Segmentation in the crustacean Artemia: engrailed staining studied with an antibody raised against the Artemia protein.

We have studied the process of post-embryonic segmentation in the anostracan crustacean Artemia franciscana using a specific antibody raised against the engrailed protein of this organism. Three cephalic segments are specified during embryonic development, before larval hatching, whilst trunk (thoracic) segmentation begins after the first stage free-swimming nauplius larva has emerged from the dormant cyst. Thus, cephalic and trunk segmentation seem to be at least in part independent and superimposed processes. Trunk stripes of engrailed expression are added one at a time as segments are generated from the posterior growth zone. The first detectable decision in engrailed expression is the establishment of a line of engrailed-expressing cells, interpretable as delineating the parasegmental boundary. The subsequent widening of engrailed stripes is not correlated with cell lineage events but is probably mediated by the combination of inheritance of the active state and recruitment of new cells into the engrailed-expressing stripe.