5. Box Jelly Fish -
These alien like entities are basically floating blobs of gelatinous death. They are among the sea’s most venomous creatures, and their sting is often fatal. They administer toxins by use of their tentacles, which carry over five hundred thousand cnidocytes containing nematocysts, which are basically microscopic harpoon shaped mechanisms that inject venom. You do NOT want to invite these things over to your next pool party!