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Monday, July 15, 2013

Sticky side arms

Last night I mentioned seeing ctenophores (or comb jellies) washing up on the beach.  Tonight I took a closer look at a few of them.  This is how they appeared on the sand.  They ranged in size from about 5 mm to 2 cm long.

I photographed several individuals in a small aquarium.  In the image below, the light is highlighting the comb rows used for propulsion.  The mouth is at the top.

In these individuals the long tentacles were noticeably pink.

The tentacles have numerous tentillae that can be extended like a cascading net for capturing prey.  Remember that ctenophores don't have stinging cells (nematocysts) like jellyfish, but instead have sticky cells (colloblasts) for capturing prey by adhesion.

The long tentacles can be completely withdrawn within sheaths inside the main body of the ctenophore.  Compare the two pictures below.  They show the same individual — the first with both tentacles extended and the second with the righthand tentacle mostly retracted. 

If you return to the second photo of this post, you can see the pink tentacles inside the ctenophore.

I think this is Pleurobrachia bachei, the same species we often encounter on local beaches in the winter and spring.  In my experience, it's somewhat unusual to see so many washing ashore in July.

P.S.  Pleurobrachia means "side arm" and refers to the position of the long feeding tentacles.

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