With shell on seafood there is usually so much waste. Instead of throwing them in the bin, save them in the freezer and when you have 500g, make a seafood stock. There is so much flavour that comes off, and the stock is far superior than you can buy.

Ingredients

500g of the shells, heads etc you can top this up with shell on prawns if you do not have enough 
1 onion, diced
1 carrot, diced
1 celery stick, diced
50ml brandy 
1 litre water   
1 chicken stock pot

Method
In a large pot heat 2 tbsp olive oil and brown and caramelise the shellfish on a medium heat (this will take around 10 minutes). While the shells are caramelising take your wooden spoon and bash the shells to break them up.

Add the chopped vegetables and cook for a further 5/10 minutes until soft.

Add the brandy and bring to the boil to burn off the alcohol.

Add the water and the stock pot and bring to the boil, reduce the heat to low and cook for 1/1 ½ hours stirring occasionally. 

Strain, pushing against the shells and vegetables to extract every bit of flavour you can.