It's becoming quite clear that I don't have to worry about having too many sea star larvae to deal with. While the embryos from my F1 x M1 (Purple x Purple) cross had hatched this morning, nothing from the F2 x M1 (Orange x Purple) cross looked promising. I'm about ready to write off these guys and dump them all down the drain, but will give them until tomorrow to pull themselves together and do something that doesn't look all wonky.
In the meantime, it's really fun looking at the good embryos from the F1 x M1 mating. They hatched out of their fertilization envelopes and have become elongated, sort of like stubby Tylenol caplets. This elongation defines a functional anterior-posterior axis, and the animal swims with its anterior end forward.
Gastrulation is the process of forming the first larval gut, or archenteron. Remember how yesterday the embryo was a hollow ball of cells called a blastula? In these echinoderms gastrulation is simply an invagination into the blastula. Imagine poking your finger into an inflated balloon: The balloon is the blastula and your finger forms an invagination, or channel, through it. In embryos, gastrulation begins at a site on the blastula called the blastopore; this is where you'd stick your finger into the balloon in our analogy.
Most animal guts have two openings, a mouth and an anus. You understand what happens at each of those openings. The archenteron is a gut, one of whose openings is the blastopore. The fate of said blastopore is to be either the mouth end or the anus end of the archenteron. In echinoderms, the major invertebrate phylum that makes up a larger grouping of animals called the deuterostomes, the blastopore becomes the anus, with the mouth breaking through as the process of gastrulation finishes. And lest you think that possessing an anus before a mouth is somehow less evolved than the reverse would be, you might be interested in knowing that we humans are also deuterostomes. That's right, each of you reading this blog, as well as the one who writes it, built an anus first and a mouth second.
These sea star embryos swim really fast! I had to squash them under a cover slip to snap some halfway decent pictures, and even then it wasn't easy to slow them down or chase them around on the slide. You can get a feel for how fast they can move in this short video clip:
The archenteron appears to wobble because it doesn't go straight through from the blastopore to the apex of the embryo. The mouth will break through along one of the sides, resulting in a curved gut. I suspect that when I look at the embryos tomorrow they will have graduated to the status of larvae, with complete guts. Then I get to start feeding them and watching them grow.