The hatching mechanism in Atlantic halibut (Hippoglossus hippoglossus)

In general fish larvae emerge from the protective egg after secreting a hatching enzyme (HE) from diffusely located hatching gland cells (HGCs). This proteolytic enzyme is distributed over the entire inner part of the eggshell (zona radiata). In a marine flatfish halibut, (Hippoglossus hippoglossus), we have found a more specialized hatching process. A strategic location of the HGCs in a narrow belt on the anterior part of the yolk sac leads to restricted degradation of the eggshell resulting in cleavage of the eggshell into two distinct rigid parts. This hatching process--termed "rim-hatching"--results in an empty eggshell with a lid approximately 1/4 the size of the bottom shell. During the hatching process the yolk sac is reshaped. The posterior part of the yolk sac contracts and the yolk mass is squeezed forward before hatching. This mechanism ensures close contact between the HGCs and the eggshell during the release of the hatching enzyme, which is a prerequisite for restricted degradation of the eggshell.