All headings are times after spawning. The timing of events described here and in the following pages are typical of those found at a water temperature of 30^oC.

Figure 2.1a, b Time 0 - 30 minutes

The attachment of sperm to the egg takes place within the first minute of spawning. Sperm are composed of a central spherical body and a short non-flagellated spike. Attachment to the egg’s surface is via the spike end of the sperm. By the second minute post-spawning the egg becomes spherical and a clear membrane is observed beginning to envelop the egg. The eggs are approximately 250 – 330 µm in diameter. By the third minute the sperm have undergone the acrosome reaction, where the sperm fuses with the egg membrane and the male pronucleus enters the egg’s cytoplasm. If the egg has not come into contact with a sperm at the time of spawning, or the fusion of the pronuclei from the sperm and egg is unsuccessful, those eggs will be unable to hatch and so hatching rates will be below 100%.

The first and second meiotic cell divisions occur at approximately 2-5 and 8-14 minutes after spawning, respectively. These divisions result in the production of minute cells called polar bodies. Although these can be seen under the microscope the egg appears to remain as a single large 250 – 300 µm diameter cell. The extrusion of the jelly-like substance from the cortical crypts of the egg is completed within the first 8 minutes (Fig. 2.1a). The soft jelly hardens into a tough transparent shell, the hatching envelope, by 12 – 15 minutes post-spawning and probably acts as a shield against microbial attack during the 12-14 hours of embryo development (Fig. 2.1b). During egg incubation numerous ciliates can be observed under the microscopic which appear to be attracted to the egg and may be searching for perforations in the shell membrane where they can gain entry into the egg and feed on the nutrient rich yolk. By electron microscopy microbial organisms can be observed adhering to the shell surface. The fertilised egg, called a zygote, undergoes its first cell division to clearly defined two-cell stage approximately 30-60 minutes after spawning. However, some eggs remain undivided for at least 60 minutes. By 90 minutes all of the eggs which have been fertilised are into at least the first cell division.