What are the key characteristics that must be considered in buying fish and shellfish?

Chef's answer
They should have bright, clear, full eyes that are often protruding. As the fish loses freshness, the eyes become cloudy, pink, and sunken. The gills should be bright red or pink. Avoid fish with dull-colored gills that are gray, brown, or green.
Frequently asked Questions 🎓
The scallop eyes fringing the edge of its shell detect moving objects as they pass successive eyes. 3. ... At some stage in their lives, all scallops all have a ridge of tiny teeth near the notch where the byssus comes out in other bivalves. This is called the ctenolium.
Although the reproductive organs, or roe, are edible, the part of the scallop that most people in the United States eat is the adductor muscle that opens and closes the shell. ... The scallops eyes are on its shell, not on the meat inside.20 Dec 2020
Scallops are highly sensitive to shadows, vibrations, water movement, and chemical stimuli. All possess a series of 100 blue eyes, embedded on the edge of the mantle of their upper and lower valves that can distinguish between light and darkness. They serve as a vital defense mechanism for avoiding predators.
16 
Warm eyeballs allow them to see prey faster in the murky depths. Researchers already knew that swordfish (Xiphias gladius) can selectively warm their eyes and brains. ...10 Jan 2005
There are between seven and eleven distinct eye types in molluscs.
Clams lack heads, but most can react to changes in light and some have eyes. All clams have two shells joined near a hinge structure with a flexible ligament, and all are filter feeders. Clams also have kidneys, a heart, a mouth, a stomach, a nervous system and an anus. Many have a siphon.
A few more cooking questions 📍