Today is Monday, which means Scott and I changed the water for our Pisaster larvae. I should have taken some pictures to show you how we do it. Maybe next time.
The largest and most developed larvae are now 2.2-2.5 mm long, not including the long brachiolar arms, which is about as big as they're going to get. They are still eating and developing their juvenile rudiments. Unlike the sea urchin's pluteus larva, these brachiolaria larvae lack any kind of skeletal structure and are entirely squishy--they bend and flex along any axis and can scrunch into surprisingly tiny balls. Those long arms are flexible as well, and sometimes the larvae swim around with their arms tucked or rolled up. I haven't been able to catch them in the act with the camera, but both Scott and I have seen the larvae react to encountering a surface by flipping the long arms around as though doing the backstroke.
If a picture is worth a thousand words, then how much is a video worth?
The weird alien-like effect is enhanced by the dark background I film them against. Except for their guts and the tips of their arms, the larvae are entirely transparent, which makes it difficult to photograph them. The black velvet that I use as a background, combined with lighting from an oblique angle, maximizes contrast and makes the transparent bodies more visible. The little illuminated "stars" in the background are actually part of the texture of the velvet.
To capture this video I shrunk the larvae's universe into a single drop of water on a depression slide. This means they couldn't swim too far out of the field of view and would have to bump into each other. Don't worry, though, after the photo shoot I returned the larvae to one of the jars and let them swim away. They'll be just fine.