Have you seen a Curacha? I think it's been almost 10 years since I've encountered these crabs! I was in Zamboanga city at that time too so it was quite a shock to see these in the grocery :)
The Curacha (or spanner crab in English) is a strange looking crab.
Quite pricey too at P549/$12.50 a kilo! Too bad I didn't get to take more detailed shots but here are some links to check if you're interested in reading more about this odd (but yummy! hehe) crab.
Curacha from 80breakfasts.blogspot.com
Curacha from Marketman.com