We've been making our way around the sushi bar here at slashfood, and preparation for Valentine's Day simply begs for a study in bivalves - clams, scallops, and those ever-so-aphrodisiac-al oysters.

"Gai" is a general Japanese term that refer to most of the clams. I very rarely see the kind of smaller clams that we put into chowders or batter and fry on a sushi menu. However, mirugai and hokkigai are regular offerings. Oysters are called "kaki."

• Mirugai - is a giant clam, often called "geoduck," and the part that is served as sushi is the enormous siphon that extends out of the clam. If you've ever seen the light brown, sometimes tinged with gray, mirugai sushi, it usually looks like a cross-section of an animal's snout (at least to me it does). The flesh is very chewy, sometimes rubbery, which may turn some people off, but those who eat it really love it.
• Hokkigai - is sometimes called "surf clam," sometimes called "red clam" on the menus. The clam is half red, half white. The flesh is chewy, as most of the fish in this family, but far less chewy than mirugai.
• Hotategai - are scallops. We are all familiar with the enormous scallops that are the diameter of a silver dollar. When raw, scallops are sweet and tender. Smaller scallops are often served in hand rolls, mixed with a creamy sauce.
• Kaki - these are oysters. I don't usually see oysters served as sushi, nigiri or otherwise. They are served just as they served raw anywhere else - on the half shell, though in a sushi bar, with a Japanese-flavored sauce. You can always take a raw oyster as a "shooter" - in a shot glass, with sake.

Previous lessons at the School of Fish:
Saba - Holy mackerel!
Hirame - white fish, sometimes halibut
Tai and Suzuki - snapper and seabass
Maguro - tuna
Toro - fatty belly of tuna
Shiro Maguro - "white" tuna, aka albacore
Ahi tuna
Hamachi - amberjack, aka yellowtail
Sake - salmon