Greenfish(Stichopus chloronotus)

Mommy... where do baby sea cucumbers come from?

The species Stichopus chloronotus is a dioecious species meaning the male and female reproductive parts are contained in separate individuals; however, the two sexes cannot be determined by external characteristics. For sexual reproduction within holothurians there is a single gonad located towards the anterior part of the coelom.  Within the gonad reside either one or two tufts of tubules. Stichopus chloronotus has two tufts of tubules, one on each side of the mesentery. The tubules open up into a gonoduct which leads to a gonopore, or genital orifice, located on the top of the organism slightly behind the tentacles.  This pore is where the gametes are released during spawning.  While spawning, the sea cucumber lifts its anterior end vertically off the ground while leaving the posterior end on the ground. When the gametes are released, they are expelled from the genital orifice in a continuous flow, somewhat resembling a thin white thread.  This spawning process only occurs two times per year, usually sometime around mid- November and late January, both of which are during the hot seasons of the Indo-Pacific region. After expulsion of the gametes occurs, fertilization takes place externally in the water.  Following about three days of development the embryo becomes a free-swimming larva. Eventually the larva develops into a juvenile with tentacles and podia.

 Asexual Reproduction

Not only is this species of sea cucumber able to produce sexually, it is capable, like a few other species in the Aspidochriotida, of performing asexual reproduction. Besides Stichopus chloronotus, seven other species of Aspidochirotida are capable of this unusual reproductive strategy. Asexual reproduction in these species is performed through the process of transverse fission.  In most sea cucumber species capable of asexually reproducing, a method is used where the organism’s anterior and posterior end rotate in opposite directions. After a while, the two ends slowly move in different directions, eventually causing the body wall to tear and split the organism into two separate individuals. However, Stichopus chloronotus uses a different method of transverse fission in order to divide itself.  More specifically, the organism starts by creating a constriction in the center of its body. While the posterior region of its body stays stationary the anterior end begins to move forward. This results in a more intense constriction in the center of the body. As the anterior end moves further away from the posterior, the constriction in the center, or fission site, begins to turn in to almost nothing but a fluid like substance. Following this, the two halves separate with ease. Based on a study, this entire process only takes a few minutes. Following the fission, it takes around a day for the tissue at the fission site to heal. The body wall of Stichopus chloronotus is often referred to as “catch connective tissue” and it is very thin and fluid-like. It is believed that this tissue is the reason why it is possible for Stichopus chloronotus to undergo transverse fission with more ease than other species capable of the phenomenon. Furthermore, it is thought that this tissue also helps with the speedy recovery that Stichopus chloronotus is capable of after fission has occurred. Transverse fission usually occurs during cool portions of the year (around June in the Indo-Pacific region) and it is also thought that it mainly takes place a night.

Now learn more about interactions Stichopus chloronotus has with other species!