If you were an early human assigned the task of drawing up a "wish list" of your ideal high-protein food, you'd probably want something that's easy to find, prolific in number and relatively defenseless. The many varieties of shellfish in coastal regions fit that description handily, so it's little wonder they've been a valued food source throughout human history. The vast majority of shellfish are edible, within certain limits, and most are remarkably tasty as well.
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The culinary kings of the shellfish world are the crustaceans, a group of armored, bug-like predators. The crustaceans include crabs, langoustines, prawns, shrimp, lobsters and their freshwater cousins, the crawfish or crayfish. As a group, they're renowned for their rich and sweet flavor and are among the most eagerly consumed shellfish. A few species are of little commercial or culinary importance because of their small size or mediocre flavor, but most are choice.
Cephalopods are a shellfish turned inside-out, with a soft exterior and a vestigial shell on the interior. The best-known of these piscine paradoxes are the squid and octopus, though the cuttlefish and nautilus are also valued in some parts of the world. The nautilus is an exception among the cephalopods, sharing their tentacles and mobility but retaining an external snail-like shell. As a family, they're tough and chewy unless prepared well, but tasty nonetheless.
The term "bivalve" applies to that large, familiar group of molluscs that live within a pair of shells, which open for feeding but close for self-defense. This group includes oysters, clams, mussels and scallops, all staples of the seafood industry. Freshwater clams and mussels are widespread but rarely eaten, thanks to their relatively small size, bland flavor and high likelihood of contamination.
A number of shellfish don't fit neat categories but are also edible and sometimes highly prized. Aquatic snails, from tiny winkles to larger whelks and conch, tend to have a chewy texture but rich, sweet flavor. Spiny sea urchins are prized for their colorful and distinctively flavored sexual organs. Even some species of barnacles, those rock-clinging paragons of the sedentary life, are harvested in many countries as a food source. Spain's gooseneck barnacles are especially choice.
Don't Go There
Only a few shellfish are outright dangerous. Mediterranean sea slugs, for example, and some tropical snails contain potentially lethal neurotoxins. A more immediate threat comes from familiar, everyday shellfish such as clams, oysters and mussels. They can host bacteria that cause a number of highly unpleasant and potentially fatal conditions, ranging from typhoid fever to paralytic shellfish poisoning. Additionally, most molluscs are filter-feeders, straining nutrients -- and with them, heavy metals and other contaminants -- from the surrounding waters. Before harvesting or eating any shellfish from the wild, it's prudent to check with your local authorities to ensure that it's both safe and legal.