sea urchins

I know next to nothing about fish tanks. A friend talked me into having
one. It is a marine tank set up with live rock, live sand, 1 large
clown fish (black and yellow), 3 black & white striped fish, 2 nemos,
an orang,white and black wrasser, an elecric blue (damsel?) and a sea
urchin. The large black and yellow clown and blue fish have been in the
tank now for almost a year. Everything has been great until about 4
months ago when my friend brought me the sea urchin. Within weeks I
couldnt find my favorite shrimp that I had had for five months, a
yellow wrasser had disappeared also and this morning I found the sea
urchin dining on one of my black and white striped clown fish. Last
night when I checked the tank they were all healthy. My friend said the
sea urchin only eats dead fish but my shrimp, my yellow wrasser and the
black and white clown fish never at any time showed signs of sickness.
This is the first time I actually caught the urchin dining on the fish.
Is he the culprit or could there be another also?

I dont want to lose any more as I have sort of grown attached to them.

nicodemus