Egg activation in the black tiger shrimp Penaeus monodon

Abstract

This report describes morphological changes in the eggs in the black tiger shrimp Penaeus monodon upon contact with seawater, the process known as egg activation. Eggs from wild P. monodon broodstock were collected at 15-s intervals post-spawning during the first 15 min, and at 15-min intervals thereafter for 2 h. The samples were fixed and processed for light, scanning and transmission electron microscopy. As soon as the egg was released into seawater, the cortical rods began to emerge from the crypts on the periphery of the egg, and elevated the thin investment coat that covered the surface of the egg. Sperm in the first phase of the acrosome reaction were observed on both the egg and the surface of the investment coat. The rods protruded from the surface and were completely expelled out within 45 s. I0mmediately after complete extrusion, the cortical rods began to break up and formed the jelly layer around the egg. By this time, the interaction between the sperm at the second phase of the acrosome reaction and egg began. The hatching envelope had started formation at 1-min post-spawning, and was completed within 13–15-min post-spawning. The first and second polar bodies extruded from the egg at 3–5- and 10–15-min post-spawning, respectively. It was apparent that after the hatching envelop had formed, additional sperm could not enter the egg. This study suggests that the critical period for the egg–sperm interaction in P. monodon is within 45-s post-spawning.