“My goodness, Gammarus, what big eyes you have!”
There are a lot of possible answers to Red Riding Hoods question. You might have big eyes to help you navigate in the world, to find resources, mates, and all sorts of things. If you see variation in eye size in populations, it will be tricky to figure out what the selection pressure on eye size is, because eyes do so many different jobs.
In the case of one amphipod crustacean, the answer to Red’s question seems to be:
“All the better to see predators with, m’dear.”
Gammarus minus is a small, freshwater crustacean. Douglas Glazier and Travis Deptola were lucky enough to find five springs where this little bug lives. Two of them were nearly predator free. Amphipod paradise! Three other springs, though, were also home to a fish predator with the suspicious-sounding name of the slimy sculpin. With a name like that, you know that’s a fish up to no good. Two of the springs have quite large numbers of sculpins, and one had sculpins, but far fewer. The sculpins appear to be the major predators in these springs, though the authors note that the two with the highest number of sculpins also had the occasional trout.
Glazier and Deptola went through all five springs in one day, catching sculpins. They standardized the area and time spent looking for the fish. Not only did they count all the sculpins they found, they took and weighed them all in the lab.
They also caught amphipods from each stream, took them back to the lab, and measured the size of their eyes and their body length.
The authors show that the eyes are smaller in the spring without sculpins, and in the one with the smallest number of sculpins, than in the two springs with lots of sculpins.
It’s tricky to interpret this result. It would be cleaner if eye size fell right along a “sculpins / no sculpins” dividing line. Instead, you have to start thinking about what the threshold of selection pressure is, how many fish prey upon how many amphipods, etc.
The authors don’t detail any relevant behaviour here, either of the crustacean or the fish. It’s reasonable to think that the amphipods avoid the fish by visual cues, but the authors do say that this sculpin is “relatively inactive benthic fish.” I could imagine some fish would have predation strategies that slightly larger eyes would not help to avoid. A little more detail on how this fish feeds might help persuade that bigger eyes can help the amphipod.
This set of springs is a lovely little natural experiment, but I hope that follow-ups start to bring some more lab experiments to bear on relationship of predation and eye size.
Glazier D, Deptola T. 2011. The amphipod Gammarus minus has larger eyes in freshwater springs with numerous fish predators. Invertebrate Biology. DOI: 10.1111/j.1744-7410.2010.00220.x
Gammarus photo from here. Sculpin photo by Noel Burkhead on Flickr; used under a Creative Commons license.