The nauplius is the most primitive crustacean larval type found in existing crustaceans. It is the first larval stage that emerges from the egg and the earliest free-swimming phase in crustacean development. Generally, it has an oval shaped body, three pairs of appendages, and a single median eye near the front. In terms of sheer numbers, the crustacean nauplius is considered by some to be the most abundant type of multi-cellular animal on earth and is an important food source for fish and predatory invertebrates.

It is widely distributed larval form amongst crustacean species, making the nauplius extremely useful as a key feature that unites the entire subphylum Crustacea. However, many crustaceans have lost this larval stage, or pass through it prior to emerging from the egg.