I caught one of my first instar D. Diadema eating a sibling earlier today. They just left the mom maybe a week ago or so ago and I had not separated them yet. I know it happens and they were pretty crowded (about 40 of them in a 1 gallon container) but I never witnessed it before. I've separated them 3 to a 32 oz deli which is how I have done it for the last 4 clutches and I have yet to experience any cannibalization this way. I keep them like this for two or three more molts and then I rear them individually. I tried keeping a dozen of them communally once but after losing 4 in three months or so I decided to separate them. Interestingly the ones I tried raising communally were about a third again as big as the siblings I raised individually but they had gone through the same number of molts. I figured because they had more food available they just grew more with each molt.