Fertilization of sand dollar eggs (Dendraster Excentricus)

  • Saad Bhamla

Today is day 1 of the Hopkins marine biology course.

We injected 5 molar KCL into the oral cavity of the Sand dollar organism and within minutes, from the non-oral side (opposite), either eggs or sperm were released. There is no way to physically distinguish between male and female sand dollars, but you can identify the female if it’s releases pinkish eggs while makes release white sperm.

This is a sand dollar, FYI.

And here is a video of movements on a sand dollars body. It’s covered in soft spines.

We kept them separate and them added a dilute sample of sperms into an eggs slide. I didn’t get a chance to record the sperm swimming, but they were gorgeous. Long flapping flagellae that propelled them as soon as they hit the water. It’s like they all hear an invisible beacon and zoom towards an egg.
This is an unfertilized egg.

In the image above, notice the pink dots- they are embedded in a transparent matrix. It’s not clear to me why they are pink and what they’re role is?

Here is a video after I added the sperm into the  droplet containing eggs. The tiny moving dots are actually the sperm. Notice the circular membrane around the nucleus of the egg- that is a mechanical the egg releases once it’s fertilized to prevent more sperms from further fertilizing it. (See sketch)

There are so many questions I have racing in my head at this point.

The sand dollar produces like a million or more eggs and sperm and just releases into the ocean waters. How do they find each other at fertilize? Is there a synchronized time for release? Do they do it at a specific time of year/day? What about ocean currents? Do they exploit the environment to ensure successful fertilization?
Onto the sperm/ within seconds they had permeated the jelly matrix and fertilizer the egg. And seconds later, the membrane was formed. How is the membrane triggered? How is it that the sperms swim so easily through the jelly matrix which is designed to slow them down ? Also, what is the exact mechanism for transferring the genetic material? Do they puncture through the egg? Do they fuse the plasma wall?

More questions on the whole organism to be discussed later. Especially its ability to transfer grains of sands one by one on its back. How does it sense gravity? And what’s with the aerodynamic shape? Does it take advantage of drag somehow?



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