A nauplius (plural nauplii) is the first larva of animals classified as crustaceans (subphylum of Arthropoda). It consists of a head and a telson. The thorax and abdomen, characteristic of adult crustaceans, have not developed yet. A prominent characteristic of nauplii is that they have only one compound eye, which will divide in two in later stages. Nauplii have three pairs of cephalic appendages with which they swim; in the adult these become the antennules, the antennae, and the mandibles. The name nauplii properly refers to crustacean larvae that use appendages that stem from the head (antennules and antennae) as their main means of swimming.
The naupliar stage is the defining link among crustaceans, as they all pass through this larval stage. Some are nauplii as part of their embryonic development (as is the case for the American lobster), or as the first swimming larvae that hatches out of the egg (as in the case for most common shrimp). Nauplii larvae do not feed, but utilize their internal yolk reserves from the egg for energy. Nauplii of marine crustaceans are important components of zooplankton, providing food for many marine organisms.