Biology - Reproductive System and Mating

Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

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Polyclads are hermaphrodites with male and female reproductive systems in one body. Self fertilization is possible but outcrossing is the rule. The reproductive structures of polyclads are not well-defined organs as may be found in many other animals. Instead, they are scattered throughout the parenchyma.

When sperm are produced, they often collect in an enlarged reservoir called the seminal vesicle before being moved into the copulatory organ. Prostatic secretions that nourish the sperm and extend their life span, are added to the mixture. The arrangement of the seminal vesicle, prostatic vesicle, copulatory organ and various ducts is extremely varied and complex.

Reproductive system of Armatoplana colombiana. The anterior end of the animal is to the left.

It is precisely this arrangment that can be used by taxonomists to determine many of the polyclad species. The copulatory organ can be armed with hard, sclerotinized structures such as stylets or spines.

extruded stylet

Armed stylet extruded from the acotylean Styloplanocera fasciata.

Photo credit: Bolanos & Quiroga

Eggs are produced in ovaries and moved to uteri that usually lie alongside the pharynx. Egg-filled uteri can sometimes been seen through a transparent body wall or when a worm is turned upside down. Fertilized eggs are surrounded by a hardened shell or capsule. Egg capsules are released to the outside where they are being glued down by additional secretions from the female pore.

Melloplana ferruginea showing egg-filled uteri along either side of pharynx.

Fertilization of the eggs is internal and sperm must be delivered to the interior of the mate. It is possible that fertilization is mutal because polycalds are hermaphroditic, thus, able to function both as male and female. However, it is more likely that one worm acts as the male and the other as the female.

A few species of flatworms (especially the colorful pseudocerotids that live on coral reefs) have evolved other mechanisms to assure fertilization. They use a method known as hypodermic insemination. It means that they jab their penis through the skin of a partner and and then release their sperm into the other worm's body. The sperm then finds its way to the eggs for fertilization.

Which worm acts as the male injecting its sperm and which worm acts as the female bearing the burden of egg laying? Their roles are determined during a behavior known as penis fencing. Penis fencing was first described and documented on film by Dr. Leslie Newman, one of the flatworm wranglers (the link will take you to the PBS site "The Shape of Life" where you can see a short video clip of penis fencing flatworms; you will need the RealPlayer plug-in to view the video).

Penis fencing begins, when two pseudocerotid worms approach each other and start to rear up with their anterior ends. They begin to trust out their penis (in some species two of them), armed with a fine, hard stylet that acts like a hypodermic needle.

Each worm then tries to jab its penis into any part of the partner's body. The worm to first penetrate its partner's body will be able to release its sperm. Penis fencing may go on repeatedly, and often the wounds filled with whitish sperm can be seen on the surface of the partner (arrow).

Once fertilized, the eggs are attached to the substrate, develop, and after a few days, young worms hatch and crawl away. Most acotylean polyclads develop this way. In others though, the eggs hatch into tiny larvae covered with 4 or 8 ciliated arm-like lobes. These larvae, called Müller’s or Götte’s larvae depending on the number of lobes, will swim about in the plankton and will have to undergo metamorphosis before settling to the bottom.

The larva of a polyclad flatworm. This image is from BIODIDAC

These pages are for educational purposes only. If you use any of the information or the images in your classes, please credit these web pages. © MK Litvaitis, 2005

Photo credits to Leslie Newman, Andrew Flowers, or Anne DuPont, unnless otherwise indicated.


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