Gumbo with Crab, Prawn, Snapper and Sausage

Ingredients

1 whole raw mud crab
5 raw prawns. Peel 4 and reserve heads and shells, keep 1 whole
1 tablespoon olive oil
600ml chicken stock
70g spiced sausage, diced
1 ½ tablespoons plain flour
½ brown onion, diced
¼ green capsicum, diced