Tiny yellow discs attached to the bowl — the planulae had undergone metamorphosis!
Below is an even closer view. I chose these three individuals because they're in different stages of development. Can you guess which is youngest and which is oldest?
The youngest juvenile is on the far left, the oldest is in the middle (lowest), and the juvenile on the far right (highest) is at an intermediate stage. You can tell by the number of divisions in the basal disc, and by the development (height) of the stalk growing up from the center of the disc.
Remember that these juveniles are going to become tall, extensively branched colonies (reminiscent of a fern or a miniature tree). These are the youngest hydroids we've ever seen. They would be too small to see in the field, so we just got lucky that they metamorphosed in the lab. In the next image you can see the central stalk extending upward.
Amazingly, when I looked around, I noticed a few juveniles that had the first developing zooids! It did not appear that these zooids had formed tentacles yet, but two developing zooids are clearly visible attached to either side of the central stalk:
These little hydroids have grown very quickly. It will be interesting to see how the branching pattern proceeds, and when the first feeding tentacles appear.
A fun side note: When I first saw the these juvenile hydroids as in the last photo, one of the first things that came to my mind was that they looked like tiny chess pieces. And then later I read an article that described a stage similar to this as the "pawn" stage!
P.S. To review what an Abietinaria colony looks like when it's older, see the post from 27 June 2016.