wanted: ribbon worms

[Umbra]

That sounds like one, possibly Notospermus tricuspidatus which is a more common snail predator.

Here is an example of N. tricuspidatus in the attachments (black worm - I don't own any of the pictures). There are also other species that seem to frequently turn up such as this brown species that seems to eat crustaceans like hermit crabs and shrimp. Im interested in anything anyone can find!