So. Last week when I looked at my sand dollar larvae I wasn't at all sure what to make of them. I thought that all of the offspring from one of the matings (F2xM1) were going south and didn't know how much longer they would survive. The offspring from the other two matings seemed to be doing much better.
Fast forward a week and a half and my, how things have changed. I have some juvenile sand dollars now! And so far they are all from the F2xM1 mating, the ones that had started looking strange and that I thought might die. I'm surprised that any of the larvae metamorphosed, as my general understanding of sand dollars was that competent larvae settle among adults of their species, so that when they finish metamorphosis they would be in a suitable location to grow up. However, the animals is always right, and in this case I was happy to learn that my understanding was wrong.
This larva is almost competent. The main part of its body is almost completely filled by the juvenile rudiment (the tannish structure on the left side of the more reddish stomach) and the arms are shorter.
And here is a video of a trio of competent larvae.
Their bodies are almost entirely opaque now but they are unquestionably pluteus larvae.
As metamorphosis begins, the tube feet in the juvenile rudiment rupture through the body wall and the animal starts sticking to a hard surface, in this case a glass slide. For a while the animal is suspended between the larval and juvenile forms, in a state I call a larvenile. Hopefully the time spent in the larvenile stage is short, as to be neither larva nor juvenile is a bad thing. I've seen both sea urchins and sea stars get stuck in the larvenile stage, and they all died.
Larveniles are strange things. See for yourself.
In this video the right side of the animal (not the anatomical right but the right side of the image as it is presented on the screen) is the juvenile, and the left side is the larva. The larva half still has its fenestrated arm rods, which will eventually be dropped and left behind. It also retains for the time being the ciliated band which it used both to swim and to capture food. Another weird feature of the larvenile is the transition between the bilateral symmetry of the larva and the pentaradial symmetry of the juvenile. The bilateral symmetry has been more or less obliterated by the process of metamorphosis, but there isn't enough of the juvenile to have complete pentaradial symmetry yet.
And, finally, metamorphosis is complete and a little sand dollar walks around on tube feet.
Yesterday this animal was a larva, and today it's a juvenile. The sea urchins do the same thing. But these sand dollars have done everything faster than the urchins, and that includes development immediately after metamorphosis. You may recall that the purple urchins have only five tube feet when they metamorphose, and they struggle to coordinate them to walk. From what I can see these sand dollars have at least twice that many tube feet very shortly after metamorphosis, and they can walk much more quickly.
The tube feet themselves are different, too. Urchins' tube feet are suckered and look like little plungers. Sand dollars' tube feet have those pincher-looking tips (although I haven't seen them open up and grab things yet). Adult sand dollars live partly buried in sand and don't use their tube feet to cling to surfaces; they do use their tube feet to grab food, though.
Speaking of food, I don't know what these juvenile sand dollars will be able to eat. Fortunately I have a while to figure out what to try feeding them, as their mouths won't open up for at least a week (I hope). While it's easy to observe what happens on the surface of the animal as it metamorphoses, it's impossible to see what's going on with the internal reorganization of the body. I do know that an entire new gut will have to be formed before the animal can eat. In the meantime it will have to survive on energy stores stashed in all that opaque part of the body.