Seeing (wannabe) stars

So. I have a batch of larvae from a spontaneous spawning of the leather star, Dermasterias imbricata, that occurred four weeks ago tonight. Until now I’ve never had an opportunity to work with this species, even though we have quite a few of them at the marine lab. I had my own for several years, until they became casualties of the plague about a year into the current sea star wasting syndrome event. In any case, this is the first time I’ve been able to spend time with larvae of this species. At the very least I wanted to see how big they would get and how quickly they would develop, compared to the species I’m more familiar with, Patiria miniata (bat star) and Pisaster ochraceus (ochre star).

When the Dermasterias spawned, the first thing I noticed was that the eggs are huge. I measured them at 220 µm in diameter, which is big even compared to what I’ve seen in other stars. Hatch rates were pretty good, and four days later the larvae were already in the 400-430 µm range. Since I have no experience culturing this species, I thought I’d divvy up my larvae and put them into three feeding treatments to see which larval diet resulted in the best overall success. According to the literature, Dermasterias larvae can be raised on a mixture of the unicellular algae Dunaliella tertiolecta (green) and Isochrysis galbana (golden). My three feeding treatments are: Dun only, a Dun/Iso mix, and Iso only.

A week into the experiment there was a clear difference between the larvae eating only the green food, and those eating either a mixture of green and golden or only the golden. Larvae from all food treatments were about the same size, but the ones eating only Dunaliella had noticeably green guts.

Fast forward two weeks, and the larvae were 20 days old. By this time they had progressed from the bipinnaria stage to the brachiolaria stage. The interesting thing was the absence of green pigment in any of the guts, even those that were eating only green food. The D. tertiolecta larvae looked good, actually. They were a little smaller than the other larvae but were perfectly formed.

Obviously all of the larvae are assimilating enough of their food to grow and develop normally. I looked at them today but didn’t have time to take pictures. Qualitatively there is no difference between the Dun larvae and the Dun/Iso larvae. In the Iso jars, however, there are many larvae at earlier stages; some are still at the “jellybean” stage. I don’t know if this is because these larvae are developing more slowly, or because of some nonrandom distribution of earlier stages into those jars when I was setting up the feeding treatments.

Next week I’ll measure the larvae again, and will have three data points to track growth trajectories.