Seafood replacements products (which include plant-based products or those made from cell culture) are gaining popularity and traction across the world. Fish, meanwhile, are the most numerous vertebrates killed by humans.
There is not, and may never be, an unambiguous and universally-accepted metric for whether an organism can suffer or not. Nonetheless, our perceptions of animal behavior and cognition are related to our decisions about using animals.
Parasites are organisms that live on or in another host organism and redirect its resources for themselves. They are nearly as old as life itself, having existed since before the days of the last universal common ancestor of all life.