Sorting out the different varieties of orange can be challenging; mandarins and clementines make it even more difficult because of how closely they resemble each other. The resemblance is no coincidence -- the two are closely related. However, despite their many similarities, these fruits have some important differences.
Mandarin: Ancestral Orange
"Mandarin" is a catch-all term for many subspecies of Citrus reticulata. This sweet fruit is one of the four citrus species from which all others are descended. There are many varieties of mandarin, but most are comparatively small, with sweet flesh and juice. Each variety has its own flavor, but they are typically very sweet, without the acidity of an orange. One distinctive feature is that mandarins have a very loose peel that is easy to split and remove. They are sometimes called "mandarin oranges," or "loose-peel oranges," but this is a misnomer; oranges are descended from mandarins.
Clementine: Hardy Hybrid
Small, sweet and with a loose peel, the clementine appears very similar to the mandarin; the two are often confused and may even be sold together. In fact, the clementine is a hybrid of a mandarin and another species, probably an orange. Unlike most mandarins, clementines are seedless.
Clementines originated in North Africa but now grow around the world. Because they come into season in mid-November, they are a traditional Christmas treat in many parts of the world. Clementines are typically sweet, but, unlike mandarins they often have a slight bite of citrus acidity.
Satsuma: Seedless Mandarin
Citrus unshiu or the satsuma mandarin -- generally simply called a satsuma -- may be the type of mandarin easiest to mistake for a clementine. Like clementines, these small mandarins are seedless. The combination of sweet flavor, lack of seeds and loose peel makes satsumas a popular snack.
Like clementines, they are also resistant to cold, making them common during winter months. Although eaten around the world, satsumas are particularly popular in Japan -- the first examples in the U.S. were imported from Japan in the 19th century.
Tangerines and Confusion
Many of the first mandarins in North America came from Tangiers in Morocco. The name "tangerine" originates from this history; today, "tangerine" usually refers to any mandarin with a dark reddish-orange peel. Despite the difference in appearance, these fruit are actually the same species; the terms are interchangeable, and are often used inconsistently. Some mandarin hybrids have names derived from "tangerine." For instance, a tangelo is a hybrid of a mandarin and a grapefruit, while a tangor is a hybrid of a mandarin and an orange.