Brachyura larvae
Recognition of the main decapod larval forms

According to Williamson (1982) and Gore (1985), the first larvae produced by decapods usually consist of a free-swimming or motile planktonic form that hatches from the egg. These larvae hatch as different types, ranging from morphologically relatively simple forms termed nauplii, among decapods found in dendrobranchiate shrimps, to more complex forms found in pleocyematids, termed prezoea, zoea, naupliosoma or phyllosoma. Subsequent stages are either a megalopa, glaucothöe, puerulus or decapodid (Table 2 Number of larval stages). The collective term decapodid is used here as by Williamson (1982) and Felder et al. (1985). Note: the key to recognize zoeal larvae of major decapod groups can be found on page Bl.10 Key to recognize zoeal larvae of major decapod groups.

The distinction between each larval phase is mainly based on the appendages used for locomotion. The phases are (Table 2 Number of larval stagesand Table 3 Larval phases):

—Nauplius (several stages)
It is characterized by a single small median eye and propulsion is by means of three pairs of head appendages (antennae and mandibles). This type of larva belongs to members of the Dendrobranchiata.

—Protozoea (three stages)
This larva has a pair of compound eyes, an antenna that is segmented to the base, locomotion that is shared between the second antennae and exopods of the first two maxillipeds, and a telson with two blunt cylindrical rami. This kind of larva belongs to representatives of the Dendrobranchiata (Penaeoidea, Sergestoidea).

—Prezoea (one stage): It is the last embryonic stage, which precedes the first stage zoea; its free life is usually only a matter of minutes before molting again. This stage is totally enveloped by a cuticle and, unlike for other phases, the appendages do not have setae. Movement is limited to abdominal flexion. This kind of larva can appear in Brachyuran development.

—Zoea (several stages, see Table 3 Larval phases)
It is similar to the protozoea, from which it differs by the means of locomotion. In early zoeae, propulsion is by means of exopods of maxillipeds and in some older zoeae also by exopods of pereopods. The pleopods can be present but are not natatory. Zoeae are found as older larvae of dendrobranchiates or as early larvae of pleocyemates (Caridea, Thalassinidea, Stenopodidea, Brachyura, Portunidae, Astacura, Nephropoidea, Anomura, Diogenidae, Anomura, Hippidae, Anomura, Paguridae, Anomura, Porcellanidae).

—Mysis (several stages)
With setose swimming exopods on the maxillipeds and pereopods; pleopods are present starting with the second stage but lack setae and are not yet functional. Locomotion is by exopods of the thoracopods. This larval form can be found among penaeideans after the zoeal phase.

—Phyllosoma (several stages)
The body is dorsoventrally flattened, anteriorly pear-shaped, and wider than long or as long as wide; third maxillipeds with or without exopod, abdomen one tenth to one fourth of total length, and locomotion is by cephalothoracic maxillipeds with well developed and numerous natatory setae on the exopods. These larvae are found among scyllarid (Palinura, Scyllaridae) and palinurid lobsters.

—"Eryoneicus" (several stages)
Body not extremely compressed; eyes rarely covered by carapace; at least two pairs of functional thoracopods; antennal exopod unsegmented or segmented at tip only; abdomen one third of total length (excluding rostrum) (Nephropidae and Polychelidae: Palinura, Polychelidae). There is some debate if this larva is equivalent to a zoeal or post-larval phase (Gurney, 1942; Williamson, 1982).

—Decapodid (usually one stage, see Table 3 Larval phases)
This is the transitional stage between the zoeal and juvenile growth phase. The locomotory function has shifted to setose pleopods for swimming but pereopods for crawling are also present.

—Megalopa (Table 3 Larval phases))
This term is usually used for brachyuran (Megalopa dorsal view) and some anomuran decapodids. It looks like a small crab with an extended abdomen bearing ventral pairs of pulsating setose pleopods while swimming. The pereopods are also well developed, the first pair being chelipeds.

—Glaucothöe
This usually refers to an anomuran pagurid decapodid. The first pereopod is chelated; the 5th pereopod and, sometimes also the 4th, are distinctly smaller than the other pereopods.

—Puerulus
This is usually used for a palinuroidean decapodid. It looks like a small spiny lobster.

—Penaeoidean decapodid
The most characteristic feature is that swimming shifts during this stage from the thorax to the abdomen.