Theoretically mussels can be picked from the beach and then eaten the same day but you need to be careful about the sea in which they live. If refuse or sewage is discharged into the sea then it is likely that the mussels will have absorbed this and so they may be hazardous to health.
If you are absolutely certain that the sea contains no sewage (and I am not sure how you could be) then you can collect mussels from the edges of the sea, but they should only be collected during the months of October to April (not during the warmer months).
You should only collect mussels which are alive. To check this, prise it open a little, if the shell closes then it is alive, but if it stays open or opens without any resistance at all then it is dead. They can then be cooked as per bought mussels: but exercise care when picking mussels!
If you are absolutely certain that the sea contains no sewage (and I am not sure how you could be) then you can collect mussels from the edges of the sea, but they should only be collected during the months of October to April (not during the warmer months).
You should only collect mussels which are alive. To check this, prise it open a little, if the shell closes then it is alive, but if it stays open or opens without any resistance at all then it is dead. They can then be cooked as per bought mussels: but exercise care when picking mussels!