We had a bit of a mystery pop up while in Mexico recently for our annual whale shark research project, one with a funky twist and room for a a pretty good sequel. As those of you who’ve followed the work may know (and if you haven’t, you can read about it here), whale sharks come to the Yucatan every summer to feed on fish spawn. Specifically, they’re nom-nomming their way through vast quantities of embryos of the little tunny, Euthynnus alleteratus, a smallish tuna called “bonito” in Mexico (a name that refers to a different small tuna in English speaking countries: Sarda spp.). It’s a spectacular biological event, wondrous to behold, unless you’re a baby tuna, in which case it’s a disaster of holocaust proportions.
While we were tootling around the warm surface waters gathering whale shark photo ID’s, deploying satellite tags and generally having good clean wholesome scientific fun, we noticed that some of the fish eggs were not floating freely but were gathered together in diaphanous sheets. (See what I did there? I could have said see-through or translucent, but I didn’t; I said diaphanous. I’ve always wanted to use the word diaphanous in a blog post; it brings to mind images of soft and dangly arm wattle silhouetted through your grandma’s nightie…) ANYHOO, here’s what they looked like:
In this still shot I cranked the levels a bit so that you could see the eggs arrayed neatly in the jelly matrix:
So what’s the deal here? Are tuna laying eggs in sheets, now? Probably not, no, but with a bit of digging, we found a good theory and have started on a path to the truth.
There’s only one group of fishes that’s known to lay its eggs in sheets like these, and they aren’t tunas. In fact, they’re about as far from tuna as you can get. It’s the anglerfishes, or members of the order Lophiiformes, and the sheets they lay are more properly called “egg veils” (another diaphanous fabric, although usually obscuring something a little younger and less wrinkly than does nanna’s nightie). Familiar anglers include the goosefish, a table favourite in the northeast sold as monkfish, and the (only slightly) more hideous deep sea anglers such as the one featured in that N-word movie:
The thing is, we were in tropical Mexico, at the surface, in 150ft of clear pelagic water. Ain’t no anglers here, right? Not so fast! There IS a tropical pelagic anglerfish, and here it is:
It’s the sargassum fish, Histrio histrio, a small relative of the anglerfish that lives its whole life camouflaged among the rafts of floating sargassum seaweed that form rich little islands of diversity floating in the warm, still plegaic zones of the world. At just a few inches long, it’s not a big fish, but big enough to make the egg veils we were seeing and a very likely candidate given the abundance of sargassum in the area; the tuna eggs are not the only things pushed together by breezes and offshore currents.
So there’s a good hypothesis to solve the mystery of the, ahem, diaphanous egg sheets. The sequel to this story will be the confirmation (or not) of this idea by DNA analysis. For that, our Mexican colleague Rafael de la Parra will use the lab at ECOSUR, and we’ll send some matching samples to the Smithsonian’s Lab for Analytical Biology in DC, and both labs will do some DNA barcoding. Regardless of the outcome, the important thing is that something that seemed simple – tuna spawn, whale sharks eat spawn, end of story – turns out to have hidden diversity and a surprising twist of biological complexity. And I love the fact that you can only make these sorts of discoveries by getting out there, getting wet, and sticking your face in the water. It might not be high tech, but to me it’s science at its simplest and purest.