Of spheres and ribbons

A cross-section of Porites lobata, photographed at 30x 
magnification. This individual is a female.

One of the things we wanted to do before spawning started this year was check all of our colonies for gametes. In many corals, you can tell whether they have eggs ready to spawn just by looking at them. Branching corals like Acropora have large, obvious eggs right under the surface of their tissue, so if you snap off the end of a branch, you can see the eggs in cross-section. Porites is not quite so simple. Our study species is mounding, so there's no such thing as just snapping off a branch. Still, one previous researcher was able to find eggs in the tissue by looking under a microscope, so we decided to check our colonies. We collected small chips of tissue with a hammer and chisel in the field and brought the samples back to the lab to check. 

It was pretty exciting when Matthew found eggs in the very first colony. It looked exactly like the previous study, too. They were spherical, covered in zooxanthellae (those are the algal symbionts that live in coral tissue), and just barely hanging onto the coral's skeleton. We were able to positively identify several females in our set of samples. 

Things got a little trickier when it came to identifying males. Sperm are so small that you need a compound microscope to see them, and you can't exactly stick a hunk of coral under the objective of a compound scope. The best alternative was to look for sperm-generating tissue in the coral itself. I thought I had found gonads in several of our samples - there was a white, ribbon-like tissue that really, really reminded me of gonads in other invertebrates. It occurred at the base of some polyps, which I suppose would be a logical place for sperm to be generated. I thought we could use it to identify males. 

The only problem is that we found the "gonad" tissue in some females, too. Not that it would be entirely unexpected for females to also have some gonadal tissue generating their eggs, but it meant we couldn't use the white tissue to identify males. 

The underside of a Porites lobata tissue chip, photographed at
55x magnification.

Matthew had the idea to take a sample of the white tissue and look at it on a microscope slide. What he found surprised us both. The tissue was chock full of nematocysts - you know, the little stinging cells that all corals and their relatives have. The tissue was not gonad at all, but instead looked like a nematocyst factory. 

The good news is that even if we're not able to identify individual colonies as males, we can definitely identify females. A full 20% of our samples had eggs in them, so it's pretty reasonable to expect another 20% to have sperm. The remaining 60% might not spawn this month - our species also spawns in May. Either way, it's very reassuring that we at least found the eggs, because it tells us that at least some corals in our dataset are ready to spawn but have not yet. In the next couple days, they should give us what we want: their babies!