The juvenile sea urchins I've been raising this year are now nine months old. Back in June I put them on three different macroalgal diets and have been measuring their test diameters monthly. I do the measuring in the first week of every month, and today was the day for November. Over the past few weeks I lost a lot of my Ulva urchins, for no reason that I could discern. Judging from the poop production they were definitely eating, but on some days there would be a handful of corpses in the bowl when I changed the water. They all seemed healthy and happy today, including this beautiful creature:
Seriously, this has to be the most gorgeous photo of a sea urchin I've ever taken. This individual is the largest of my Ulva urchins, with a test diameter of 12.7 mm. I love the coloration of this animal: the younger spines are green, the older spines are pale purple, and the tube feet are beautifully transparent and tipped with purple suckers.
By contrast, the urchins eating Macrocystis continue to be a more uniformly golden color:
This Macrocystis urchin is actually a tad bigger than the Ulva urchin and has a test diameter of 13.0 mm. It looks smaller because its tube feet are fully extended, so I had to zoom out a bit to get the entire body in the frame. It was also crawling around very fast and I had to hold it down to get it centered, then remove the forceps and take the picture quickly before it walked out of the picture. Every photo of this individual that I managed to get is a little blurry because of the movement.
Last but not least, the urchins eating coralline algae are hanging in there. None of them died in the past month and they are growing. Their color patterns are qualitatively different from the those of urchins eating Ulva or Macrocystis. To my eye there is more contrast in the coralline urchins; they all seem to have prominent dark coloration in the lines that radiate outward from the apical region. The other urchins have it too, but in the coralline urchins this dark pigmentation is concentrated into more clearly defined streaks and contrasts more strongly with the paler background color.
This animal, with a test diameter of 6.08 mm, is about half the diameter of the largest of its full siblings in each of the other food treatments. Food quality definitely has an effect on size, as these data indicate:
It remains to be seen whether or not I'll be able to provide Ulva and Macrocystis to these animals throughout the winter. If we get the strong El Niño storms that are predicted, the nearshore algae could be wiped out for a while. I'll make sure that if I run out of one food then urchins in the other treatment will also fast until I can feed both of them again. In the meantime, because the coralline urchins are so far behind in their growth, I'll continue to give them access to food. I don't want any of them to die of starvation, and the coralline eaters are the most vulnerable, I think.