So, those bits of Ectopleura crocea that I grabbed from the harbor on Monday are voracious eaters. I didn't feed them yesterday because I didn't have time and the students spent the afternoon looking at them in lab, and I hoped that they'd be alive today. Some of the stalks had dropped their hydranths (the distal part that bears the feeding tentacles, mouth, and reproductive gonophores) but most of them were alive and lovely. I had a nice fresh batch of brine shrimp all hatched out and ready to go and thought I'd see if the hydranths could eat them. Little did I know that this simple exercise would occupy most of my day.
I started by squirting some of the brine shrimp onto the hydranths, which for the most were pretty lackadaisical about catching them. Then I decided to feed them the mashed up brine shrimp that I'm still feeding the tiny Melibe and WOW! that did the trick! Maybe it was the scent of the macerated brine shrimp that triggered the feeding response. I was fascinated.
They are beautifully and unexpectedly animated animals.
After I watched them feed for a while, it seemed to me that the outer ring of tentacles catches and holds onto prey, while the prehensile manubrium swings around and brings the mouth into contact with the food. In the meantime, while all this brine shrimp catching is going on there are other larger crustaceans crawling all over the hydranths, even onto the tentacles, without getting stung. I think their exoskeletons must be thick enough not to be penetrated by the hydroid's cnidocytes (stinging cells).
Having discovered the trick to making the hydranths eat, I squirted brine shrimp mush on them and left them alone for about 20 minutes. When I came back they had eaten and I could see brine shrimp in their guts, so I gave them more. The feeding response was pretty much as vigorous as the first one had been. So I kept feeding them throughout the morning and early afternoon.
If I didn't have other things to do, I could watch these all day. I hope that if I can keep feeding them this much they will regrow their dropped hydranths. Although I'm not sure how realistic it is to think that I can go through this routine every day. And do I really need a few dozen more mouths to feed on a regular basis? I seem to accumulate animals like other women accumulate shoes. On the other hand, I don't expect the Ectopleura colonies to last long in the lab so even my "forever" relationship with these particular animals will likely be over in a week or so. I can probably keep up this level of effort for that long.