When horseshoe crabs come up, I always try to mention that they are harvested (and usually killed) for their blood in the most unethical way possible. They basically pump it directly from their (version of) hearts, and as long as they're "alive" when they throw them back in the water, it's counted as ethical harvesting. Usually, they just die a few hours/days later from having their hearts punctured and much less blood available than they'd like.

Also, their blood is wholly unnecessary for medical usage. Medical companies use the amebocyte lysate in their blood to detect endotoxins, but it's pretty cheap and easy to synthesize amebocyte lysate in a lab without involving crabs. It is only being held back by the FDA because, before we could synthesize it, the blood made harvesters wealthy enough to lobby for the FDA to refuse approval of the synthetic version when it came along so they can keep pumping that blue gold. Feel free to correct me if I got anything wrong. I'm not an expert.