Yasmeen Hussain: Ciliate vs. Urchin Egg

One summer day, I was counting sea urchin eggs and saw one moving. Now, the classical delineation of sperm and eggs is that one of them (sperm) moves and the other (egg) does not. When eggs start moving, I get a little concerned. I moved the hemacytometer (a neat device invented over a century ago that helps people count the number of objects per unit volume) with this egg to our lab’s bigger microscope, which has better optics and higher magnification. This is what I saw: https://youtu.be/1GxgL-PIYdc

A ciliated, single-celled organism appeared to be either trying to break into the egg and eat its nutritious insides or is feeding off of the egg’s jelly layer, which is full of polysaccharides, peptides, and other potentially nutritious substances. While it moves around the egg (in my head this makes a slurping noise), the egg appears to be moving. This is pretty fun!
But, you may ask, why am I in lab, counting sea urchin eggs on an otherwise perfectly nice day? I’m trying to understand how sperm chemotaxis, how sperm find eggs using chemical signals from the eggs, affects fertilization success, which I determine by counting how many eggs get fertilized. I could just put a drop of sperm into a vial of eggs, or put two spawning sea urchins (that white stuff is sperm) in a tank together, but that wouldn’t be very precise. The number of sperm in each drop, and the number of eggs from each female, can differ wildly, and my advisor showed in a PNAS paper that the ratio of sperm to eggs can make a big difference in the number of eggs that get fertilized. Knowing this, before I mix the sperm from one male urchin and the eggs from one female urchin together, I count the number of sperm in an average milliliter collected from the male and I also count the number of eggs in an average milliliter from the female. Then, I can calculate the appropriate amount of sperm and egg needed for a consistent sperm:egg ratio and mix them together in that proportion. When I look at the embryos a couple of hours later (see picture), I can count what percentage of eggs were fertilized.