You are what you eat, part the third

To recap:  Way back in January I spawned some sea urchins. The resulting progeny are now almost 7.5 months old, counting from the day that they were zygotes. Once they metamorphosed and became established as post-larval urchins in June, I divided them into three feeding treatments:  the kelp Macrocystis pyrifera, the green alga Ulva sp., and a red coralline alga. Since then I've been counting and measuring them monthly, and today I completed the fourth data collection.

I could tell by looking at the bowls that the Macrocystis and Ulva urchins continue to grow much more quickly than the poor urchins stuck on the coralline diet. The Macrocystis urchins are, overall, bigger than the Ulva urchins, despite the qualitative observation that the Ulva urchins appear to be eating more. However, I am not monitoring the amount of food that is eaten by any of the urchins.

Test diameter of juvenile sea urchins (Strongylocentrotus purpuratus) as a function of diet. 2 September 2015. © Allison J. Gong
Test diameter of juvenile sea urchins (Strongylocentrotus purpuratus) as a function of diet.
2 September 2015.
© Allison J. Gong

In the past month I lost almost half of the coralline urchins; I'm down to six. Mortality for the other groups remains low. I think the Macrocystis and Ulva urchins have for the most part gotten big enough that, barring any unexpected disastrophe (yes, I made up that word), they shouldn't experience much mortality.

Population sizes of juvenile sea urchins (Strongylocentrotus purpuratus) as a function of diet. 2 September 2015. © Allison J. Gong
Population sizes of juvenile sea urchins (Strongylocentrotus purpuratus) as a function of diet.
2 September 2015.
© Allison J. Gong

In terms of color, I think the differences between the Macrocystis and Ulva diets have become more pronounced in the last month. Today I tried to photograph the two groups of urchins under the same lighting conditions, with mixed success. There's some variation within groups, of course, but overall the Macrocystis urchins have a more golden color on both the test and the spines. . .

Juvenile sea urchins (Strongylocentrotus purpuratus) eating the kelp Macrocystis pyrifera. 2 September 2015. © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus) eating the kelp Macrocystis pyrifera.
2 September 2015
© Allison J. Gong

. . . whereas the Ulva urchins have more purple coloration:

Juvenile sea urchins (Strongylocentrotus purpuratus) eating the green alga Ulva sp. 2 September 2015 © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus) eating the green alga Ulva sp.
2 September 2015
© Allison J. Gong

And, just to make sure that I hadn't inadvertently biased the light in favor of one group at the expense of the other, I manhandled all of the urchins to one side of their respective bowls and took a picture of the two bowls side by side. Let me tell you, it was like herding cats. I'd get one group all bunched together then start working on the other, and the first ones would immediately begin wandering away from where I'd put them. This is the best shot I managed to get. Without reading the caption, you can still figure out which group is which, right?

Juvenile sea urchins (Strongylocentrotus purpuratus) feeding on Macrocystis (left) and Ulva (right). 2 September 2015 © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus) feeding on Macrocystis (left) and Ulva (right).
2 September 2015
© Allison J. Gong

We're coming into the time of year when it might be difficult obtaining food for these urchins on a regular basis. Everybody may have to go on a diet for a few months. As long as I can get my hands on both Ulva and Macrocystis I'll keep feeding them, and when I run out of one food the other group will have to fast also. I think they're well enough established by now that not having unlimited food won't do much harm.

I just had another thought. I could put the Ulva and Macrocystis urchins back on coralline rocks and see how they do over the winter. Something to think about.

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