Thirty-one days ago, on 20 January 2015, I spawned purple sea urchins (Strongylocentrotus purpuratus) and generated six jars of larvae. I've been examining the larvae twice a week ever since. At first they were doing great, developing on schedule with no appreciable abnormalities or warning flags. Then, at about Day 24, the cultures began crashing for no apparent reason. At first I expected to see lots of malformed, shriveled, or underdeveloped larvae, but the thing that I don't understand is that for the most part they look great. They're eating, pooping, growing, and (apparently) doing everything that they should be doing.
Case in point:
This larva is PERFECT. It has all four pairs of arms now, and they are growing symmetrically. The stomach (the inverted-pear-shaped structure in the middle of the cup-shaped region) is pigmented with the red food it has been eating, and there are no skeletal rods protruding beyond the tips of the arms. This individual doesn't give me any clues as to why the culture it came from took a nosedive this past week. The other larvae that I sampled from this jar today also look good. There aren't many left in the jars from this spawning, but if they all look as promising as this one then I still have hope that some will be able to metamorphose successfully.
So what gives? I suspect that Day 24 has something to do with it. I'm working on a hypothesis and need to let it percolate inside my brain a bit more. When it's ready I want to test it, although that will have to wait until next year, as we've reached the end of this year's spawning season.