Subject: Why starfish eat other starfish on the same species?

I'd encounter this problem in an aquarium in National Science Centre Malaysia. 
One on the starfish eat his own friend in the same species. Then me and my 
friends that doing the same practical project in the Science Centre transfered 
the injured starfish into another aquarium in hope that the starfish would 
generate itself.Here, I want to know why it eat the same species starfish? 
Would it be a disease on the other starfish or because of nutrient less in the 
aquarium. Thank you.

Re: Why starfish eat other starfish on the same species?