Kanten is a seaweed-based gelling agent derived from tengusa テングサ, a specific type of red seaweed. It is semi-translucent, and firm when it solidifies. Kanten is 80 percent fiber, and has no odor or taste.
It is used to make Japanese confectionery, and unlike gelatin kanten-based confections don't dissolve at room temperature. Kanten is often confused with agar agar. Both are seaweed-based, but they come from different algea. Kanten can be swapped 1:1 with agar agar in a recipe, although the resulting texture will not be the same. Kanten produces a firmer texture than agar.
There are 3 main groups of edible seaweed: Red algea, green algea and brown algea. Most edible seaweeds are marine algae whereas most freshwater algae are toxic. Seaweed contains high levels of iodine and calcium. It is possibly a source of vitamin B12 (see ref), but the amount is variable and therefore, not dependable.
Because it comes from the sea, seaweed contains sodium. It should be avoided by anyone on a sodium-restricted diet. Wakame has the highest sodium content, with kelp and laver having significantly less.