ULTRASTRUCTURE OF THE CYST SHELL AND UNDERLYING MEMBRANES OF THE BRINE SHRIMP ARTEMIA-FRANCISCANA KELLOGG (ANOSTRACA) DURING POSTENCYSTIC DEVELOPMENT, EMERGENCE, AND HATCHING

Cyst components and their products (shell, cuticles, membranes, embryo , prenauplius, nauplius 1) were examined with electron microscopy and identified in all stages of postencystic development through to the em ergence of the prenauplius and the hatching of the nauplius 1. At pren auplius emergence, the shelf (tertiary envelope) cracks open in a stra ight, smooth, 180 degrees are, while the first embryonic cuticle (EC1) separates in a jagged fashion, along a fracture seam between distinct polygonal plates. These plates differentiate within fibrous lamellae lying in concentric spheres between the outer and the inner cuticular membranes of the EC1. The second embryonic cuticle (EC2) forms after c yst uptake of water and initially adheres to the inner cuticular membr ane (ICM) of the EC1. If hatching is interpreted as when the nauplius 1 is free from the EC1 and EC2 and able to swim, then we identify thre e methods of emergence but only one of hatching, as follows. From its shell, (1) the prenauplius emerges in a tapered EC2 bag, caudally atta ched to (Or free of) the ICM, and then escapes from the EC2 as a swimm ing nauplius 1 (the only method of hatching); (2) the nauplius emerges halfway from the shell but without the ICM or EC2 over its head (it c annot escape from the shell and dies); or (3) the prenauplius emerges in an oval bag composed of the ICM and the EC2 (and the nauplius 1 dif ferentiates within. but fails to escape and dies). During postencystic development and before the formation of the EC2, the embryo first sec retes a fine, granular, extracellular matrix next to the ICM, and then between folds of the expanding cellular surface of the developing emb ryo. Next, the embryo forms the EC2, and the extracellular matrix is n ow between the ICM and the EC2, becoming exposed to the hatching mediu m only when the ICM breaks. During late embryogenesis, the EC2 forms o ver differentiating paired anterior appendages (and elsewhere), extend ing its surface area beyond that still adhered to the ICM. Finally, be fore hatching occurs, the EC2 inflates during prenauplius emergence, p eeling off the surface on which it was formed; soon, the EC3 exoskelet on becomes continuous on its surface and mature, thus completing diffe rentiation, and the nauplius 1 escapes the EC2 and becomes free swimmi ng.