Metanauplius is an early larval stage of some crustacean

Crustacean

Crustaceans 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 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s such as krill

Krill

Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...

. It follows the nauplius stage.

In sac-spawning krill, there is an intermediary phase called pseudometanauplius, a newly hatched form distinguished from older metanauplii by its extremely short abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

. In some species, this form is not considered a separate developmental stage as it develops into a metanauplius without molting

Ecdysis

Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

; in other species such as Nyctiphanes couchii, it can be separated from the metanauplius stage by the molt of a very thin cuticle soon after hatching.

Broadcast-spawning species of krill do not have a pseudometanauplius stage. They generally hatch in the nauplius 1 stage, but recently have been discovered to hatch sometimes as metanauplius or even as calyptopis stages.