Saltwater Aquarium/Unknown creature.
QUESTION: I have 75 gallon salt water aquarium. I have noticed an unknown creature that likes to live inside of the hole if rock. Now I have another one. When they get out of the hole the still like to stick to the rock. Also they sift the sand.
ANSWER: Hi Lana,
This looks like a Medusa worm, they are sea cucumbers (Holothuroids). These animals have a wide range of feeding habits. Some species, are generalist detritivores, which cruise the sea floor at night collecting tiny organic detritus from the reef rubble and base rock structure. Other species, are highly specialized feeders which are only associated with living sponges, where it removes tiny organic particles from the surface of the sponge and appears to require the sponge metabolites to survive.
These animals are impossible to positively identify species by anyone other than an expert, and even then it usually involves knowing exactly where the animal was collected, and likely killing the animal to be certain of the species identification.
If he seems healthy and happy in your system I would not worry to much about him, however there has been talk in forums of these guys killing entire systems. So when getting new fish consider what type you get as some may pick at him and if he is a toxic species you don't want him to kill your system when your not around or home to save the system.
I hope this gives you an answer on these fascinating creatures!
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QUESTION: Looks like it is medusa worm. But how they get into my aquarium? Should I worry about them causing problems for my fish?
For the most part if you have only seen it shifting sand I would not worry about it bothering your fish as I said just make sure none of your fish pick at it. A lot of strange creatures hitchhike on live rock or new coral or even snail shells. It's hard to say how long you actually have had the medusa worm but keep an eye on its size if it's starts getting to big maybe see if any other hobbyist in your area maybe interested in housing it for you. :)