Chaetognatha - Arrow Worm

Charles Krebs · Post by **Charles Krebs** » Fri Feb 24, 2012 2:31 pm

This unusual looking creature showed up in my plankton net. Shaped just like an arrow... long thin, and completely straight. (And in regular light absolutely crystal clear). A striking feature of this one was the "tail" that showed fins very reminiscent of a fish.

Chaetognaths are carnivorous, preying on other planktonic animals. While not obvious in the first image (taken looking at the top of the head), there are large grasping hooks on each side of the head.

Apparently there have been significantly different opinions in the scientific community on where these belong taxonomically.

I was not able to work on an "eye" image with this one, but it is quite interesting and better developed than a simple eye-spot. The extreme crop below shows what I mean... and if I get another one on the scope I'll try to concentrate on that feature.

Gerd · Post by **Gerd** » Fri Feb 24, 2012 3:22 pm

Wow, Charles, a fantastic set of pics of these difficult objects, congrats !
These arrow-worms are not easy to catch with a plankton-net,
i got a lot of samples from the north sea, but never any Chaetognaths !

Greetings,
Gerd

Franz Neidl · Post by **Franz Neidl** » Sat Feb 25, 2012 3:00 pm

Hello Charles,

congatulations also from my side.
The eyes are so well developed because the Chaetognatha are voracious predators. (I have mainly two problems with the Arrow worms: they are too long for my microscope-objectives and they show very seldom clearly the two groups of the lateral - very impressioning - spines on the head).

Franz

Wim van Egmond · Post by **Wim van Egmond** » Sun Feb 26, 2012 3:34 am

Very nice images, and indeed a difficult subject to photograph. They are fragile, don't seem to last long in a sample. And they are not easy to see when you use binoculars. And when you find them they don't like to swim into the pipette. Great work, Charles.

Wim

Charles Krebs · Post by **Charles Krebs** » Sun Feb 26, 2012 2:56 pm

Thanks all.

Franz... I was initially more impressed with the "tail", but now I wish I had been able to spend more time trying to do a side shot of the head... the grasping spines are very interesting.

Jean-marc · Post by **Jean-marc** » Sun Feb 26, 2012 5:37 pm

a bit too late, but very beautiful.

JM

Mitch640 · Post by **Mitch640** » Sun Feb 26, 2012 6:13 pm

Great images and now after reading about them, an even more amazing animal. Says they can grow up to 4.7 inches? Glad my swimming days are over.