Because I was so surprised at how quickly my sand dollar larvae (Dendraster excentricus) were developing, I checked my notebook from the invertebrate embryology course I took while in grad school to see if what I'm observing now is normal for these animals. It turns out that yes, Dendraster does develop at a much quicker rate than its cousin the sea urchin. And now that I think of it, when I took that 5-week embryology course the sand dollars were the only echinoids that we followed all the way to competence; we spawned and observed urchins as well, but none of them were as far along as the sand dollars by the time the class ended and we "graduated" our larvae off the dock.
Yesterday my Dendraster larvae were five days old. They already had two well-developed pairs of arms and were working on the third pair.
These larvae are big, too--500 µm long. Of course, they started from eggs that were over twice the size of urchin eggs, but they've still grown a lot in only five days. The fourth pair of arms will be the preoral arms. At the rate these larvae are developing, I wouldn't be surprised if these arms show up in the next few days.
As beautiful as those long arms are, they may be a little too long. The larvae swim and gather food using a band of cilia that runs up and down all the arms; the entire body is ciliated, but the ciliated band is the primary locomotory system. I remember the instructor of my embryology course telling us that echinoid plutei will respond to lack of food by growing longer arms, which increases the length of the ciliated band and thus (presumably) the animal's ability to capture the food that is available. There are two pieces of circumstantial evidence that my larvae may be a little food-deprived: (1) the really long arms; and (2) the lack of visible food cells in the stomachs. In urchin plutei that are feeding well I can see food cells churning away in the stomachs. These Dendraster plutei have beautifully transparent bodies, but I don't see food in the guts. On they other hand, they are growing, so obviously they are eating. Just in case they are short of food, though, I'll increase their food ration for the next few days and see how the animals respond.
In the meantime, I continue to be fascinated by the intricacy of the larval skeleton and the complexity of the skeletal rods themselves. Next time I'll try to take photos of these.