甲壳亚门是由非常大的一组的节肢动物门形成的，通常被当作是一个亚门，包括常见的物种，例如螃蟹，虾，龙虾，淡水龙虾，磷虾，和藤壶等等。这些物种通过对非常不同的环境和方式的适应而极其相异。有人将它们称为是“水中的昆虫”。

其中有67,000个已经被描述物种，大小尺寸范围从0.1 mm（0.004寸）的Stygotantulus stocki，到具有了一个腿长跨度达到3.8米（12.5尺）和重量达到20公斤（44磅）的甘氏巨螯蟹。

Crustaceans (Crustacea /krʌˈsteɪʃə/) form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles.

The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.