J.R. Rosowski, D. Belk, M.A. Gouthro, K.W. Lee-1997
Journal of Shellfish Research, 16 (1): 233-249
Abstract:
Cyst components and their products (shell, cuticles, membranes, embryo, prenauplius, nauplius 1) were examined with electron microscopy and identified in all stage of postencystic development through to the emergence of the prenauplius and the hatching of the nauplius 1. At prenauplius emergence, the shell (tertiary envelope) cracks open in a straight, smooth, 180 degrees arc, 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 cyst uptake of water and initially adheres to the inner cuticular membrane (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 three methods of emergence but only one of hatching, as follows. From its shell, (1) the prenauplius emerges in a tapered EC2 bag, caudally attached to (or free of) the ICM, and then escapes from the EC2 as a swimming 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 cannot 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 differentiates within, but fails to escape and dies). During postencystic development and before the formation of the EC2, the embryo first secretes a fine, granular, extracellular matrix next to the ICM, and then between folds of the expanding cellular surface of the developing embryo. Next, the embryo forms the EC2, and the extracellular matrix is now between the ICM and the EC2, becoming exposed to the hatching medium only when the ICM breaks. During late embryogenesis, the EC2 forms over differentiating paired anterior appendages (and elsewhere), extending its surface area beyond that still adhered to the ICM. Finally, before hatching occurs, the EC2 inflates during prenauplius emergence, peeling off the surface on which it was formed; soon, the EC3 exoskeleton becomes continuous on its surface and mature, thus completing differentiation, and the nauplius 1 escapes the EC2 and becomes free swimming.
(School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, Nebraska, 685588-0118, USA)
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