The dark line that runs from head to tail in a shrimp is commonly called the “sand vein.” Grocers, recipes and restaurant menus often label shrimp that have are “deveined,” which means someone has manually removed the sand vein from the shrimp. The line really isn’t a vein though. It’s the shrimp’s alimentary canal, which is its digestive tract. Identifying the dark line naturally raises more questions than it answers. Queries about canal contents, food safety and deveining have straightforward answers as well.

The canal runs along the outer curve of the shrimp, from the head to the tail. The sand vein is semi-translucent like the rest of the shrimp’s flesh. The contents of the vein are what makes it appear dark. Depending on the volume of material, the canal may be a consistent line or a series of dots and dashes. If there isn’t any material in the canal, the organ won't be visible.

The alimentary canal is the shrimp’s intestinal tract, so it contains whatever the shrimp ate before it was caught. Shrimp are bottom-feeders. They are not picky eaters. As they root around the ocean floor looking for food, they consume a substantial amount of sand, which is why the canal is colloquially called the “sand vein.”

It is safe to eat the sand vein as long as the shrimp is properly cooked. While cooking, the shrimp should reach an internal temperature of at least 145 degrees Fahrenheit. The flesh should turn white and opaque, and the surface should turn pink. Though it is safe to eat, some people feel that the canal alters the flavor of a shrimp or gives it a gritty texture, due to the presence of sand. Cooks often devein shrimp simply because the dark line is eye-catching and unappealing.

Many people only devein larger shrimp since the canals of smaller shrimp are so thin and difficult to remove. Shrimp shells are so thin that you can cut through them to remove the vein if you prefer to leave the shells intact. It is possible to devein shrimp after you cook them, but it is easier to pull the canals out beforehand. To remove a sand vein, insert the tip of a paring knife at the broad end of the outer curve. Pull it down to the tail to expose the vein. Slide the tip of the knife, a toothpick or your finger under the vein. It should come out easily in one or two pieces. It’s easier to remove the vein under cool running water so that the water can wash away debris. For a small fee, most grocers and fish mongers will devein fresh shrimp for you at your request.

A shrimp also has a “blood vein” along its inner curve parallel to the sand vein. This inner vein is the ventral nerve cord. It is thinner and lighter in color than the alimentary canal, but it looks similar. For that reason, some people choose to remove it too. It is trickier to extract, however, because there is more tissue connecting it to the shrimp’s body. Like the sand vein, the blood vein is safe to eat in properly cooked shrimp.