Nauplii, Antennular Ontogeny, and the Position of the Ascothoracida within the Maxillopoda

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Abstract
The external anatomy of ove 25 species of ascothoracid nauplii, and the later ontogeny of their antennules, are reviewed and compared to other maxillopodan subclasses. The brooded nauplii are of the nonbranchiopod type. Although lacking frontolateral horns, generalized ascothoracid nauplii (Lauridae, Petrarcidae, Synagoga) are most similar to early instar cirriped nauplii, but many of the features in common are probably symplesiomorphies. Ascothoracids and cirripeds may have a common basic naupliar ontogeny, but features of later cirriped instars, many of which are associated with feeding, are delayed or entirely suppressed in ascothoracids due to brooding. A planktonic metanauplius from the South China Sea that combines ascothoracid features with late cirriped naupliar limbs is tentatively identified as a postbrooding ascothoracid, perhaps of the Lauridae. Lack of postmaxillular limb buds in metanauplii is a possible synapomorphy of the Ascothoracida, Cirripedia, and Facetotecta (Hansen''s y-larvae). The 6 apparent segments in a generalized ascothoracid naupliar antennule are not the same as the 6 in the adults. The 3 distal segments fuse (the adult''s movable claw does not represent a segment) and 2 new segments appear basally, so that there are 8 segments altogether in ontogeny. Eight-segmented antennules are proposed as a new diagnostic feature of the Maxillopoda. The positional homology of the ascothoracid claw and the cirriped cyprid attachment disc is shown by following supposedly homologous setae in the nauplii through later development. The antennular claws of cypris y and of branchiurans may also be of similar origin. These findings, together with other published data, tend to contradict a spermatology-based phylogeny of the Maxillopoda, but do not wholly resolve the Ascothoracida-Cirripedia relationship.

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