Embryonic Coat of the Grass Shrimp Palaemonetes pugio

The embryo of the grass shrimp, Palaemonetes pugio, is surrounded during development by a protective extracellular coat designated as the embryonic coat (EC). At hatching, this EC is composed of four embryonic envelopes (EE), each of which is composed of multiple layers. The outermost layer of the EC, the outer investment coat (OIC), is derived primarily, if not completely, from pleopods of the female. The first envelope (EE1) forms as a bilayered envelope, EE1 a and EE1 b, immediately after oviposition. The OIC becomes closely associated with EE1 and remains in close contact with EE1 until hatching occurs. An additional layer, EE1 c, is added to the inner side of EE1 between 3 and 5 d after oviposition. Three more embryonic envelopes, EE2, EE3, and EE4, are formed between the embryo and EE1 by 7 d after oviposition. Formation of embryonic envelopes continues until 10 d after oviposition; by this time each envelope is morphologically distinct in composition, with "outer" and "inner" sides clearly identifiable. All but the innermost embryonic envelope (EE4) are shed by the embryo about 6 h before hatching. Permeability of the EC during the 12-d incubation period is found to decrease between 0 and 5 d after oviposition, and then increase until hatching. Fluorescently labeled lectins react positively with the OIC, indicating the presence of glucose and N-acetylglucosamine residues. Thus, the palaemonid EC is a dynamic structure throughout embryonic development.