I think this is a tough ID. Let's see what you think.

[aarafael]

I started a new 60 gal marine tank about 2 weeks ago. There's nothing in it but live rock at this point. Im patient.

 

So last night, this thing is swimming around wildly in the tank. I tried to get some pictures but it was moving too fast. I did get some pictures but they're blurry and indistinct. I took some short videos but i still dont think they will help alot. So im going to try to describe and see if it sounds familiar to anyone.

 

 

It really looks like a tadpole or a sperm with an elliptical head and a tail propelling it. Its not tiny. About 1 to 1.5 inches long and 1/4 - 3/8 in wide. The tail doesnt whip back and forth but has a sort of skirt around it like a cuttlefish's body. This "skirt" ripples in a wavy motion to propel the thing. Its white or cream -ish in color with no eyes but a darkened spot at the head.

 

I tend to think it's some sort of larval form of something else as it doesnt look like much and it's blind. It zoomed around the tank mostly randomly and seemed to be looking for a hole or crevice to squirm into for a home. A couple times it ran into something and seemed to try to wiggle it's way in before giving up.

 

It is daytime here now and the creature is nowhere to be found. Ill look for it again tonight when i turn the lights out. It appeared last night after the lights had been out for about 20 minutes.

 

I can post the pictures i have but i dont think they're much use.

[Merthynia]

Does he look like this? Sounds like a polyclad flatworm. If that is the case GET IT OUT. It will happily eat clams, snails, etc.

 

Here is someone else's post about catching them.

http://www.nano-reef.com/topic/250732-polyclad-flatworm/

 

[RayWhisperer]

Judging by your description, I'd guess it's a epitoke of some type of eunicid.

 

https://en.m.wikipedia.org/wiki/Epitoky

[aarafael]

definitely not polyclad flatworm unless it's some very immature form