Hi Marc,
What you have seen has actually been discussed here many times.
This is the spawning pattern of several different types of polychaete worms. When the worm becomes gravid it undergoes several morphological changes that allow it to swim better. There are two basic patterns. In the first, the worms will swim to the surface and the body will rupture releasing gametes into the water. This type of spawning is naturally cued to lunar cycles and at the right time, large numbers of worms will be up in the water spawning and disintegrating releasing large numbers of eggs and sperm. Fertilization occurs in the water, and the larvae develop in the plankton. These worms are referred to as heteronereids or epitokes, and this pattern happens in the polychaete families Nereidae, Eunicidae and some others.
In the second pattern, the female worm grows a clone, typically from the back end of the worm. This clone has the swimming modifications, but it lacks a gut. It is filled with stored food (fats). Fertilization occurs in the sediments or rocks, and the eggs are glued to the clone which breaks free from the female and swims away. Typically the clone lives for about two weeks or so, and during this time it disperses to new habitats. The baby worms grow to a small juvenile stage fastened to the belly of the clone. When the clone "runs out of gas" the babies are released into their new habitat. These worms are typically referred to as swarmers, and this pattern is characteristic of worms in the family Syllidae.
Do a search of this forum on "epitoke" or "swarmer" and you will find some of the earlier threads. Unfortunately all of the earlier images have been deleted.
Here is image of a syllid swarmer that I took while it was swimming in the plankton (and this was fun to try to get while we both were floating in the water). This animal was about 2 inches long.
This was a much smaller worm collected with a plankton net; the eggs are the dark blurry mass in the center.
In some species the swarmers stay in the sediments. This small worm was discovered in some sediment samples my students were working through during my invert class last summer.
