What is the difference between tangerines, mandarins, clementines, and regular oranges? Like apples, they share many similarities but also some distinct differences.
Tangerine (Citrus tangerina) - In the 1800's this fruit was exported from the port of Tangier (which is in present day Morocco) and brought to European countries. Its namesake, Tangerine, stemmed from the name of the seaport it came from. Tangerines tend to be more sweet than the typical orange, but more sour when compared to the next on our list, the Mandarin.
Mandarin (Citrus reticulata) - As you can guess from the name, their origin is rooted in China. When compared to tangerines and regular oranges, the mandarin orange is sweeter and less sour. Its shape is also not perfectly round, more like an oval, which is one way to instantly identify them at the grocery store before even seeing their label. If you want to know what is the sweetest orange out of them all, it would be this.
Clementine (Citrus clementina) - This is a hybrid between a sweet orange and deliciosa which is a variety of Mediterranean citrus fruit very similar to a Mandarin. Having only been around for about a century, it is especially popular in California's agriculatural production. Being seedless, they are sterile and cannot reproduce on their own - farmers must graft the shoots onto other varietals in order to create new trees. The fact that they are seedless and small is what makes them a popular snacking food, as they're easy to peel and you don't have to hassle with removing seeds. Their flavor can be described as tangy, tart, and a bit less sweetness versus a Mandarin.
Orange (Citrus x sinensis) - What we refer to as a common orange covers a number of types including the common sweet orange, navel oranges, and blood oranges, along with other lesser-known varieties. It has been found that it shares about 75% of its genes with the mandarin and the other 25% with polemo (Citrus maxima) which looks like a big grapefruit and in its natural (non-hybrid form) is rarely cultivated for consumption (1).
As far as antioxidant content, the USDA testing did not differentiate between tangerines versus mandarin oranges, presumably because the difference in their ORAC is not statistically significant enough to categorize separately. They did however do separate testing for normal oranges and navel oranges which calculated out at 2,103 and 1,819, respectively. Therefore it appears that the healthiest and most nutritious - at least in terms how how much antioxidants they have - would be the common orange which has around 16% more than navel and 30% more than tangerines and mandarins (when all are compared on an equal weight basis).
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010