nauplius

Nauplius is the free-swimming larval stage of many crustaceans, most notably copepods, barnacles (cirripedes), and branchiopods. The term, derived from Greek, describes the larval form that hatches from the egg and precedes later larval stages in the respective life cycles.

Morphology: The nauplius typically has a simple, helmet-shaped body with a single median naupliar eye. It bears three pairs of appendages on the cephalic region—antennules (first pair), antennae (second pair), and mandibles (third pair)—used for swimming and feeding. In many species a protective carapace covers the anterior segments, and the body lacks fully developed thoracic limbs in this stage.

Development: In crustaceans that pass through a nauplius, this stage is the first larval instar after hatching. There may be several successive naupliar instars (often labeled N1, N2, etc.) before metamorphosis to the next larval form, such as protozoea, zoea, or copepodid, depending on the group. Some nauplii are lecithotrophic and feed little or not at all, while others are planktotrophic and feed on microalgae and detritus.

Ecology and significance: Nauplii are typically planktonic and occupy coastal and open-ocean environments, contributing to early-life stages of many crustaceans. The presence, number of naupliar instars, and morphological features of the nauplius are often diagnostic for identifying crustacean groups and understanding their larval development and evolutionary relationships.