Friday 1 April was the last day of my spring break, and tomorrow I go back to teaching. Spring break felt very short this year, and I was busy the entire week. I decided to spend my last day of freedom doing my favorite lab-related things: looking through microscopes at tiny organisms. I had already planned on spending a few hours dealing with my two batches of larvae, and figured I might as well make a day of it and collect a plankton sample on my way in.
It was a beautiful morning out on the bay.
Alas, as gorgeous as the outdoor scenery was, I couldn’t linger long once I’d collected the plankton sample so I headed to the lab. If you’ve ever wondered what a marine biologist’s desk looks like, here’s mine:
The dissecting scope on the left belongs to me, as it was a graduation gift I bought for myself when I finished graduate school. The compound scope on the right belongs to the lab, but I’m the person who uses it most frequently. I find that, when looking at something like plankton, it’s easiest to start by looking at a bit of the sample in a small dish under the dissecting scope; then, when I find interesting critters I can pipet them out and put them on a microscope slide for observation under the compound scope. It may seem a little awkward, but this switching back and forth between “forest” and “tree” views works for me. And honestly, any field biologist worth her salt should be able to switch focus from “big picture” to “small detail” fairly easily. How else would she be able to develop a solid understanding of the system(s) she studies?
Now back to the plankton. Right off the bat I could see with the naked eye some big (by plankton standards) crustaceans zooming around. It wasn’t easy chasing them down with the pipet, but after a while I caught one and dumped it on a depression slide. It was a mysid shrimp.
Those big compound eyes are stereotypical of many crustaceans–think crabs, lobsters, large shrimps, etc. Looking carefully at the tail of this particular individual, can you see two small circular structures? Those are statocysts, the organs that give the animal information about its orientation with respect to gravity. The presence of two statocysts in the uropods (the appendages on the most posterior segment of the body) tell me that this animal is a mysid, rather than one of the gazillion other shrimplike crustaceans living in the sea. I saw at least half a dozen mysids in this plankton sample.
Pelagic crustaceans tend to be quite spastic, and mysids are no exception. Their thoracic appendages beat almost constantly to generate a current that brings particles close to the ventral midline, where they are passed forward to the head and sorted as either “food” or “not-food” and disposed of accordingly. The action of the thoracic appendages also moves the animal slowly through the water, but for quick swimming the mysid rapidly flexes its abdomen and moves away in short bursts.
Overall, this wasn’t the most interesting plankton sample I’ve ever collected. When my students and I collected and examined a sample a week earlier, we saw much more animal diversity than I saw the other day. We had some strong winds on Monday-Thursday of last week (I’m writing this on Sunday) and the surface water temperature dropped to 12°C; I thought this would be the start of the spring upwelling season. If it was, then the phytoplankers hadn’t responded when I collected this plankton sample on Friday. In any case, it appears that the spring phytoplankton bloom hadn’t yet begun. I expect that in another week or two I’ll find more diatoms in the plankton.
After lunch it was time to tend and observe my larvae. There’s not much to report about the Dermasterias (leather star) larvae. If you remember, I’ve split these larvae into three different food treatments: (1) Dunaliella only; (2) a combination of Dunaliella and Isochrysis; and (3) Isochrysis only. At this point, 38 days into development, there is no discernable difference between treatments 1 and 2. The larvae in treatment 3, however, don’t look so good. They are stunted and appear to be regressing to earlier developmental stages.
On the other hand, the Dendraster (sand dollar) plutei continue to astound and fascinate me. They are stunning!
They are happy and healthy and seem to be doing well. Their posterodorsal arms have grown and their pre-oral arms (the fourth and last pair to form) are poking out. The larvae are eating all the food I’m giving them and are putting it to good use. At this rate I expect to see their rudiments developing soon.