Two months ago now I gave my juvenile sea urchins a job. It's the kind of job they're perfectly suited for: eating algae. I measured them all and randomly divvied them up into three food treatments. One group remains on the pink coralline alga they'd all been eating once they graduated from a diet of scum, one group gets to eat the soft green alga Ulva sp., and the third group is eating the kelp Macrocystis pyrifera. I fully expected that the urchins on coralline algae would grow much more slowly and experience higher mortality than the other groups. And now I have data to validate my intuition!
It has been clear from the get-go that the Ulva and Macrocystis urchins are growing faster than the poor guys relegated to coralline algae. The coralline urchins are hanging in there, though, and are even growing a bit. They are also dying, a lot.
During the first month of the experiment I was surprised to see the high attrition rate of urchins eating Macrocystis. I think these early deaths were due to the fact that Macrocystis, once it starts to go bad, goes bad fast. Even with daily water changes to rinse out the poop, the Macrocystis bowl tended to get dirty faster than the others, so poor water quality may have killed the urchins. The copious slime from the Macrocystis itself doesn't help, either. Eventually I will be able to graduate the urchins to containers that will allow flow-through water, but for now most of them are too small to be kept in screened containers because they would escape through the mesh.
Overall, the Ulva urchins seem to be the happiest. I haven't lost any this past month and they eat and poop a lot. These individuals have the good fortune that Ulva doesn't foul the water as quickly as Macrocystis does. They are extraordinarily beautiful, too, and are becoming much more colorful:
I've always wondered about the biochemical magic that allows this species of sea urchin to eat algae (primarily kelps, but also some red and green algae) and end up so unabashedly purple as they grow to adulthood. I know from experience in the intertidal that juveniles of S. purpuratus usually go through a green stage when they're in the 1-2 cm size range, before they become purple. And once they're purple, they stay purple. Part of the reason I wanted to do this feeding experiment is to see how the juvenile diet affects color of the animal. These urchins are all from the same mating, so they are full siblings. Presumably there would be some color variation even among a cohort of full-sibs, but if I can distinguish differences between urchins eating Ulva and urchins eating Macrocystis, then perhaps these would be at least partly due to diet?
The difficulty is in photographing individual urchins under the same lighting and background conditions so that color can be somewhat objectively registered. I'm going to have to become a much better photographer, and the urchins are going to have to be more willing to sit still and pose for me. In the meantime, it is easier to compare overall color between the two groups, rather than individual urchins. Looking at the two bowls side-by-side, I get a better feel for the gestalt of each group; can you see the difference? Before you read the caption, can you guess which is the Ulva group and which is the Macrocystis group?
To my admittedly very subjective eye, the urchins on the left have more dark pigment and the ones on the right have a more overall golden color. The golden color makes sense because Macrocystis is golden in color (even though taxonomically it is considered a brown alga). But the darker purple in the urchins eating green algae? That makes less sense to me. In any case, I'll have to wait and see how the color develops in both groups of urchins. I suspect that in the long run they'll all end up purple, because that's what they do in the field, but they may take different routes getting there. Stay tuned!