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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.

Gastrulating embryo of Pisaster ochraceus, 4 June 2015. © Allison J. Gong
Gastrulating embryo of Pisaster ochraceus, 4 June 2015.
© Allison J. Gong

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.

I've been fielding questions about my recent sea star spawning work from people I've shared this blog with, which is a lot of fun! To streamline things and make the info available to anybody who might be following, I decided to put together a very brief FAQ-like post to address the most recent questions.

Question:  Can you watch the eggs divide in real time?

In a time-lapse sense you can watch cleavage divisions occur, but not in real time. What I can do is set up a slide on the microscope and leave it there for a while. The gradually warming temperature speeds up development to the point that I can sort of see the division in real time. Of course, the danger is that the embryo will cook on the slide. I generally figure that once I've pipetted some embryos onto a slide and dropped a cover slip on top of them, they're goners (it's not really possible to remove the cover slip without damaging the cells underneath it) so I feel marginally less bad about sacrificing a few to the gods of observation.

Questions:  I’m fairly certain that the stars can go back to the sea, but are you able to keep their eggs with them, too? How difficult is that transport?

Actually, my scientific collecting permit specifically states that I'm not allowed to return animals to the wild. If I needed to, I could apply for additional permits but it has never been necessary for the work I do. Surplus eggs and larvae, therefore, are discharged into the seawater outflow at the lab and do return to the ocean but the parents remain in my care.

Question:  Are orange and purple stars usually able to cross with each other?

As far as anyone has been able to determine, the color of stars has zero effect on whether two individuals' gametes are able to do the nasty together. The sea stars that I'm working with--Pisaster ochraceus, the ochre star--are broadcast spawners, meaning that each individual spews his/her gametes into the water, where fertilization and development occur. The stars are also synchronous spawners, meaning that if one individual in an area begins spawning other stars in the immediate vicinity will also spawn. After all, it does take two to tango, and to spawn while nobody else does is a tremendous waste of energy.

So yes, a purple star and an orange star should be able to mate without any problems... at least not any problems due to the parents' colors.

Question:  If so, what color do they end up being, statstically?

This is a very interesting question. Two of my colleagues are going to spawn Patiria miniata (bat stars) next week to address this. Their plans are to cross a Blue female with an Orange male, an Orange female with a Blue male, and both pure-color matings. They did a preliminary version of this experiment a couple of years ago but didn't end up with enough juveniles at a size that color could be ascertained; thus they couldn't calculate any statistically meaningful color ratios.

Questions:  Do you suppose that the wasting disease could be now in the genetic makeup? Any thoughts (unofficial of course) about this?

My thought is sort of the opposite, actually. The animals that we brought in from the field are all survivors of SSWS; if anything, I'd expect them to be resistant to whatever causes the plague, and to (hopefully) pass on this resistance to their offspring. Of course, there's no way of knowing if and how exposure to SSWS affects the quality of the gametes. It's quite possible that these survivors are less fit after the SSWS outbreak than they were before.

Question:  Purple Male with Purple Female developed well and purple Male with Orange female didn’t…some sort of incompatibility?

Well, given what I saw today the Orange (female) x Purple (male) cross almost certainly did not work. Fertilization occurred, but almost none of the embryos had any indication of normal development. Since we know the Purple male was able to mate successfully with the Purple female, we can infer that his sperm were fine. It could be that there was something going on with the Orange female's eggs; there were a lot of them, but maybe their quality just wasn't very good. Or perhaps we somehow mistreated and wrecked them the other day.

Any other questions? Use the Comments section to ask them, and I'll address them in a future post.

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