Carrageenan is derived from edible red seaweed — a harmless-enough beginning, as many varieties of seaweed are quite nutritious. The seaweed's gelatinous properties are utilized to thicken and provide a smooth texture to many canned dog and cat foods.
For hundreds of years, "carraigín" has been made in the kitchen by boiling Irish moss as a base for a pudding-like dessert. But the industrially produced carrageenan of today is a far cry from those homely beginnings. Now it's a highly-processed ingredient that's extracted using strong alkaline solvents.