Oikopleura's fishing house
- by Wim van Egmond, The Netherlands

  When I was examining some marine water under a stereo microscope I suddenly saw something I had always been intrigued by. It was a larvacean called Oikopleura. Larvaceans are primitive chordates related to tunicates. They look like tunicate larvae but retain this larval shape when they become adult.  

Oikopleura without its housing


In fact I did not see what I wanted to see. The most intriguing bit about this larvacean is its housing. And this housing isn't visible since it is completely transparent. But I detected the housing since I saw the small larvacean wiggling its tail to create a water current within this housing. Oikopleura is a tunicate, a primitive chordate, and its house is a very complex fishing device. It is very difficult to study since it's very delicate. If you try to transfer it onto a glass slide for examination under the compound microscope it will certainly be destroyed. But I thought it would be interesting to try and observe it better. A watch glass would make it easier than the plastic container it was in now. I took a pipette with a rubber sucker and attached the rubber sucker to the wrong end. Now I had a pipette with a very wide opening, so I tried to suck up the delicate organism and it worked. To prevent damaging it I had already put some water in the watch glass.

It worked. The larvacean was still alive and its house was intact. But it was impossible to photograph since it was not visible at all. The only way to see the shape of it was to follow the water current. I could see two fine nets because they were full of tiny particles. The fishing net is so fine meshed that scientists have used it to trap and study the tiniest marine creatures. The house is quite big compared to the small inhabitant.


A drawing of Oikopleura in its fishing house.

This drawing was based on the work of Dr. Lohmann, a German planktologist who discovered the feeding apparatus. The red line shows the water flow (through one entrance) created by the undulating tail of the larvacean. Large particles are blocked by protective grids. The filters on the left, filter tiny organisms from the water which are then passed to the mouth. The filters are so fine-meshed that they can filter out particles as small as 1/1000 of a millimeter. Finer than any plankton net. Dr. Lohmann was able to discover many unknown tiny life forms (flagellates etc.) by examining these filters.

The opening at the front (bottom right) of the house is an emergency exit!

  Although I knew it would be impossible I tried to transfer the creature to a microscope slide. And indeed it did not work. The house was immediately destroyed into a shapeless mass. I made a series of pictures of the homeless Oikopleura and I tried to reconstruct its housing in a drawing so you'll get an idea.

I do think it must be possible to make pictures of the whole device. It has to be done in a very small aquarium with illumination from the back (like darkfield illumination). By carefully adjusting the position of the lamp the housing should light up. A strong macro lens that enables a magnification of 3 to 1 should be enough. Plenty of specimens should be available. Larvaceans rebuild their house every day. If you give them time it should be possible. But I did not succeed. I only found a few larvaceans, but was already pleased that I made this observation. And it is not that bad that there are still many challenges left in microscopy.

  More about the anatomy of larvaceans, and some of their relatives like the salps can be found in the article Tunicates extraordinaire: Salps, Larvaceans by Jean-Marie Cavanihac.

A close up of the head of Oikopleura, the large sphere is its ovary.


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All photographs Wim van Egmond

Published in the January 2001 edition of Micscape Magazine.

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