They look like spiders, but sea spiders aren't true arachnids. They're considered primitive marine arthropods called pycnogonids. Sea spiders are carnivorous and eat mostly worms, jellyfish and sponges. So far humans aren't on their menu.
"Weird one from the bottom of the sea," fisherman Roman Fedortsov captioned this photo. This looks like another kind of basket star complete with spines and hooks on its tendrils which it uses to catch its prey.