Oysters are an interesting part of the Mollusc tribe. They are bivalves, meaning that they have two half shells, which are jointed together on one edge and can open and close as the animal desires, or more properly, is instinctively demanded to do.
Unlike their cousins, clams, oysters are from infancy pretty much fastened onto some sort of support, so they do not move. Clams are sort of solitary, and like to dig into sandy beaches. Another relative, the scallop, is so free to move that jet propulsion is the norm for them!
Let us examine some of the natural history of these interesting (and often delicious) animals. We will point out that edible oysters are quite different from the pearl oysters.
Oysters are of worldwide distribution for the most part. They are filter feeders, meaning that they continuously take in water (and algae and other food materials), filter out the nutrients (and in many cases reject things that are not nutrients), then return the filtered water to the sea. This feeding habit has some drawbacks, especially to we humans eating them, because sometimes they filter pathogens and become contaminated. However, they are important from an ecological standpoint because they filter out lots of sediment.
We shall confine our discussion to oysters native to and farmed in the United States except for a brief mention of pearl oysters. In the US, three species are used: the "Atlantic", also called the "bluepoint" or the Eastern one, Crassostrea virginica (formerly Ostria virginica) found on the Atlantic and Gulf of Mexico coasts (2 to 4 inches diameter), the Olympic oyster, Ostrea lurida of the northern Pacific coast (1 to 1.5 inches diameter), and the Japanese or Pacific oyster, Crassostrea gigas (formerly Ostrea gigas), around 6 inches diameter. The first two are native species, and the last one is from Japan, farmed in the Pacific northwest region.
Oysters are interesting critters. Like most bivalve molluscs, in the larval stage they have two adductor muscles (the muscles that keep the shell closed), but as adults only the posterior one remains. That is why you see two in muscles, but only one in oysters. Those muscles are strong! If you have ever shucked raw oysters, you know how much effort is required to open them. Some old studies show that it takes 17 days for the adductor muscle to fatigue and the shell to open with a 1 kg force on it, about 2.2 pounds! It takes a little less than an hour with 10 kg, 22 pounds, so when you are shucking oysters you have to use a LOT of force. When you shuck oysters, you really are tearing the muscle, and that requires betwixt 12 to 15 kg (26 to 33 pounds of force.
Most folks think that oysters go off very rapidly, but that is not the case, if they are kept alive. Atlantic oysters have been known to stay alive, and hence good, for four MONTHS, out of water, at 34 degrees F. However, the fresher the better as far as flavor goes. The bottom line is that they are good if they are alive, but better fresh. The way that they stay alive is interesting.
Since they have no active defensive mechanism, the way that they survive attack is to seal their two shell halves together. They do such a good job of it that the seal is air and water tight. Little energy is required to keep the shell closed if there is no force trying to open it, so they need no oxygen for a long time. A big Atlantic oyster uses around 2 to 6 mL of oxygen per hour in warm water, but almost none at all just above freezing. They stop syphoning at about 43 degrees F, but in the water may stay open at those temperatures. You have to handle oysters gently to keep them good for very long, because bumping them around may cause them to open, even if quite cold. Then the essential liquid (that contains oxygen) leaks out, and, having no way to replace the liquid, the oyster dies quickly. So, keep them cold and do not handle them until you are ready to open and eat them, either raw or cooked.
Fresh, shucked oysters are good for only a very few days, and have to be kept extremely cold, just above freezing, of they will go off rapidly. The problem with preshucked ones is that the entire container goes off at the same time, where with live ones in the shell, only the occasional one that opens is bad. Oysters are still best near the seacoast, but with rapid transportation at low temperatures they are still good far inland, but rather expensive because of transportation costs.
In the United States, oysters are both wild caught and farmed. Wild caught ones are harvested by dragging the bottom of the oyster bed with a rake, sometimes by hand if shallow, but more often dredged with stout steel teeth and then brought up to the ship. Farmed ones may be harvested by dredging, but sometimes are also put in porous bags which hang above the seafloor. Since they are filter feeders, their food comes to them, so they do not have to be attached to the bottom. After a few years, when they are big enough to harvest, the bag is simply hoisted and the oysters removed. This is more expensive, but losses from predators are less. Starfish love oysters! They open them by attaching their sucker like "feet" onto the shell and exerting a force until the adductor fatigues and the oyster opens. Then the starfish pushes its stomach around the oyster, having them on the half shell!
Oysters are sexual perverts. Well, not really, but their reproduction is sort of interesting. For the first year or so they are males, spawning by releasing sperm into the water. As they get larger, they cross the gender line and release eggs as females. Talk about versatility! This brings up an interesting aspect to oyster farming, using polyploid strains, artificially created, that can not reproduce. They release sperm and eggs, but they are sterile and thus invasive growth can not occur where alien species are farmed.
There are lots of diseases in oysters, and they have to be controlled to keep the beds viable. Unfortunately, it is hard to treat the oceans with antibiotics, so other control measures are used. The oldest is to dig up infected beds and destroy the oysters, but prevention is better. Modern methods that work well is to breed strains of oysters that are naturally resistant to whatever diseases are the most troublesome in a given area.
Pollution is another problem with oyster beds. Before the environmental laws got teeth in the 1970s and later, pollution was a real problem here. It still is a factor today, but not nearly as bad as it was before. This is in the United States. China, which produces around 80% of the world supply of oysters, is not nearly so careful. Fortunately, most Chinese oysters eaten in the United States are cooked, which eliminates any danger from microbial contamination. However, filter feeders also can concentrate dangerous chemical pollutants that are not eliminated by cooking. Since oysters are not very high in fat, chlorinated organics like PCBs and TCDD are not highly retained, but that is small solace because of the extreme toxicity of these materials. Water soluble pollutants are not highly concentrated because of the large amount of water passing through the animal, so the concentration of water soluble pollutants is not much higher in the oyster than in the seawater.
Raw oysters can carry bacteria that are dangerous to humans, particularly a couple of species of the Vibrio, only discovered in 1976. This is a very serious infection, particularly in persons with less than perfect immune systems. The very young, the very old, and people otherwise immunocomprimised should not eat raw oysters, in my opinion, but I love them! Oysters also can concentrate toxins from algae, but his is not very common. Oysters from US waters are generally safe unless the immune system is compromised for the most part. The states that have oyster beds do a good job of monitoring the waters and the oysters.
The old saying that oysters are only safe to eat in months with the letter "r" in the name is just an old saying. Wikipedia says that this saying comes from the fact that oysters spoil quickly in warm months, but that is nonsense. If kept on ice, it does not matter what the temperature is. The real reason is that in the warmer months the oysters are busy spawning, and their energy and body mass is dedicated to that. Summer oysters tend to be more watery and less flavorful than ones harvested during colder months.
Speaking of flavor, why do oysters (and other molluscs) have that unique flavor? The answer is glycogen, the so-called animal starch. This is also the way that people store sugar reserves short of converting it to fat. Glycogen has a slightly sweet flavor. By the way, summer oysters have very little glycogen. Oysters also use glutamic acid to balance their salt content. Hmmm, glutamic acid and sodium ions, what does that produce? MSG! They also have unusually high concentrations of a couple of other amino acids that contribute to their flavor. The saltier the water, the more flavor that oysters have. Because of the huge inflow from the Mississippi River, the best oysters are caught at some distance from the mouth. In New Orleans the best oysters are called salty oysters.
Of all the molluscs, oysters are by far the most delicate and most tender. If cooked, they need to be cooked extremely carefully. Lots of people do not like oysters because they have only had them overcooked. Except for oyster stew, the less cooking the better. I love fried oysters, but unless I cook them myself of go to a place that really knows what it is doing, they are tough, have a bad smell and flavor, and are essentially not fit to eat. The secret of perfect fried oysters is very hot fat, a good breading, and cooking only long enough to brown the breading. In addition, they should be cooked only a few at a time if the cook is going to eat any, because on standing they get overdone and then cold. The breading actually insulates the delicate oyster from some of the heat from the oil, sparing the inside from overcooking.
By far my favorite way of eating oysters is freshly shucked, on the half shell. Lemon wedges should be handy, and some New Orleans style hot sauce. By the way, folks in New Orleans do not use much Tobasco, preferring the Crystal brand. If you can find Crystal Extra Hot, you have it made! Take the oyster, place it on a plain saltine cracker, and add some hot sauce and lemon juice. MMMMMMMM! I prefer mine so minimally processed, but you can actually mix the lemon juice and hot sauce, and soak the oysters in the mixture in it for some time, keeping it cold.
The acids in the sauce and juice acutally begin to coagulate proteins in the delicate oysters, "cooking" them without heat. The delicate flavor is not damaged, but the texture gets increasingly firm as they soak in the mixture. Studies done at LSU indicate that this treatment actually kills any pathogens in the oysters if they are left in the mixture long enough. I just prefer mine extremely fresh.
Let's hear it for oysters, one of the world's greatest foods!
Pearl oysters are not really edible, belonging to a different genus, Pinctada, form true pearls. Edible oysters sometimes form a concretion that is hard, but never of gem quality. Perhaps next week we can talk about them more.
Well, you have done it again! You have wasted many more einsteins of perfectly good photons reading this fishy piece. And even though Liz Trotta tries not to act like the harpie that she actually is when she reads me say it, I always learn much more from writing this series than I could possibly hope to teach, so keep those comments, questions, corrections, and other feedback coming. I shall remain here for comments as long as they are coming, and shall return tomorrow at around 9:00 PM Eastern for Review Time.
Featured at TheStarshollowgazette.com. Crossposted at Docudharma.com and at Anetmedius.com