Little Marisco the Hitchhiker

Apr 29, 2004

I don't have a camera so I'm going to describe it as best I can.

About two months ago I purchased some live rock for my new tank and to my surpise there was already a little animal within half an hour of being in the tank. He lives in what looks like a snail shell cemented to the rock (same color too), and near the top is an opening that literally "screws" open whenever the animal wants to poke his head out. There appears to be a mouth with a little tentacle on each side. I'm certain he has eyes because he reacts when I walk in front of the tank or wave my hands. When he's hungry he puts out a thread that catches any food or waste floating around in the tank, and then pulls on the string until the it reaches his mouth. Quite nifty.

Anyway I've become very attached to the little booger and even consider him part of the "crew" I use to keep my tank clean. I even gave him a name, Marisco, which means "seafood". Any ideas as to what I've got?


Staff member
Sep 4, 2006
I have lots of worms that use a small white thread to catch food that way but their homes are small particles of substrate sand that they either eat and excrete or attach with some kind of animal produced "glue" to make a white tube (it looks like a very small feather duster tube but made of sand) but the housing you describe is quite different. I tried to take a video once of some of them catching and reeling up food but my octopus got in the way of the camera :hmm:

I can't think of anything I have had that lives in something that resembles a snail shell except snails and hermits. Are you sure that its house is the shell? I have a hard shelled feather duster that attached itself to a snail so that its home is entwined in the shell. Eventually the snail died, likely from the weight of the feather duster as it grew and kept it from righting itself (I would find it flipped over often). The duster is still doing very well but its tube and the snail shell are not separatable and are the same color so that it might look like the feather duster lived in the snail shell.

How big is the part of the critter you see (or do you ever see it?). From your description, I am guessing that it is one of the huge number of worms (feather dusters being only one of them) that live in the ocean substrate.

It may not have eyes as there are many animals that have photo sensing cells and can detect shadow (change in lighting) without actually being able to detect shapes (sea stars and feather dusters come to mind).


Oct 16, 2008
It could definitely be a polychaete worm, but the shell description is throwing me off.
Are there "bristle" like structures on the animal?
Does its mouth have jaws?
Apr 29, 2004
Here's something I drew in paint quickly. The whole animal - shell and all - is about the size of a grown man's fingernail.


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