I've got a Eunicid and Mantis sharing a (sectioned) tank. They are really fun to watch. You know you are constantly being watched by the mantis if you are anywhere in the area of the tank. The Eunicid is always fun for gross out response when someone sees it

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Some grad level marine biology students from Auburn saw some of the critters I have and mentioned that they'd love to get their hands on some for the lab. They came by to get some planaria back in January (seem to come in on a good number of frags, so easy to come by), and got excited to see chitons, etc. I've been trying to round up anything I can for them since.