here's a view of both the crabs before the more yellowish female made her escape. The one with the darker spots on the legs is the male. They both disappeared with the next wave.
Here's a closer look at the crabs. They are quite different from the kind that is more usually encountered. I'm not sure what kind of moon crab they are. Chay Hoon saw another one with a more reticulated pattern on the body which is probably Matuta planipes.
Here is the more commonly encountered moon crab (Ashtoret lunaris). Moon crabs have all their walking legs modified into paddles. These are not so much for swimming but for digging. With four pairs of little spades, the Moon crab disappears instantly into the sand.
Moon crabs are scavengers and this one was having a nibble at a recently dead, much larger crab.
Here's a closer look at the Moon crab.Wow, the shores of Changi are very much alive!
