This was a bit of a tough choice for me, as I have an oddly large assortment of animal mating pictures (although I always kick myself for not having the camera on me the day I saw barnacles mating). I decided to go with this 'morning after' picture, as I find this type of procreation to be very bizarre.
These are Nereis sp. worms. They normally live on the benthos, but when these particular worms get ready to mate, their bodies undergo a startling transformation. Their internal organs degenerate, and their body cavity fills with gametes. They develop stronger podia (paddle-like feet) for swimming and better sensory organs. Then, when the moon is right, they swim up into the water column. Males will find a female and swim in circles around her... then they both explode.
So above you can see chunks of worm, white strands of sperm, and reddish-yellow eggs (both of which are released from the body cavity by the explosion).