We saw at least 8 of these masses, all about the same size and texture. At first they looked like fish eggs, but you might be able to tell that although they stayed together, they were "mushy" or malleable. Quite peculiar! With a 10x hand lens we thought we might be able to see tiny eyespots, so there was still potential for this to be an egg mass of some sort. We decided to bring it back for a look under the microscope.
Before I show you those pictures, I'll mention that these masses were buoyant. We placed a few in a ziploc bag with seawater and they floated at the water surface. Here's a picture I took later so you can see what the masses look like under water.
This is the first view under the microscope with low magnification:
The mass was made up of many thousands of tiny developing larvae! In the image above (and those that follow), each larva has a greenish body and dark eyespots. Here's a much closer view:
Now you should be able to see a head with four eyespots (two large, two small) followed by a segmented body. You can also see three sets of setae or bristles on each side of the body.
To make this even easier, we took a few pictures under a compound microscope (see below). Note that we were able to do this because the larvae were fully developed and some were starting to leave the egg mass and were actively swimming around. In the following images, note that the bristles can be also be splayed out to the side.
These are the larvae of a polychaete worm (known as trochophore larvae). The larvae use cilia to swim. If you look very carefully between the bases of the bristles, you might be able to see the cilia — very fine hair-like structures that beat very rapidly to propel the larvae through the water.
So the mystery blobs are worm egg masses! We don't know what species of worm produced these egg masses, but we'll be trying to figure it out. If you have any ideas, let us know!