It’s high time I explained what exactly I’m doing chest-deep in a pond every morning.
My life’s focus these days is a little fish called the threespine stickleback. They’ve been studied systematically for many years, starting with Niko Tinbergen’s work on their territorial behaviour. Today they are considered an emerging model system: there are extensive, well-established lab resources (including a sequenced genome) for studying them, and research on these fish is used to answer more general questions in biology, particularly evolution.
I am tempted to say that the stickleback is more than just a fish. This is not just me being flippant; they are actually more than one species. The large saltwater stickleback population that stretches around the northern hemisphere is subdivided into clusters that don’t interbreed (i.e. species), and marine stickleback have repeatedly colonized lakes and streams, evolving into new species as they did so. It is these repeated, parallel, and most importantly recent—often they occurred less than 12,000 years ago as the last ice sheets retreated—speciation events, and their accompanying changes in morphology and behaviour, that have made sticklebacks into such an important study organism for evolutionary biology.
It’s two of the lake species that my research at the ponds focusses on. In five watersheds that we know of, the sticklebacks that colonized freshwater became not one but two species. One, the benthic species, lives near the lakebed and is larger; the other, the limnetic species, lives in open water and is much smaller. Since these species are so young, they can hybridize, though in the wild they rarely do. My research focuses on why they don’t; in other words, why they remain separate species.
Sticklebacks are odd fish. The males build nests, for one, and guard their eggs and later their fry. As the water warms in spring, each males stakes out a territory in the littoral zone. His throat turns bright red and his body a bright blue-green. He gathers bits of vegetation, sand, anything he can find, like a bird collecting twigs for its nest, and glues them together with a protein made in his kidneys. The nest has a tunnel through the middle; sometimes he swims through it to shape it. Here, he hopes, a female will lay her eggs.
Many people have noticed that in at least some of the “species pairs”, the benthics hide their nests under dense vegetation while the limnetics nest in plain sight on bare sand, rock, or sunken logs. My experiment this spring and summer aims to test whether this difference in habitat actually affects a female’s choice of mate. In other words, if I put a male of the “right” species on the “wrong” habitat, will a female still mate with him?
This is why I’ve been moving algae around for the past few weeks. I’ve built 1×1 metre pens in the shallow ends of two ponds. Half of them are left open while the other half has been covered with plants. I collected fish from one of the species pair lakes, and they’re being held in aquaria until I’m ready to experiment on them. I’ve now put the first two males into enclosures, and they’ve both built nests. As soon as I have some females that look like they’re ready to lay eggs, I’ll put one in each enclosure and see whether they spawn. If being on the “wrong” habitat decreases the chance that a pair spawns, then this difference in habitat plays a role in reproductive isolation, the lack of hybridization between two related species.
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