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I have been watching this comb jelly ever since I bought my plate coral in august. It never leaves the plate coral, staying on the underside until the night when it crawls partially on to the top of the coral. I have narrowed it down to some species of platyctenida, but I was wondering if anyone could I.D. further. It's white and clear but with shiny white spots on it. Roughly 5-6" long (when it's not hiding like in the picture). I outlined it on the picture here, hopefully that helps. Thanks!
 

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Killer goby,
You're bang on with identifying it as platyctenida. It's in the class of tentaculata and phylum of ctenophore. They are considered ectosymbiotic and often will take up shop under a plate coral without harm. I have one myself and really enjoy turning on the penlight at night and watching this jelly "fly fishing" with its long tentacles and comblike side branches. I've observed it stretching these over 14" long and touching nearly every coral in my nano tank without effect. The tentacle is sensitive to light and touch and if spooked will retract very quickly. I view it as harmless and welcome its presence. It's on my "good" list of over two dozen types of hitch hikers I am hosting. I'm also glad to meet someone else with one, they are either rare or largely unnoticed in the reef tank. I've spoken with many experts who had little experience or knowledge of the critters! Consider yourself fortunate to have one and let me know if want to compare notes on its behaviour and outcome.

Jerry
 

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This is a great find in an aquarium. Many of these get mistaken for sea slugs and are killed.

There is a great write up on the sea slug forum if you would like to learn more. They are frequently colored similarly to the organisms they are living with (e.g. leathers, plate corals, sea stars, etc.) and thus blend in.
 

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On further research I've been able to narrow this down to a specific species called Vallicula Multiformis. In one paper I've found that they are not in fact filter feeders but dissection has found they consume amphipods and copepods. So for my tank they most certainly will not starve! Another paper indicated it depends on high water flow to provide drag to extend its two tentacles (seemed fairly obvious to me). Below is the best image I can find of one.

 

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Very cool!

It is also thought that some live on the mucus produced by the organism they live with.

Unfortunately, not a whole lot is readily known about them.
 
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