Tracing blue blood

Horseshoe crab (and some other critters, like octopi and tarantulas) have copper-based heme instead of iron-based.

Cool. Where do they get the copper?

We get iron (about 4g for an adult man) from meat and some plants (preferably meat), but that's kind of a zero-sum game--somehow iron had to enter the cycle from non-organic sources. Maybe plankton pick up enough trace iron and copper dissolved in the water to allow overall biosphere growth and make up for losses.

The crabs get some from momma, but they have to get more to grow. They eat mollusks, which have copper-heme too, so maybe that's their main source. What do the mollusks eat that give them copper? Octopi eat fish, and many other mollusks eat algae. Some algae need trace amounts of copper, and fish seem to need a little too.

It looks like the devil is in the details here--I don't know how well a snail will take up trace elements from algae (and it looks like a moderately hard problem to solve).

I wonder if anybody has studied copper takeup and use in the food chain.