Blood Worms. They look like gentle earthworms until they extend their mouths and show their teeth made of atacamite, "a copper-based chloride biomineral." Their poisonous bite is painful for humans and kills whatever prey they find in the shallow marine waters where they live.
As Don Garvey describes, they are used as bait for fishing:
I wanted to show friends what blood worms were like but couldn't find a good video on YouTube... so I made my own. If anyone is interested, I'll make a video of one biting me in the near future. I use these worms as bait for Striper Fishing (Striped Bass.)
Well Don, by all means, please make that video and send it to us.