What are they up to?

These sand dollar (Dendraster excentricus) larvae that I’ve been raising will be 21 days old tomorrow, and they are still on the fast track. They’re developing much more quickly than any of the sea urchin cohorts I have raised. Some of them already have juvenile rudiments with tube feet visible. With the urchins (Strongylocentrotus purpuratus) this is the age when I worry about the cultures crashing for no apparent reason, and so far these sand dollar plutei look great. I hope I didn’t jinx them by writing that. In any case, the sand dollars are known to go through larval development more quickly than their sea urchin cousins, so my larvae appear to be playing by the book, at least as far as timelines go.

Just for kicks I took the largest full-sib cohort I had and split it into two batches. One batch I’m feeding the recommended combination of Rhodomonas sp. (red) and Dunaliella tertiolecta (green), and the other I’m feeding Rhodomonas sp. only. I’ve been able to raise urchin larvae through metamorphosis on a diet of Rhodomonas so I assumed that this food might work for the sand dollars as well. It turns out, however, that the Rhodomonas-fed larvae look a little strange now.

Their bodies have become more opaque and compact; they’ve shrunk to a length of 450-500 µm. I wonder if this is the first stage in metamorphosis. I didn’t see a well-defined juvenile rudiment in any of the larvae I examined but that doesn’t mean it isn’t there. And although they look weird and deformed, they don’t necessarily look bad. They just don’t look . . . right.

On the other hand, there may indeed be something wonky going on. I have a jar of siblings of these larvae being fed a red/green diet, and they look totally different.

This is a beautiful 8-armed pluteus larva. It looks great! The arms are nice and long but none of the arm spines are poking through the ends. They appear to be eating well and have grown to a length of 700-800 µm. This is a ventral view, and that oblong blob on the left side of the pigmented stomach is the juvenile rudiment.

Here’s a close-up view of the rudiment:

See how the rudiment is crowding into the stomach? And if you squint you might be able to talk yourself into seeing a couple of round blobs in the rudiment. These would be tube feet, which I can see as I focus the microscope up and down through the animal’s body but which don’t show up very well in a photograph.

The next day that I change the water and have a chance to look at these guys under the microscope is Friday. It’s only three days from now, but given how quickly the larvae are developing, a lot could happen between now and then. I’m a little nervous.