In vivo investigation of oocyte transit and maturation in a broadcast‐spawning holothurian

A sequential in vivo approach was used to examine the transformations undergone by oocytes during transit in the gonoduct of the sea cucumber Holothuria leucospilota, from ovulation until fertilization competency. Spasms of the ovarian muscle bands, during the pre‐spawning locomotor activity of the females, coincided with the extrusion of oocytes from the follicle cells (ovulation). No germinal vesicle breakdown (GVBD) was visible and the oocytes were not fertilizable. As the animal began to display the anterior sweeping movements characteristic of spawning, the oocytes streamed out of the gonad and were stored in the gonad basis. The oocytes, which were still non‐fertilizable, were then pressed forward through the first (proximal) section of the gonoduct. GVBD was completed during this rapid transit, but oocytes could not be fertilized unless they had soaked ≥20 min in seawater. In the second (distal) section of the gonoduct, most oocytes were readily fertilizable; fertilization rates increased noticeably after the formation of a bulge beneath the gonopore, which favored the entry of seawater. Hydration of the jelly coat was apparent (i.e., a 60% increase in oocyte surface area). Gamete release occurred in one powerful spurt ∼85 min after the onset of ovulation. This oocyte maturation sequence is expected to occur in holothurian species with similar anatomy and spawning behavior.