In the parlance of invertebrate zoologists, competence is the state of development when a larva has all of the structures and energy reserves it needs to undergo metamorphosis into the juvenile form. In the case of my sea urchins, this means that they have four complete pairs of arms, each with its own skeletal rod, and a fully formed juvenile rudiment, which contains the first five tube feet of the water vascular system. A continuous ciliated band runs up and down all eight arms and provides the water current used both for swimming and feeding. The larva will have been eating well and its gut will be full of food. It will have lost the transparency it had when it was younger and will appear to be more solid-looking in the central area.
The first batch of larvae that I began culturing this season are now 42 days old. Some of these are competent, or very nearly so. Last week I isolated about a dozen of these big guys into a small dish, making it easier for me to observe them closely every day. Today they looked decidedly opaque and dumpy, and although some of them were still swimming others were heavy and tended to rest on the bottom of the dish.
Here's a photo that I took yesterday:
General orientation: This is a ventral view. The animal swims with its arms forward, which defines the anterior portion. Thus the bottom of the cup-shaped body is the posterior. This larva measures ~900 microns along the anterior-posterior axis. Plutei have bilateral symmetry that goes all to hell during metamorphosis, from which the urchin crawls away with typical echinoderm pentaradial symmetry. This wholescale change in body organization is one of the truly amazing things about metamorphosis in these animals. It boggles my mind every time I think about it.
You can see that this pluteus has eight arms. The oblong reddish structure in the middle is the stomach, which has taken on the color of the food the animal has been eating. The strange mixed-up looking structure adjacent to the stomach on the animal's left side is the juvenile rudiment. Focusing up and down through the rudiment shows that it contains five tube feet. After metamorphosis, the juvenile urchin will use those first five tube feet to walk around as a benthic creature, having spent all of its life up to this point as a member of the plankton.
Today I captured about 20 seconds of a larva feeding. This individual is a day older than the one in the photo above and has more of that opacity that I associate with competence.
This is a dorsal view; if you imagine that you're looking at the animal's back, you see that the rudiment is indeed on its left side. The larva's ciliated band is moving a lot of water, and the little specks that you can see flying around are food cells. There wasn't enough water in this drop for the pluteus to do any actual swimming, but at this point it's pretty heavy and would tend to sink to the bottom.
Some time in the next several days these guys are going to start metamorphosing. I will be examining them every day; keep your fingers crossed that I catch them in the act!