Instead of iron and hemoglobin, what do snails, spiders, and octopi use to transport oxygen?

Copper and hemocyanin.

In humans and other mammals, hemoglobin (also spelled haemoglobin) is used to transport oxygen around the body. The oxygen binds to hemoglobin's central Iron atom. The molecule also takes carbon dioxide back to the lungs.

Snails, spiders and octopi use a similar related compound called hemocyanin which has a central atom of copper capable of binding oxygen or carbon dioxide as part of the transport process. The blood of these animals appears blue.

https://www.mcgill.ca/oss/article/did-you-know/snails-spiders-and-octupi-all-have-blue-blood