Structure of the egg envelope of the haddock, Melanogrammus aeglefinus, and effects of microbial colonization during incubation

Abstract

Haddock (Melanogrammus aeglefinus) eggs from an experimental culture were examined by electron and light microscopy to determine the structure of the egg envelope as preserved by various fixatives and to observe the effects on the envelope of microbial colonization during incubation. The egg envelope consisted of a featureless thin surface coating that readily dissociated into globules and was lost under standard fixation; a thin homogeneous surface layer penetrated by pores; a well-defined intact outer lamella 0.5-1.0 µm thick immediately beneath this; then about three more vaguely defined lamellae of similar thickness beneath which were several broader fibrillar layers. The total envelope thickness was 6.5-8.5 µm. Different fixatives primarily affected the appearance of the thin surface layer, which was well-preserved only by freezing or by fixation in glutaraldehyde with Alcian blue. Egg surfaces were colonized by microorganisms, regardless of early disinfection by various agents, and eggs at hatching were densely populated by bacteria, notably the filamentous genus Leucothrix. However, bacteria did not penetrate the egg envelope beyond the outer layers, where their presence was associated with surface erosion. A dense epiflora did not necessarily prevent normal development of the embryo, although very heavy infestations, as seen on eggs that failed to hatch, may cause suffocation. The envelope structure is compared with that of another gadoid which has been extensively studied and cultured, the Atlantic cod (Gadus morhua).