These little jerks are actually plants that will lash out and grab you (and each other, if there's more than one in the room), forcing you to kill them before you can move on. They're pretty weak, so this normally wouldn't be a big deal except that they're only found underwater. Hope you brought a gill!
River squid are found in the flooded sections of the Colossus Silo, as well as under the warm waters of the Weezer Dam.
They carry no loot and they're only really interesting if you want to be sprayed with sap or something.