Are eating raw oysters a violation of the first Precept? The first Precept is to abstain from Killing a "living being". 

Is there a consistent definition of Living Being that we should not kill to avoid violating the first Precept?

I have heard of a definition such as a Living Being means they have the Five Aggregates, so that Plants / Germs do not count as they do not have all Five and cannot be reincarnated into.... etc. But I don't know if this definition is widely accepted in either Theravada or Mahayana.

If this definition is accepted, how do we know what beings do not have all Five Aggregates? If not, is there any other definitions that we can use?

Would also want to know if Clams, Oysters, Corals...etc... are considered as having all of the Five Aggregates too.

Five Aggregates: 
1. form (or matter or body) (rupa), 
2. sensations (or feelings, received from form) (vedana), 
3. perceptions (samjna), 
4. mental activity or formations (sankhara), and 
5. consciousness (vijnana)
The  word 'panatipata'  is  derived  from  two  words: 'pana'  which means  'living  being'  and 'atipata'  which  means  'striking  down', hence  killing  or  destroying.  According  to  the Atthasalini or Expositor, for killing to take place five conditions must be met.  
  
  
    
    The being must be alive.  
    There must be knowledge that it is a living being.  
    There must be intention to cause its death.  
    Action must be taken to cause its death.  
    Death must result from such action.  
    
    
      If  all  these  conditions  are  fulfilled,  then  the  precept  has  been broken.
    
    
  


~ Cited from: Five Precepts (Pncasila)

If all the above conditions are not met, the action doesn't become a kammapata (action which gives birth in another existence) but become only a kamma (action which gives its result during the lifetime of a living being in an existence).

Suppose you eat something. But you don't know it's a living being. First precept is not broken for such action.

Suppose you see something on your desk and think of it as a bug. You hit that with your hand with the intention of killing that bug. But later you get to know it's not a bug but something which was not alive. First precept is not broken for such action. But you accumulate bad kamma for such action.

Suppose you see a flying mosquito and you hit that mosquito with a mosquito racket with the intention of killing it. But mosquito is not killed (let's assume the battery in the racket was drained). First precept is not broken for such action. But you accumulate bad kamma for such action.

Suppose you gave medicine to a ill dog with the intention of saving that dog. But then that dog die of that medicine. For such action you don't accumulate bad kamma. Actually you accumulate good kamma for such action.

Note: This is what I understood. I may be wrong but not Dhamma.