Notes from a California naturalist

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.

Stay tuned!

Remember that one batch of sand dollar larvae that were looking weird on Monday? Well, they still look weird. In fact, all of the larvae looked the same yesterday as they did on Monday, which seems strange, considering how quickly they galloped through development for the first three weeks of larval life. It’s as though they’ve entered some stasis period during which developmental progress slows way down. Or maybe I just can’t see the signs of change.

If I had seen these larvae for the very first time yesterday, I might not suspect that anything was strange. But having watched them twice weekly since fertilization and knowing how different they looked a week ago, my Potential Weirdness-o-Meter™ is redlining. These larvae have definitely changed in a week, and not in the way that I’m used to echinoid larvae developing. With their much shorter arms and overall stunted appearance, these guys appear to be regressing. However, they aren’t dying and they don’t really look bad. As I said on Monday, they just look . . . weird.

Remember how I said I’d split this cohort of larvae into two batches and fed them different things? At first I thought this strange appearance was due to the change in diet from a Rhodomonas/Dunaliella mixture to Rhodomonas only. The larva in the photo above was from the Rhodomonas-only jar, and perhaps its odd appearance could be explained by some deficiency in the monoculture diet. Then I continued on my rounds and looked at the larvae from the same mating that were still on the Rhodo/Dun diet.

All the larvae in these photos remained on the mixed diet, and they look pretty much the same as their siblings eating the monoculture diet. So I don’t think the change in diet explains the appearance of the larvae.

Okay, then. If it’s not the food that accounts for what these larvae look like, maybe it’s something about the mating itself. These larvae, from both food treatments, are all full siblings from one mother mated with one father. As full sibs they share, on average, 1/4 of their DNA with each other, which could account for the similarity in their appearances. Perhaps this “strange” look is due more to genetics than to the environment (i.e., food).

I can test this hypothesis by examining larvae from the other crosses. Rather fortuitously, as it turns out, when I spawned the adult sand dollars a little over three weeks ago now, only one male contributed enough sperm for me to use. Three females spawned usable amounts of eggs, so I set up three matings:

• F1xM1
• F2xM1
• F3xM1

The female designated F2 gave the most eggs, and her offspring are the ones that I split into the Rhodo-only and Rhodo/Dun diets. Note that all of the larvae in this little experiment have the same father. This gives me the opportunity to test for maternal effects on development; in other words, having controlled for the effects of different fathers–ha! I make it sound as though I did that on purpose–I can now assume that differences (in growth rate, survivability, and successful metamorphosis if we get that far) between the different matings are at least partially due to differences in egg quality among the three mothers. Or to differing gamete compatibilities between each female and the one male.

So now let’s take a look at the larvae from other matings. We’ll start with F1xM1:

This larva looks normal to me, or at least what I’ve come to assume is normal. And wow, that was one filthy cover slip,wasn’t it?

The offspring of the F3xM1 mating look very much the same:

And here’s a short video of that same pair of larvae. They look like they’re singing a duet. If I were the clever sort I’d dub in some music; alas, I’m not that clever. Does somebody want to do this for me?