EARLY POST-EMBRYONIC DEVELOPMENT OF MARINE CHELICERATES AND CRUSTACEANS WITH A NAUPLIUS

Crustaceans that hatch as a nauplius-like larva, as well as xiphosuran and pantopodan chelicerates, are surveyed for five characters: presence or absence of arthrodial membranes separating somites; ventral configuration of the protopod of the second limb; number of transformed (segmented) limbs and limb buds; addition of segments to transformed limbs; fate of limb buds. An arthrodial membrane separates somites 7 and 8 of xiphosurans, a small knob articulates on the protopod of the second limb, and there are nine pairs of limbs but no limb buds. During early development, no arthrodial membranes are added, nor are segments added to limbs 1-9; limbs 10-14 are added as transformed limbs, not as limb buds, after several molts. On the post-embryonic larva of the presumed ancestral pantopodan, arthrodial membranes did not separate adjacent somites, the proximal segment of limb 2 was simple, there were three transformed limbs and no limbs buds. During subsequent molts, arthrodial membranes separated somites 4-7, buds of limbs 4-7 were added in register with each molt, limb buds were reorganized in register into transformed limbs during the following molt, and two segments were added to each transformed limb in register during the next molt. Somites of most crustacean taxa that hatch as a nauplius-like larva are not separated by arthrodial membranes on early post-embryonic stages; exceptions are posterior somites of branchiurans, mystacocaridans and cephalocaridans. Limb 2 (antenna 2) of branchiopods, copepods, thecostracans, mystacocaridans and cephalocaridans bears a naupliar arthrite on the ventral face of the coxa, on branchiurans there is an attenuation, or spine-like outgrowth, on the ventral face, and on ostracodes, remipedes, euphausiaceans, mysidaceans and dendrobranchiate decapods there is no ventral structure. Most of these crustaceans hatch with three transformed limbs; exceptions include myodocopan ostracodes with six and branchiurans with seven. The number of limb buds varies from none on remipedes to three on branchiurans and cephalocaridans. Limb buds of podocopan ostracodes, branchiurans, mystacocaridans and cephalocaridans are reorganized into transformed limbs after one molt; reorganization of limb buds of branchiopods, copepods, thecostracans, remipedes, euphausiaceans, mysidaceans and dendrobranchiates are delayed several molts. Early post-embryonic development of pantopodan chelicerates is more similar to that of podocopan ostracodes than it is to that of any other crustacean taxon, and no crustacean exhibits an early post-embryonic development similar to that of xiphosurans. Within the crustacean lineage, articulating somites, a naupliar arthrite or ventral attenuation on the coxa of antenna 2, and delayed reorganization of limb buds are apomorphies. Absence of a naupliar arthrite on ostracodes is inherited from the last stem lineage arthropod but its absence on remipedes and malacostracans is derived, and perhaps more than once.