MOLLUSCS
Molluscs include all the different mussels, clams, oysters, squids, scallops, and other animals that all together comprise over 100,000 species. A big family of animals indeed, and it's very old as well. Our human ancestors have eaten them for ages, and we continue the tradition! Why are the molluscs so many? Simply, they have succeeded in surviving and proliferating during the many years of the Earth's history. Molluscs achieved this thanks to their unique anatomy. In general, they can do the same things as many other animals, that is, eating, sensing, and moving, but it's the organization of these functions within their body that's so special. Their precious living functions are well shielded from outer attacks within a shell that is by itself functional and used for e.g. regulation of water filtering. Since they are so relatively small and can filter the food and oxygen so efficiently, molluscs are very fast-growing group of animals, which is of course valuable not only among them ut also among us, predators. The more of the good the better!
Molluscs have an adductor muscle that is able to open and close the shells, and it is most prononunced in scallops - which is the very part we like to eat. It is wise to bear in mind though that a part of this muscle system is not very tender, and has to be in fact cooked for a long time until it is deemed edible. This part, the "catch" portion, is normally cut away and only the tender part of the muscle is preserved and cooked. The smaller molluscs like clams and mussels have smaller adductor muscles, and their innards are usually eaten as a whole (contrary to scallops). This includes every small muscle and sex organs, as well as semi-digested food. Yep, we eat all that! All these components affect of course the texture of the mollusc when it is cooked - and the longer you cook them the more chewy they become. Therefore, be careful not to overcook your muscles, they take literally a minute to cook in most cases - usually when the shells open when you steam them in a pan it is time to remove them from the heat and serve. Try not to reheat them later on, but if you have to - do it quickly. An exception to this rule is squid and octopus. Since they contain lots of connective tissue they need to be cooked for a long time before they become tender (they are chewy in their natural state).
The flavor of the molluscs is very unique and inviting, and they actually taste at their best when eaten raw. In chemical terms, this has to do with how the molluscs handle the big osmotic pressure created by the sea water. They use several flavorful amino acids and, indirectly, some sugar molecules, to counterbalance the osmotic pressure. Compared with fish, they use amino acids a lot. That's why they taste so nice. If eaten raw, as we do with oysters, the flavor is most intensive. If we cook them, this natural taste is somehow reduced because the flavor molecules become "entangled" within the coagulated proteins, although the taste is also modified and additional taste molecules are being formed. For the best experience of taste, you need to eat molluscs in season, that is when they are not spawning.