EMBRYONIC COAT OF THE GRASS SHRIMP PALAEMONETES PUGIO

The embryo of the grass shrimp, Palaemonetes pugio, is surrounded duri ng development by a protective extracellular coat designated as the em bryonic coat (EC). At hatching, this EC is composed of four embryonic envelopes (EE), each of which is composed of multiple layers. The oute rmost layer of the EC, the outer investment coat (OIC), is derived pri marily, if not completely, from pleopods of the female. The first enve lope (EE1) forms as a bilayered envelope, EE1(a) and EE1(b), immediate ly 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 af ter oviposition. Three more embryonic envelopes, EE2, EE3, and EE4, ar e formed between the embryo and EE1 by 7 d after oviposition. Formatio n of embryonic envelopes continues until 10 d after oviposition; by th is time each envelope is morphologically distinct in composition, with ''outer'' and ''inner'' sides clearly identifiable. All but the inner most 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 increa se until hatching. Fluorescently labeled lectins react positively with the OIC, indicating the presence of glucose and N-acetylglucosamine r esidues. Thus, the palaemonid EC is a dynamic structure throughout emb ryonic development.