Heat a pan and roast the whole spices. Cool and then grind to a fine powder. Heat vegetable oil in a pot, fry the onion until golden, add tomatoes and cook until soft. Add ginger and garlic and fry for 5 more minutes.
Add turmeric powder, red chilli powder and the crab. Pour in water and coconut milk, and cook for 10 to 15 min. Remove the crab from the pot, blend the sauce and strain.
Heat oil in the pan, add razor clams, scallop, squid and tiger prawns and stir fry for a minute and a half. Mix in the sauce and serve immediately.