I was very fortunate when i had happend to have my eye on my reef aquarium when the large Diadema (long spine) urchin started spawning. It released large puffs of white sperm into the water column at regular intervals. This process went on for at least 3 minutes.
I managed to take a sample from the water near his body and here is what I found:
Diadema urchin sperm, 40x, DIC
Diadema urchin sperm, 40x, PC
Images made through a microscope. All subject types.